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UNIVERSITÉ DU QUÉBEC À MONTRÉAL REGIONAL AND GLOBAL PATTERNS IN CARBON EXPORT FROM LAND TO WATER THESIS PRESENTED IN PARTrAL FULFILLMENT OF THE DOCTORAL DEGREE IN BIOLOGY BY MINGFENG LI JANUARY2016

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Page 1: Regional and global patterns in carbon export from land to water

UNIVERSITÉ DU QUÉBEC À MONTRÉAL

REGIONAL AND GLOBAL PATTERNS IN CARBON EXPORT

FROM LAND TO WATER

THESIS

PRESENTED

IN PARTrAL FULFILLMENT OF

THE DOCTORAL DEGREE IN BIOLOGY

BY

MINGFENG LI

JANUARY2016

Page 2: Regional and global patterns in carbon export from land to water

UNIVERSITÉ DU QUÉBEC À MONTRÉAL Service des bibliothèques

Avertissement

La diffusion de cette thèse se fait dans le respect des droits de son auteur, qui a signé le formulaire Autorisation de reproduire et de diffuser un travail de recherche de cycles supérieurs (SDU-522 - Rév.0?-2011 ). Cette autorisation stipule que <<conformément à l'article 11 du Règlement no 8 des études de cycles supérieurs, [l 'auteur] concède à l'Université du Québec à Montréal une licence non exclusive d'utilisation et de publication de la totalité ou d'une partie importante de [son] travail de recherche pour des fins pédagogiques et non commerciales. Plus précisément, [l 'auteur] autorise l'Université du Québec à Montréal à reproduire , diffuser, prêter, distribuer ou vendre des copies de [son] travail de recherche à des fins non commerciales sur quelque support que ce soit, y compris l'Internet. Cette licence et cette autorisation n'entraînent pas une renonciation de [la] part [de l'auteur] à [ses] droits moraux ni à [ses] droits de propriété intellectuelle. Sauf entente contraire, [l 'auteur] conserve la liberté de diffuser et de commercialiser ou non ce travail dont [il] possède un exemplaire. »

Page 3: Regional and global patterns in carbon export from land to water

UNIVERSITÉ DU QUÉBEC À MONTRÉAL

PATRONS RÉG IONAUX ET MONDIAUX DE L'EXPORTATION DE CARBONE DE LA TERRE À L'EAU

THÈSE PRÉSENTÉE

COMME EXIG ENCE PARTIELLE DU DOCTORAT EN BIOLOGIE

PAR MJNGFENG LI

JANVI ER 20 I6

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iii

ACKNOWLEDGEMENTS

1 wo uld like to express my deep and sincere gratitude to the many peo ple who he lped

me to bring this research project to fruition, directly or indirectly. First of a li , 1. wo uld

like to thank my PhD supervisors, Professer Paul A. del Giorgio and Professe r Yves T.

Prairie, for providing me the opportunity of PhD in aquatic biogeochemistry. 1 am

deeply grateful for their warm-hearted help, insightful profess ionalism, valuable

guidance and tinancial support throughout the project and my entire PhD program.

Their patience, understanding and encouragement carried me on through these

important years of my !ife and let me learn how to shape myse lf a good scientist in

future. Without them, this dissertation wo uld not have been possible. ln a word , my

gratitude to them is rea lly unutterable !

1 would like to thank the committee members, Ors Changhui Peng, Jonathan J. Cole

and Jean-Franço is Hélie, for their very helpful , va luable comments and suggestions to

improve the quality of thi s thesis.

1 am also deeply indebted to a li my colleagues, the lab members of the the lndustrial

Research Chair in Carbon Biogeochemistry in Boreal Aquatic Systems (CarBBAS)

and the UN ESCO Cha ir in Global Environmental Change, held respective ly by Paul

del Giorgio and Yves T. Prairie, fo r their help in the fie ld , in the lab, and in my daily

!ife : Thanks to Jean- Franço is Lapierre, Dominic Yachon, Audrey Campeau, Matthew

Bagard , Marie-Ève Ferland , Franço is Guillemette, Lisa Fauteux, Véronique

Ducharme Riel, Nico las Fortin-St-Gelais, Jerome Comte, Lennie Boutet, Marilyne

Robidoux, Juan-Pablo Nina -Garcia, Caro lina Garcia-Chavez, Cynthia Soued, Ryan

Hutchins, Shoji Thottathil , Roy Nahas and Sara Mercier Bla is. Thanks to postdoctoral

fellows, Mart in Berggren, Cristian Teodoru, Tehri Ras ilo, Clara Ruiz-Gonza lez,

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iv

Tonya DelSontro and Adam Heathcote for va luab le experience and new ideas they

gave us. Special thanks to Ali ce Parkes and Annick St-Pierre, as strong pillars of our

research team, who are be able to run fieldwork, laboratory, logistics, financial and

human resources, easily and smooth ly, thus guaranteeing an excellent environment

for our work and study. 1 also greatly thank Profs. Alison Derry and Beatrix Beisner

for their kind help with my PhD study. 1 wou Id also like to extend my appreciat ions to

the funds that materialize our research group of multiculturalism because nothing

would have been possible without the significant financial support. Thanks to Fonds

de recherche du Québec - Nature et technologies (FRQNT), Natural Sc iences and

Engineering Research Counci l of Canada (NSERC) and Hydro-Quebec, and Groupe

de Recherche Interuniversitaire en Limnologie et en Environnement Aquatique

(GRJL) .

Finally, 1 wo uld like to thank my family, espec ially my wife, Jinxia Wang, who

always understands, encourages and supports me to finish my PhD study, and my

little son, Alexander, and my daughter, Blair, who bring a lot of laughter and energy

to my life, further strengthening my confidence in my PhD study. Thanks to my

parents for their giving me encourages and blessings over telephone, rnaking my life

sunny and full of love.

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CONTENTS

LIST OF FIGURES ................. .. .... ..... .... ............ .. .. .. .... .... ......... ................................ .. ix

LIST OF TABLES .. ........................... .. ..... ... .... ................ ........... ... ................... ....... ... . x ii

ABBREV IATIONS AND ACRONYMS ..... .......... .............................. ... .................... xiv

ABSTRACT ............ ........ ...................... .. ..... .. .. ... ... ......... ... .... ...................................... xv

R ÉSU MÉ .. .. ............................................ ............................. ............ .. .................... .... xvii

INTRODUCTION ...................................... ...... .. ...... .... ...... ..................... ............... .. .... . !

0. 1 T he ro le and importance of river system in carbon cyc lin g ................ .................... l

0 .2 Environmenta l contra is on riverine carbon export from watersheds at landscape

scale .............................................. ..... ........ ..... ... ............ ..... ....... .. ... ... ..... ................ ....... 3

0.3 Tota l carbon export from watersheds to r iver systems ...................... ... ................. ! 0

0.4 Objectives of the thesis .. ..................... ............... ... ............... ......... .... ... .. ...... ... ...... . ll

0.5 Genera l scopes and approaches ... ....... .... ... ....... .. ...... .. ............................. ........... ... 12

0.5.1 Regional riverin e carbon export fi·om notthern catchments ... ...................... 13

0 .5.2 A meta-analysis up-scaling to g loba l r iver in e carbon export to the oceans .. l 4

CHAPTER 1

THE RELATIVE INFLUENCE OF TOPOGRAPHY AND LAND COYER ON

JNORGANIC AND ORGAN IC CARBON EXPORT FROM CATCHMENTS IN

SOUTHERN QUEBEC, CANADA ... .. ... .... .... ... ................ ..... ..... ... ... ........................ . !?

1. 1 Abstract. ... ............ ... ..... .... .. ....... ........ ........ ..... ............. ........ .......... ............ .. .. ... .. .. .. 17

1.2 lntroduction .............................. ........ .. .............. ... ... .. ... ...... .. .... ..... .... ....... .. ........ ..... l 8

1 .3 Materials and methods ................................................... .... ...................... .... .... ...... 21

1.3. 1 Study area ................................................................................. .................... 2 1

1.3.2 Samp1ing, ana lyses and ca lcu lations ..... .... ...... ... ....... ................... ... .... ...... .. . 23

1.3.3 Catchment topography and land cover .......... ........... .. .............. ...... ........ ... ... 27

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1.3.4 Statistical ana lyses ... .. .... ....... .... .................................................................... 28

1.4 Results ...... ... ........... ............. ............................... ........................................... ........ 29

1 .4.1 Carbon export. ..... ...................................... ................................................... 29

1.4.2 Factors intluencing carbon export .................................................... .......... .32

1.4.3 lnter-annual variation in carbon export .... .. .... ............................................. .36

1.5 Discussion ......................................... ...... .. .. ....... .. ....... ..... . .. .... ..... ... ...... .... .... .... .. .. . 38

1.5.1 Drivers of terrestrial carbon expo rt to aquatic systems ...................... .. ....... .40

1.5.1. 1 Drivers of DfC Export... ................... .. ........... .... .......... ... .. ......... ... .... .. .4 1

1.5. 1.2 Drivers of DOC Export... .................................................................... .43

1.5.1.3 Drivers ofTC Export. ................ ................. .. ... .. ......... ...... ... ............... .44

1.5.2 Influence of land cover on the fonns of carbon exported ................... .. ....... .46

1.5.3 Inter-annual variatio n in carbon export... ....................................... .... ...... ... .48

CHAPTER 11

MAGNITUDE AND COMPOS IT ION OF CARBON EXPORT D FROM BOREAL

CATCHMENTS TO RIVER SYSTEMS IN NORTHERN QUÉBEC,

CANADA ................... ... ............ .................... ... ........ ...... ................ ... ............ .............. 5 1

2 .1 Abstract. .. ....... ....... ......... .......... ............ .. ... ..... ....... ... ........ ................ .... ... ...... .. .... .. . 5 1

2.2 Introductio n .. ..... .. ..... .......... ... .... ... .. .... .... ... .......... ... .............. ... ... ..... ............. .......... 52

2.3 Materials and methods ............. ......................... .. ... ..... ........ .. ................................. 55

2.3. 1 Study area ..... ...... ......... ....... .... ............. ... .............................................. ........ 55

2.3.2 Samplin g and chem ica l ana lyses .................. ........... ......... ....... .......... ........... 57

2.3.3 Discharge and DOC and DIC flux calculat ions ................... .......... ............... 57

2 .3.4 Aquatic gas emiss ions ................................................... ... .... ......................... 60

2 .3.5 Watershed properties ...... ........................................ ..... ............................. .. ... 60

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2.3.6 Statist ica l Ana lyses .............. .......... .... ........ ...... .... ......................................... 6 1

2.4 Resul ts .......... ... .... ... ...... ....... .... ........... .................. ................... ............................... 6 1

2.4. 1 Riverine DOC and DI C expo rts .................................. .. ..... .... .... .... ... .... .... .... 6 1

2.4.2 Watershed aquatic C02 and CH4 emissions ........... ............................ ... .. ...... 63

2.4.3 Tota l C export from borea l catchments ....... .... ........... ... ..... ........................... 64

2.5 DI SCUSSIONS .............. .................................. ...... ............................................... 69

2.5. 1 River-mediated DOC, DIC and C02 loss from landscapes ............. ...... ....... 69

2.5.2 Magn itude and composit ion of tota l C export .................. ... ................. .... .... 73

2.5.3 Cross-regio na l d ifferences in the magnitude and compos itio n of C

export ...................................................... ..................................................................... 75

2.5.4 Im pli cat io ns to terrestr ia l C budgets ...................................... ...... ........ ... ...... 76

CHAPTER Ill

A GLOBAL ANALYS IS OF RI V ERINE CARBON EXPORTTO THE OCEANS

3. 1 Abstract .. .. .. ........................................ ...... ...... ....... ... .... ............ .......................... ... 81

3.2 lntroductio n ..... ... ... ....... ...... .. ... .. .. .... ... ....................... .... ........................................ 82

3.3 Resul ts and discuss ions ................... ................... .... .................................... ........... 84

3.3 .1 Dri vers of riverine C expo lt ...... ............... ...... ......... .... ...... ...... .. .. .. .. ..... ... ...... 84

3.3.2 A new estimate of g loba l riverine C export to the oceans ...... ...................... 89

3.3.3 Past and future effects ofanthropogenic disturbances on g loba l riverine C

export ........................................................ .......................................................... 93

3.4 Methods ................. ... ......... .. .... .......... ....................... .. ......... ..... .... .... .... .... ... .......... 95

CHAPTER IV

CONCLUSION S .. .............. .................... ................ ..... .............. .. ....... ........................ 97

4. 1 Main co ntributio ns .............................. .................. ... ..... ... ...... ...... ... ........ ............. 97

4.2 Main innovatio ns ................................................ ...................... ..... ...... ............... 1 0 1

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4.3 lm portant implications ... .... ....... ... ... .. .. .... ............ .. ... .... ..... ..... .... .. ........... .. ... ....... ! 03

REFERENCES ...... ...................... ... ..... .... .... ...... ..... ..... .... .... .............. ... ........ ....... ..... ] 07

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LIST OF FIGURES

Figure O. 1 The biogeochemistry of carbon in rivers .............. ... ..... .. .............. ..... ... .... 6

Figure O. 2 The research areas for the regional studies on riverine carbon export (ET:

Eastern Townships; JB: James Bay; AB: Abitibi. JB and AB are closely

neighbored in northern lowland ofQuebec) ... .......... .. ....................... ..... l4

Figure O. 3 The distributional map of the catchments (in brown) for the

meta-analysis, covering 56% of global terrestrial area and 74% of global

exorheic area (exc lud ing Antarctica) ........... .. ............ ......... ... ..... ... ......... l5

Figure 1. 1 DOC export versus DlC export for the 83 catchments. Exports are

expressed as the average of 2004 and 2005 measurements in grams of C

per square meter of total catchment area per year. ........ .... ........... .. ........ 30

Figure 1. 2 Carbon expotted as DIC (solid squares) and DOC (open circles) in g m·2

yr·' for the 83 catchments, average of 2004 and 2005 measurements, as a

function of total catchment area. Significant correlations with catchment

area are shawn for DIC export (thin tine) , DOC export (thick tine) and

TC (dashed tine, points not shown) ................................................ ..... ... 31

Figure 1. 3 Principal component analysis of DIC, DOC and TC exports (average of

2004 and 2005 values) and key topographie and land caver variab les for

the 83 catchrnents .................................. ....... ................... .......... ... .. ..... .. . .33

Figure 1. 4 Variance partitioning in the multiple linear regression models of DlC

and DOC export, showing the percentage of variability explained by

each component variable and the remaining variabi li ty, unexplained by

the rnodels ..... .. ......... ..................... ............................. ... ............... .. ..... .... 35

Figure 1. 5 lnter-annual variation in DIC and DOC exports for 32 basins. For each

site, export is reported as the difference between the export in the given

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year and the 3-year average export for that site (in g m-2 yr-1). The mean

di ffe rence is the small square within the larger 751h percentile box, with

9Yh percentile whiskers, a median line, and asterisks for maximum and

minimum va lues. Diffe rent letters within a panel indicate significant

diffe rences among yea rs (one-way ANOVA and Tukey-Kramer post-hoc

test) ... ... .. .. ..... .. ....... ... .. ...... ... ... .... .. ... .... ................ ....... .. ............ ... ........ .... 3 7

Fi gure 1. 6 Carbon export as the sum of DIC expott and DOC export in g m-2 yr-1

and the DIC/DOC export ratio versus %vegetation fo r two example

watersheds, an in fl ow to Lac d' Argent (panels A and C) and an in fl ow to

Roxton Pond (panels B and 0 ). The current vegetation cove rage of the

catchment is indicated by an "X" in each pane1... .................... .... ......... .48

Figure 2. 1 Figure. 1 a. Reg ional daily runoff (mm d-1) for the sampling year (201 0),

with the error bars (SE), averaged (from the daily discharges of six

streams and rivers during the water year of our observation (J une 1 st,

2010 through May 3 JS1, 2011 ). b. Relationships between estimated and

measured discharge for the 44 studied rivers .. .. .. ............ .. ........ .. .. .. .. .. .. . 59

Figur e 2. 2 TC export (a) and contribution of different carbon species to TC (b) for

each month in the water year. TC export is expressed in g C per m2

catchment a rea fo r each month .... .. .... .... ........ .... .... .. .............................. 65

Fi gur e 2. 3 a. The average contribution of the various C species in the an nuai total

C export from these borea l watersheds; b. The contribution of DOC

export and of aquatic co2 emiss ions to total c export from these borea l

catchments, as a fun ction of the proportion of water in each catchment

(% water). The latter is mostly driven by the presence of lakes. The

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catchments of rivers of order 1 and zero co ntained no la kes and were

aggregated as o/owater = 0 ...... .. ......... .. .. ....... ........... ... ...... ............. ........ . 66

Figure 2. 4 Co mpariso n of the average contribution of di ffe rent C fonns in the

annua l tota l C export be tween South Abitibi and James Bay regions ... 69

Figure 3. 1 The Maps of Global Distributions of DIC and DOC Expotts ... .. ...... 89-90

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LIST OF TABLES

Ta bl e 1. 1. Stream and catchment characteristics of the 83 study sites. Statistics for

discharge and water chemistry were determined by first averaging a li

measured va lu es from 2004 and 2005 for each of the 83 streams, then

calcu lating the minimum, maximum, mean and standard deviation of

these values (n=83). Stat ist ics for topography, land · cover, geo logy and

soil were obtained from digital elevation models, and maps of

topography, land cover, rock type, surficia l deposits, and soi l order,

using GIS (n=83) ... .. .... .. ..... ....... ................... ...... ... ..... ................ .. .......... 24

Ta bl e 1. 2. Tab le 2. Multiple linear regressio n models predicting DIC, DOC, and

TC export in g m·2 yr· 1 (n=83 for each). Estimates of coefficients and

corresponding p va lues are given for a li variab les offered during the

step-wise selection process. Variables were included in the mode! if

p<0.05 (in bold) and the corresponding R2 values are shown .... .... .. ..... 34

Ta bl e 1. 3. Local climate, gauged daily discharge, as weil as discharge and ore and

DOC concentrations measured in situ at the 32 s ites in 2003, 2004 and

2005 ............................... .......................... .. ................................... .. .... .. . 36

Ta bl e 2. 1. The physical , hydro logica l and chemical characteristics of the 44

catchments and rivers samp led .............................................................. 62

Ta bl e 2. 2. The ranges, means and medians of the flu xes of DOC, ore, POC, C02,

CH4 and TC exported annually from the 44 catchments (units: g C m·2

yr- 1 on catchment area basis) ....... .... ...... .... .. ......... ....... ........................... 63

Ta bl e 2. 3. The comparisons of physical, chemical and biolog ical factors between

James Bay and Abitibi du ring the measurement per iod (June 1 st, 2010

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to May 3 Pt, 20 1 1 ). The va lues were annua lly averaged fro m the 44

streams and ri vers we observed ...................................... .. .. .......... ....... .. 68

Table 2. 4. Co mpariso n of latera l and gaseo us carbon expo rts fro m watersheds in

the previous studies .. .. .... .............. ... ...... ... ........ .... .. .. .. ........... .... ........... .. 70

Tabl e 3. 1. Multip le linear regress ion mode ls pred ict ing DOC, DI C and POC

export in g m·2 yr·1• Est imates of coeffic ients and corresponding p

va lu es a re g iven fo r all var iables offered d uring the step-w ise select ion

process. Variables were inc luded in the mode! if p<O.OOO 1 and the

correspo nding R2 va lu es are shown ........................... ..... .. ....... ... ... .. .. .... 88

Ta bl e 3. 2. T he co mpar isons of our estimates of g lo ba l ri verine carbo n expo rt to the

oceans w ith those in prev io us studies ........ ... ... ... ........... ..... ......... .. ... .. ... 92

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BIC

BSI

CarBBAS

CH4

CID

co2 COD

DIC

DOC

ETC

GES

GHG

PI C

POC

Pg

TC

TIC

TOC

ABBREVIATIONS AND ACRONYMS

Bayesian Information Criterion

Bas in shape index

Carbon Biogeochemistry in Boreal Aquatic systems

Methane

Carbone inorganique dissous

Carbon diox ide

Carbone organique dissous

Disso lved inorganic carbon

Disso lved organic carbon

Exports totaux de carbone

Gaz à effet de serre

Greenhouse gases

Particulate inorganic carbon

Particulate organic carbon

Petagram

Total carbon

Tota l inorganic carbon

Total inorganic carbon

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ABSTRACT

Carbo n ex po rted fro m terrestria l ecosystems to river systems is a cri t ical co mpo nent

of the g lo ba l carbo n cyc le. How much carbo n is ex po rted fro m watersheds, in what

fo rm , and when the ex ports occur, as weil as the future respo nse of riverine carbon

export to c limatic change driven natu ra lly and anthro pogenica ll y, a re major issues of

biogeochem.istry. However, few studies in the past s imulta neo usly explored ri verine

carbo n expo rted in di ffe rent forms and thu s we still do no t have an integrated

perspective of magnitude and regulatio n of tota l riverine carbon expo rt at the regional

and g lo ba l sca les. T he research presented in thi s dissertatio n a ims to explore the

co mpositio n and drivers of tota l carbon export from land to ri ve rs, from watersheds to

no rthern reg ions to the g lo ba l sca le, and to identify the natura l and anthro pogenic

contro ls on the g lo ba l riverine carbon export to the oceans in the context of c limat ic

change. In the thes is project, we used the data co llected by the CarBBAS group over

the past 5 years fro m 127 rivers and streams in Q uebec, and have com bined these

w ith a newly co llated g lo ba l data set of publi shed carbo n co ncentratio ns and/or

ex ports fo r 566 ri vers dra ining a to ta l of 74% of g lo ba l exorheic area .

We fi rst explo red the influ ence of topography and land cover on the co mbin ed

ino rganic and organic carbon export from temperate catchments in so uthern Q uébec

(Chapter 1). O ur resul ts show that whereas both are primar ily driven by regio nal

runoff, to pography is s light ly mo re important than land cover in expla ining the

va riance in DI C export across watersheds, whereas la nd cover is much mo re

important than to pography in determining DOC expo rt. T he inter-annual di ffe rences

in C expo rt are driven mostly by shi f'ts in annual precipitat io n and reg io na l runoff.

Further, the proportion of the catch ment covered by natura l vegetatio n had a negative

effect on DI C export but a pos itive effect on DOC expo rt, sugges ting that a change in

land cover that reduces vegetation (e.g. defo restatio n) wo uld lead to modest decreases

in TC expo rt, but la rge inc reases in the DIC/DOC export ratio. As a fo llow up of

these stud ies in temperate regions, we furthe r quant ifi ed river-mediated expo rt of

disso lved organic and ino rganic C (DOC and DIC), as we il as the in tegrated aquat ic

emiss io ns of both C02 and C H4, fro m 44 borea l catchments that range wide ly in s ize,

to pography and land cover (C hapter 2) . The resulting tota l C export was seaso na lly

ve ry varia ble, dri ven mostly by the annu al runoff cyc le, and averaged 15.5±5.3 g C

m-2 (watershed) yr-1• DOC dom ina ted, on average, this tota l C export over the annua l

cyc le (5 8%), but aquatic C02 emiss ions were a majo r co mpo nent of export in a li

catchments (average 20%). Ou r results confi rm that DOC and DI C exports are most ly

driven by runoff but further regulated by fundamenta lly di ffe rent env iro nmenta l

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fac tors, and that wetlands are a majo r so urce of DOC expo rted to rivers, but further

demo nstrate that lakes w ithin the catchment are a strong DOC sink, such that the net

expo rt of DOC results fro m the ba lance between the two. T he total an nua i C ex ported

via rivers is within the range of net ecosystem prod uction, and has the potentia l to

fu ndamenta lly a lte r our perception of the ro te of these boreal landscapes as so urces or

s inks of atmospheric C02.

The meta-analys is of g loba l river ine carbon export to the oceans has shown that

beyond the expected hydro logie contro l over mate ria l flow, DOC export is mostly

dri ven by a co mbina tion of natural var iables, such the extent of wetlands and the

average organic carbo n content of the catchment so ils, as weil as by anthro pogenic

a lteratio ns of the landscape, such as the extent of cro pland s and, to a lesser degree,

the presence or la rge reservo irs. ln contrast, DIC export was ma inly contro lled by the

extent of carbo nate rocks (positive) and of water bodies (negat ive). ln additio n, the

extent of cropland expla ined a substantia l amount of va riab ili ty. T hese mode ls were

then used to estimate carbon export for a li exo rheic watersheds not present in our

database to derive a new g lobal est imate of carbo n export to the oceans of 0.68±0.05 Pg yr·1, a substantia l revis ion of the often-cited va lu e of 0.9 Pg yr·1• A retrospective

analys is suggests that as much as 40% of the current C export is associated w ith the

extent of agriculture on the planet.

1 n co nclu sion, this thes is has shown that in di ffe rent landscapes and at diffe rent

spatia l sca les, carbon export is driven by a co mbinat ion of natu ra l features of the

landscape and hu man activities. tt a lso high li g hts the d ifferentia i regulatio n of the

inorganic and organic fractio ns of C export. Anthro pogenic impacts due to land

use/cover change may be replac ing the natura l driving forces as the primary

determinants of the magnitud e and co mposit io n of both current and future transfe rs of

carbo n fro m land, thro ugh the hydro logie network, and ultim ate ly reachin g to sea,

imply ing the impo rtance of land use and management in contro lling riverine carbon

export fro m terrestria l ecosystems in the context of human- induced enviro nmenta l

changes, both regio na lly and globa lly.

Key words: ri ver carbon export ; carbon cyc le; greenhouse gases (GHG); d isso lved

organic carbon; disso lved inorganic carbo n.

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RÉSUMÉ

L' expor1ation du carbone (C) des écosystèmes terrestres vers les systèmes flu viaux

est une composante fondamentale du cycle g lo ba l du carbo ne. Co mbien de ca rbo ne

est exporté des bass ins versants, sous que lle fo rme, quand les expo rtations se

produisent-e lles et qu 'e lle sera la ré ponse de l'expo rtatio n du carbo ne ri ve ra in face

aux changeme nts c limatiques nature ls et anthro piques so nt toutes des qu estio ns

biogéochimiques d'intérêt majeur. Peu d'études cependant ont explorées

simultanément l' ex po r1ation des différentes formes du carbone riverain et no us ne

disposons donc pas enco re une perspective in tégrée de la magnitude et du co ntrô le

des expor1atio ns tota les de carbone flu v ia le à l'éche lle régio nale et g lo bale. La

recherche présentée dans cette thèse vise à explo rer la co mpos itio n et les facte urs qui

contrôlent l'expo rtation tota le de carbo ne des riv ières, des bass in s ve rsants des

régions no rdiques jusqu 'à l' éche ll e glo ba le et d'id entifi er les co ntrô les nature ls et

anthropiques de l'exportatio n de carbo ne flu via le g lo ba le vers les océans le co ntexte

des changeme nts c limatiques . Dans cette thèse, no us avo ns utili sé les do nn ées

recue illies par le groupe CarBBAS au cours des c inq derni ères ann ées provenant de

127 rivières et ruisseaux au Québec co mbinées avec un ensembl e no uvellement

rassembl ées de donn ées mondia les publi ées des co ncentratio ns et/ou des ex portatio ns

de carbone de 566 riviè res dra inant un tota l de 74% de la superfic ie exo ré ique

mondia le .

Nous avo ns d'abo rd explo ré l'influence de la topographie et la co uverture te rrestre sur

l'exportatio n combin ée de carbo ne inorganique et organique provenant de bass in s

ve rsants tempérées du sud du Québec (chapitre 1 ). Nos résultats mo ntrent que la

topographie est plus impo rtante que la co uverture terrestre po ur expliquer la

variabilité du carbo ne inorganique dissous (C ID) expo rtée des bass ins ve rsants, bien

que les deux so ient essentie llement co ntrôlées par le ruisse llement régio na l, a lo rs que

la co uverture terrestre est beauco up plus impo r1ante que la topographie dan s la

déterminatio n de l' exportation de carbo ne organique di sso us (COD). Les diffé rences

interannue lles de l' expo rtation du C sont princ ipa lement rég ies par des cha ngements

dans les précipitatio ns annue lles et le ruisse llement régio na l. De plus, la proportion

du bass in versant couvert par la végétation nature lle a eu un effet négatif sur

l'exportation du CID, mais un effet positif sur l'exportatio n du COD, ce qui suggère

qu'un changement de la co uverture terrestre qui diminu e la végétatio n (par exemple ,

la déforestatio n) co nduira it à des diminutio ns modérées de l' exportatio n du C total,

ma is de fortes augmentatio ns du ratio C l D/COD des expo rtat io ns. Pour poursuivre

cette étude dans les régions tempérées, no us avo ns qu antifi é en o utre l'expo rtation par

Page 19: Regional and global patterns in carbon export from land to water

xviii

les rivi ères du COD et C ID, a ins i que les émiss ions aquatiques de C02 et de CH4 de

44 bass ins versants bo réaux de diffé rentes ta illes, topographie et de co uvertures

terrestres (chapitre 2). L'exportatio n de C total éta it très va riable entre les sa iso ns et

notamment contrô lé par le cycle annuel du ruisse llement, avec une moyenne de

15.5±5.3 g C m·2 (bass in versant) an·1• Le COD domine en moyenne cette expo rtatio n

tota le de C sur un cyc le annue l (58%), mais les émiss ions de C0 2 aquatiques éta ient

une composante maje ure de l'expo rtatio n dans tous les bass ins versants (moyenne

20%). Nos résultats confirment que les exportations de COD et CID so nt

principalement co ntrô lés par les eaux de ruisse ll ement, mais auss i par des facteurs

environnementaux fo ndamentalement différents et que les zo nes humides so nt une

so urce majeure de COD expo rtés vers les riv ières. Nos résultats démo ntrent aussi que

les lacs situés dans le bass in versant constituent un puits de COD, de te lle sorte que

l'expo rtatio n nette de COD résulte de l'équilibre entre les deux. Le C tota l expo rté

annue llement par les rivières est comparable à la productio n nette de l' écosystème et a

le potentie l de modifie r fo ndamentalement notre perception du rô le des paysages

boréaux comme so urces ou puits de C0 2 atmosphérique.

La méta-analyse des exportatio ns fluviales g lo bales de carbo ne vers les océans a

montré qu'au-de là du contrô le hydro logique attendue sur le transpo rt de la matière,

1 'exportatio n du COD est principalement entraîné par une co mbina iso n de variables

nature lles, te ls l'étendue des zo nes humides et de la teneur en carbo ne organique

moyenne des so ls du bassin versant , a in si que par des modificatio ns anthropiques du

paysage, te ls que l'étendue des terres cult ivées et, dans une mo indre mes ure, de la

présence de grands réservo irs. En reva nche, l' exportation de CID est princ ipa lement

co ntrôlée par l' étendue de roches carbonatées (positif) et des plans d'eau (négati f) . De

plus, l' étendue de te rres cultivées expliqua it une quantité impo t1ante de variabilité.

Ces modèles ont ensuite été utilisées pour estimer l'exportatio n de carbo ne pour tous

les bass in s exoré iques absents de notre base de donn ées pour établir une no uve lle

estimatio n g lobale de l'expo t1atio n de carbo ne vers les océans de 0.68±0.05 Pg an·1,

une révis io n substantie lle de la valeur so uvent c itée de 0.9 Pg an·1• Une analyse

rétrospective suggère que jusqu'à 40% de l'expo rtatio n actue lle est associée à

l' étendue de l'agriculture sur la planète.

En co nclu sio n, cette thèse a montré que, dans des paysages di ffé rents et à di fférentes

éche lles spatia les, l'exportatio n de carbone est entraîné par une co mbina iso n des

caractéristiques nature lles du paysage et les activ ités huma ines . Elle so uligne

également la régulatio n di ffé rentie lle de 1 ' exportatio n des fractio ns organiques et

inorganiques d u C. Les impacts anthropogéniques en ra iso n des changements de

l'utili satio n des terres et des co uvertures terrestres peuvent remplacer les press io ns

Page 20: Regional and global patterns in carbon export from land to water

xix

nature lles comm e princ ipaux déterminants de l'a mpleur et la compos itio n des

transfe rts actuels et futurs de carbo ne provenant de la terre, par le réseau

hydro log ique pour fin a leme nt atte indre la mer. Ce la implique l'impo rtance de

l'utili sa tio n et de la gestion des terres dans le contrô le de l'expo rtatio n de carbone

n verames des écosystèmes terrestresde dans le co ntexte des changements

env iro nneme ntaux induits par l'ho mme, rég io na lement et mo ndia lement.

Mots clés : exportatio n de carbone, cycle du carbo ne, gaz à effet de serre (G ES),

carbo ne organique disso us, carbo ne inorganique disso us

Page 21: Regional and global patterns in carbon export from land to water

INTRODUCTION

0. 1 The ro te and importance of river system in carbon cyc ling

Carbo n, as the bu ilding block of li fe , plays an im po rtant ro le in a ser ies of processes

that prov ide food, c lothing and fuel for us, and thus we are unavo idably entw ined

with its biogeochemical cyc le. Part icular ly in the past 50 years, human activ ity has

gradua lly beco me a cruc ia l driving fo rce in g loba l warming du e to the anthropogenic

impacts o n reg iona l and g lo bal carbo n cyc ling (V itousek et a l. , 1997; Ver et a l. , 1999).

Actua lly, carbo n cycling has been w idely regarded as the key to our understanding of

the earth surface system and globa l env iro nmenta l change (e.g. c limate change, land

degradat io n and biodiversity loss) because it acts as an essentia l co mpo nent linking

abiot ic to biot ic co mponents of the earth system through photosynthes is and

decomposit ion, and a lso regulates biogeochemica l cyc ling of other e lements (e.g. N, P,

S) (H imes, 1997; C hame ides and Perd ue, 1997; Falkowski et a l. , 2000; He imann &

Reichstein, 2008).

To date, numerous studies have focused on terrestria l and/or aquatic carbo n cyc le

from di ffe rent perspectives using var io us approaches and methods, a iming to have a

better understandin g of w hat are driv ing the co upling between g lo ba l change and

carbon cyc ling , how it is contro lled or intl uenced natura lly and anthropogenica lly,

and the future response of terrestrial and aquat ic eco systems to it ( e .g . Cao and

Woodward, 1998; Cox et al. 2000; Betts, 2000; Feeman et a l. , 2004; Ca llaghan et a l. ,

201 0; Grosse et a l. , 20 Il ; Neigh et a l. 20 13). Part icular ly, eco systems at northern

latitudes have beco me the focus of recent resea rch (e.g. Call aghan et a l. , 201 0 ; Tank

et a l. 20 12; Neigh et a l. 20 13). T his is not only because these eco systems are large

potentia l si nks of carbo n in the atmosphere but a lso because they are very sens itive to

Page 22: Regional and global patterns in carbon export from land to water

2

c limate change (Priee and Apps, 1996; Betts, 2000; Callaghan et a l. , 201 0). Therefo re,

understanding the ro le and importance of bo rea l ecosystems in g lo ba l carbo n cyc le is

cruc ia l to o ur assessment of future globa l env ironmenta l change . Unfo rtunate ly,

however, in most cases riverine carbo n expo rted from terrestria l systems, which plays

a key part in linking terrestria l, aquat ic and atmospheric carbo n cyc les, is rare ly

mentio ned when discuss ing terrestria l or g lo ba l carbo n budgets. ln rea lity, inland

waters ( inc luding streams, ri vers, lakes and wetlands) occupy less than 1% of the

Earth 's surface, but the ir co llective co ntribution to the g loba l carbon flu xes is

disproportionate ly important, co mpared w ith te rrestria l and oceanic ecosystems

(Battin et al., 2009) . Espec ia lly in recent years, the re lative contributio n of inland

waters to the glo ba l carbon budget has been further highlighted (Co le et a l. , 2007;

Battin et a l. , 2009; Benstead and Leigh, 20 12; Raymond et a l. , 20 13). How mu ch

carbon is expo rted fro m watersheds, in what form, and when the expo rts occur, as

weil as the future response of riverine carbon export to g lo ba l warming have thus

become hot iss ues of majo r biogeochemical inte rest. A ltho ugh the re are aspects of the

ro le and im portance of river system in regional carbo n budgets that are re lat ive ly we il

understood, there are still many uncerta inties, particula rly in terms of the magnitude

of so me of the key processes in vo lved . Fo r example, the estim ates of g lo ba l C0 2

evasion fro m inland waters range fro m 0.26 to 3 Pg C (Co le et a l. , 2007;

Aufdenkampe et a l. , 2011 ; Raymo nd et a l. , 201 3; Lauerwald et a l. , 2015). Although

the estima tes of organic carbo n transported from land to sea ranging w ide ly from 0.03

to 1 Pg C yr·1 (Williams, 1971 ; Re ine rs, 1973; Richey et a l. , 1980; Meybeck, 1982;

Ludw ig et a l. , 1996; Schlunz and Schne id er, 2000; Aitkenhead and McDowell , 2000)

have been co nverged to a va lue of around 0.4 Pg C yr·1, the exact quantity still

remains e lus ive because previo us studies were based on the limi ted data, g iven

Page 23: Regional and global patterns in carbon export from land to water

3

similar assumptio ns, and/or co mmonly biased to sorne big rivers, especia lly tropica l

and temperate (Likens et a l. , 1981 ; Meybeck, 1993 ; Ludw ig et a l. , 1996; Cauwet,

. 2002; Da i et a l. , 201 2). As for g lo ba l particulate carbo n ex po rt, the re may be mo re

uncerta inties in the estimates becau se almost a li the previo us estimatio ns were merely

based on an assumed percentage of particulate organic or inorganic carbo n (POC or

PI C) in suspended matter, lacking strong supporting data (Garre! et a l. , 1973;

Likens et a l. , 1981 ; Meybeck, 1982; Ludw ig et a l. , 1996; Cauwet, 2002) , although

Meybeck ( 1982) and Gal y et a l. (20 1 5) estimated g lo ba l riverin e POC expo rt based

on the POC and sedim ent data of 100 and 70 rivers, respective ly. It is thus necessary

to deve lo p a better understanding of the ro le and importance of river systems or

inland waters in linking atmospheric , terrestria l and aquatic carbo n cyc ling , both

regio na lly and g lo ba lly, in the context of g lo bal warming.

0 .2 Enviro nmenta l co ntra is on riverine carbo n expo rt fro m watersheds at landscape

scale

River systems co nnect the atmosphere, hydrosphere, geosphere and bios phere, so

export of carbo n fro m watersheds to river systems is wide ly regarded as an essentia l

compo nent of regio na l and g lobal carbo n cyc ling . Espec ia ll y, headwater streams, the

so urces of river networks, have tight carbon linkages to the ir surrounding terrestrial

environments fro m hill slo pes to stream channels, and are shaped substantia lly by

interactions amo ng hydro logica l, geo morphica l and bio logica l processes that are

associated c losely with the biogeochemica l cyc ling of carbo n. Hence, they are

co ntro lled greatly by land scape va riables and regulate the cyc ling of nutrients (e.g. N,

P, Fe, S) that potentia ll y subsidize the aquatic communities in the downstream

reaches or w ithin the entire watershed (Gomi et a l. , 2002; Wipfli et a l. , 2007).

Page 24: Regional and global patterns in carbon export from land to water

4

Therefo re, the study on land scape co ntra is on carbon expo rt fro m watersheds scaling

fro m a headwater catchment to the glo be is necessary to integrate inland waters into

the terrestria l and g lo ba l carbon budgets, undo ubtedly benefi cia i to better

understanding of biogeochemi cal cyc les of carbon and nutrients in the backgro und of

g loba l change.

However, landscapes di ffe rentiate at diffe rent spatia l and tempo ral sca les due to the

heterogeneo us combinat io ns of c limate, so i! , vegetation, litho logy, la ndfo rm and land

use/cover, th us resu !ting in diffe rent riverine carbon expo rt patterns that are

characterized by di ffe rent composit ion of carbon species that are exported fro m the

watersheds and then transfo rmed a long the river. ln additio n, human activity has

beco me an impo rtant driv ing fo rce fo r g lobal change and this fu rther compli ca tes the

study of the contro lling effects of landscape on riverine carbo n expo rt fro m the

land scapes due to anthro pogenic impacts on the earth surface and regio nal and glo ba l

carbo n ba lances (e.g. defo restat ion, fa rming , damming and urbanization) (e.g. Meyer

& Tate, 1983; Carig nan et a l. , 2000; Westerhoff and Anning, 2000; Danie l et a l. 2002;

Royer and Dav id 2005 ; Laudo n et al. , 2009; Wilson and Xeno poulos, 2009; Hudo n

and Carignan, 2008; Barn es & Raymond, 2009; Alvarez-Co be las et a l. , 20 12; Bau er

et a l. , 20 13). T herefo re, mo re attentio n sho uld be focused on na tura! and

anthropogenic effects on carbon expo rt fro m catchme nts to rive rs, mechanistica lly

and quantitative ly, at di ffe rent temporal and spatia l scales, and on integrating of

riverin e carbon into terrestria l and g loba l carbon cyc les.

The carbon exported from catchments to river sys tems can genera lly be c lass ified into

disso lved organic carbo n (DOC) (<0.45!-!m), resulting fro m leaching and

deco mposition of organic carbo n in plants and so ils, disso lved inorganic carbo n

Page 25: Regional and global patterns in carbon export from land to water

5

(DIC), such as HC03-, C03-2 and H2C03 (or dissolved C02), POC (>0.45!lm),

including litter, wood debris, insects and so il organics, and PIC, debris of carbonate

minerais. Ultimately, the C that is loaded from land into river networks will in part

evade as C02 and CH4 to the atmosphere, in part wi ll flow downstream to the

receiving water bodies or oceans, and the rest will enter aquatic food webs or be

stored in sedi ments in streams, rivers, lakes and/or coasta l oceans. On a catchment

basis, the carbon from landscapes is finally transported out of the catchment in

dissolved (DOC, DJC and dissolved CH4), gaseous (C02 and CH4) and particulate

(POC and PIC) forms. Accord ing to Meybeck ( 1982 & 1 993), abo ut 0.9 petagrams

(Pg) C is annual ly delivered to the oceans via rivers, of which 0.4 Pg of organic

carbon (DOC and POC) is from so il organic carbon, 0.3 Pg of ino rganic carbon (D IC

and PIC) is from erosion of carbo nate rocks in cont inenta l crust, and the other 0.2 Pg

is DIC from soil inorganic carbon. ln reality, the main carbon forms in the river are

DOC, DIC and POC since PIC and dissolved CH4 account for a very smal l fraction of

riverine carbon exported (Aucour et al. , 1 999; Billett & Moore, 2008 ; Li et al. , 20 15)

and few studies were/are focused on them for their re lat ively less importance in

ecology and environmental science. Fig ure 1 shows the biogeochemistry of carbon in

rivers, indicating the sources and fates of riverine carbon from terrestrial organic

carbon may be respired, ingested , stored, exported and flocculated , while DIC may be

lost through C02 evasio n. Recently, the g lobal C02 evasion from inland waters is

est imated from 0.26 to 3 Pg C (Co le et a l. , 2007; Aufdenkampe et a l. , 20 Il ; Raymond

et al. , 201 3; Lauerwald et al. , 201 5), while the globa l CH4 evasion is estimated as 0_ 1

Pg (Bastviken et al. 201 1 ). Moreover, due to the landscape heterogeneity, carbon

export fi-om different landscapes to river systems varies widely arOLmd the

world----for examp le, DOC export ranges from 0.5 (Mulho lland and Watts, 1 982) to

Page 26: Regional and global patterns in carbon export from land to water

6

416 g m·2 yr·1 (Charzanowski et a l. , 1983) whi te riverine DIC export varies from 0.03

(Stets & Strieg l, 2012) to 11 5. 12 g m·2 yr·1 (Tript et a l. , 20 13 ; Sarma et a l. , 20 12).

Precipitation 1 1 Atm

SOURCE

MATERIALS:

Labile Organic

Particulates

Stem flow

Throughflow

Refractory

Particu lates

/

+

' -~ ,, .1'

ALLOCHTHONOUS ' / v v

1 1

Organi~

Acids : 1 1 1

1 .1'

/ 1 1 1

~ v INPUTS:

POC: Litter, Debris,

Trees, lnsects, Pollen

DOC

+

1 1

W\V

DIC

Rock

+CaCOJ

1-jCOJ", co3-2

.1'

' / ,, Weathering ' v

PIC

TRACERSAND

SIGNATURES: --------------~-----

RIVERINE

ENVIRONMENT

Storage

!::.. 13C, !::..

14C chemistry

Entra inment, Mobi 1 ization ( overland, groundwater),

Direct litterfall , slump, and erosion

Ingestion

Export to Downriver nd Marine Environments

J

Figure 1 The biogeochemistry of riverine carbon (modified from Likens (1981))

Page 27: Regional and global patterns in carbon export from land to water

7

Over the past 30 yea rs, great effo rt has been, to vary ing degrees, made to better

understand and quantify riverine TOC (tota l organic carbo n), DOC, POC, DI C (e.g.

Schlesinger and Me lack, 198 1; Mulho lland & Watts, 1982; Hope et a l. , 1994; L udw ig

et a l. , 1996; Stets and Strieg l 20 12; Tank et a l. , 20 12; La pierre et a l. , 20 13;

Dornblaser & Striegl, 20 15; Ga ly et a l. , 20 15), and evasion of C02 and CH4 (e.g.

Kling et a l. , 199 1; Battin et a l. , 2009; Aufdenkam pe et a l. , 20 11 ; Butman & Raymo nd,

20 11 ; Lapierre et al. , 2013; Raymond et al. , 20 13; Campeau et a l. , 20 14 ; Lauerwald et

a l. , 20 15). ln part icular, the export of te rrestr ia l organic carbon (DOC or TOC) to

river systems has been the focus of attention, because it may influence aquat ic

metabo lism and nutrient cyc li ng, is lin ked to carbo n emiss io ns to the atmos phere, and

c losely re lates to water qua lity (e.g. infl uence on the mo bili ty and ava ilab ili ty of

metals and co ntaminants) (e.g . Sch lesinger and Me lack, 198 1; Mulho lland & Watts,

1982 ; Ho pe et a l. , 1994; Ludw ig et a l. , 1996; M ulho lland, 1997; La i, 2003 ;

A lvarez-Co be las et a l. , 20 12; Lapierre et a l. , 20 13; Laro uche et a l. , 20 15). The

influence of landscape var iab les on riverine DOC export a lso has been add ressed

w ide ly fro m diffe rent perspect ives, such as c limate (e.g. Proku shkin et a l. , 2005 ;

Raymo nd & Oh, 2007; Tetzlaff et a l. , 2007; Kéihle r et a l. , 2008; Lepisto et a l, 20 14),

hydro logy (e.g . Ho rnberger et a l. , 1994; He in et a l. , 2003 ; John so n et a l. , 2006;

Dawson et a l. , 2008 ; Kawasak i et a l. , 2008), geo logy (e. g. Telmer and Yeizer, 1999;

Liu et al. , 2000; lnamdar & Mitche ll , 2006; Ca i et a l. , 2008; Llo ret et a l. , 20 11 ),

topography (e.g. D' Arcy and Carignan, 1997;· Jo hnson et a l. , 2000; Pacifie et a l. ,

20 1 0), so il (e.g. Aitkenhead and McDowe ll , 2000; Palmer et a l. , 200 1; Hae i et a l. ,

20 1 0), w ild fi re (e. g . Marchand et a l. ,2009; Laro uche et a l. , 20 15) , and land use/cover

(e.g. Carignan et al. , 2000; France et al., 2000; Wilso n and Xeno poulos, 2009;

Laudon et a l. , 2009; Regnier et al. , 20 15). T he export of DIC from watersheds to

- ··-- --· -----------------------

Page 28: Regional and global patterns in carbon export from land to water

8

aquatic ecosystems, on the other hand , appears to be linked to regional geo logy,

especia lly underlying carbonate and siliceous rocks, as we il as land-use/cover plays

important roles in contro lling riverine DLC export (e.g. Liu et a l. , 2000; Raymond &

Co le, 2003 ; Zhang et a l. , 2009; Li et a l. , 20 1 0 ; Regn ier et al. , 20 15). Dornblaser &

Striegl (20 15) a Iso fou nd that DIC export great ly depends on subsurface flow in

boreal regions whi le in tropical region ore is mainly flushed from surface so il layers

(Markewitz et al. , 2001 ). The effects of other landscape variables on riverine DIC

export are not c lear or less explored . Overall , at a regional sca le, it would appear that

catchment topography and hydrology (D ' Arcy et al. , 1 997; lnamdar & Mitchell , 2006;

Mengistu et a l. 20 14), climate (Schindler et a l. , 1997) and catchment vegetat ion

(France et al. , 2000; Lepisto et a l, 20 1 4) are the most important drivers of DOC

export, whereas geo logy is mo re dominant than land-use in contro lling riverine DIC

export (e.g. Liu et al., 2000; Zhang et al. , 20 1 1; Tank et a l. , 20 1 2). Given that the

controls of c limate and hydrology on carbo n export from catchments have weil

understood relatively, topography and land-use/cover inevitably become the main

determinants of the regional differentiation of carbon exports from catchments. ln

particular, few studies have simultaneously addressed the contro ls of topography and

land-use/cover on DOC, DIC and TC exports from watersheds to aquatic ecosystems

and explored the functiona l and mechanical differences ofthe variab les in contro lling

the magnitude and composition of the carbon exported from the landscape.

Furthermore, previous studies on topographical controls on DOC export have shown

that the DOC export has strong negative relat ionships with catchment s lope ( e.g.

Eckhardt et al. , 1990; D'Arcy et al. , 1997; Hazlett et a l. , 2008) and catchment area

(Âgren et al. , 2007). However, the opposite fi nd ings that Wolock et al. ( 1 997)

reported a strong negative relationship between DOC concentration and catchment

Page 29: Regional and global patterns in carbon export from land to water

9

area while Inamdar & Mitchell (2006) separate ly reported a positive one between

them have further blurred the catchment size effect on DOC export, but indicated that

the relationship is site-specifie. As for the studies on the re lationsh ip between DIC

export and topography, it was mainly limited to the influence of topographical

position on DIC concentration (e.g. Gburek and Fo lmar, 1999; Kling et a l. , 2000;

McGlynn and McDonnell , 2003) , according ly affecting riverine DIC export. With

respect to land-use/caver effect on carbon export, it has been found that the reduction

of natural vegetation due to loggi ng, farming, pasturing o r urbaniz ing co uld

pronouncedly increase riverine DIC exp01t (e.g. Daniel et a l. 2002; Raymond & Co le,

2003; Baker et a l. , 2008; Barnes & Raymond , 2009; Regnier et al. , 20 15) but had

varyi ng influence on DOC export to aquatic ecosystems. For examp le, most studies

fo und that forest-harvesting in catchments cou ld significant ly increase DOC export to

receiving waters (e.g. Carignan et al. , 2000; France et a l. 2000; Lamontagne et a l. ,

2000; Nieminen , 2004; O ' Driscoll et al. , 2006; Laudon et al. , 2009; Winkler et a l. ,

2009). However, sorne noted that c lear-cutting caused little change in DOC export

(Hobbie & Likens, 1973; McDowell & Likens, 1988; Moore & Jackson, 1989;

Piirainen et a l. 2002), and severa! even found that clear-cutting co uld significant ly

reduce DOC export from forested watersheds (Meyer & Tate, 1983; McLaughlin and

Phillips, 2006). As for farming effects on DOC expott, sorne reported that agricu ltural

streams (affected by crops and livestock grazing) were often of lower DOC expott

than streams in forest -, wet land- and heath land-dom in ated catchments (e.g. Cronan et

al. 1999; Royer and David 2005; Alvarez-Cobe las et al. , 20 1 0), wh ile sorne others

addressed that agricultural land use had no pronounced influence on DOC export

from the watersheds (Vidon et al. , 2008; Wilson and Xenopoulos, 2009). Reverse ly, a

significant positive relationship between agricultural use and catchment DOC export

Page 30: Regional and global patterns in carbon export from land to water

1 0

was co llective ly supported by the 24-year sur.vey of cropping effect on organic

discharge from the Rhone river watershed in US A (Corre li et a l. , 2001 ), the statistica l

analys is of va ried bo rea l catchments in F in la nd (Rantakari & Ko rte la inen, 2008) and

th e fi e ld measureme nts in Zhu jiang River, China (Sun et a l. , 201 0; Zhang et al. , 20 Il) .

Nevertheless, the re lative importance of topography and land-use/cover on DOC, DIC

or TC export and the effects of landscape va riables on compositio n of tota l riverine

carbo n exported from the landscape have been rare ly repo rted . Espec ia lly in the

backgro und of glo ba l environmental change driven largely by anthro pogenic

d isturbances, it is thu s necessary to further exp lo re the re lative effects of topography

and land-use/cover change on the magnitude and co mpos ition of the carbo n expo rted

fro m watersheds to aquatic ecosystems, thus better understanding the ro le and

importance of the rive r carbon exported from the watershed in terrestria l and g lo ba l

carbo n cyc les.

0.3 Tota l carbon expo rt from watersheds to river sys tems

As mentioned above, most previo us studies have foc used separate ly o n exploring the

loading of indiv idual C components (TOC, DOC, POC and DIC) and degass ing of the

tota l carbon expo rted fro m the watersheds (Mulho lland and Watts, 1982; Hope et a l. ,

1994; A lvarez-Co belas et a l. , 20 12; Hoss le r and Bauer, 20 13). A ltho ugh a

considerable in s ight into the dynamics of spec ifi e carbon fo rms has been ga ined w ith

this approach, it neverthe less has yie ld ed a rather fragme nted view of the magnitude

and regulation of TC expo rt from watersheds, and of the re lative importance of

di ffe rent indiv idual carbo n spec ies and potentia l interactio ns between them. ln

addition, the few studies that have quantified TC expo rt and co mpared disso lved

carbon expo rt and degassed C0 2 and CH4, most of these, were limited to one or

Page 31: Regional and global patterns in carbon export from land to water

l l

severa! small catchment(s) (Hope et a l. , 2001 ; Billett et a l. , 2008 ; Dinsmore et a l. ,

201 3; Wallin et a l,. 2013). lt is currently still diffi cult to produce an integrated view

of TC exported fro m landscapes to river systems and to characterize the

spatial -tempo ral diffe rences of TC and its compositio n. To develo p a better

understanding o f the expo1t of terrestrial carbo n to river systems requ 1res

simultaneo us observation of the main components of riverine carbon fro m the

landscape across a range of land scape types, watershed s izes and c limate, ba th

theo retica lly and practica lly. Only in this way, can the magnitude and co mposition of

total carbon expo 1ted to ri ver sys tems be quantified at di ffe rent space/time scales, and

hence a better understanding of carbon expo rt from watersheds be rea lized fro m the

integrated perspective.

0.4 Objectives of the thes is

The general objective of this thesis is to explore the magnitude and co ntra is of carbon

export fro m watersheds to rivers, scaling from reg io nal to g lo ba l leve ls . lt has been

divided into 3 sub-objectives: 1) to better understand natura l and anthro pogenic

effects on carbo n export from temperate watersheds and the response o f rive rine

carbon expo rt to human activities in the co ntext of g lo bal warming; 2) to identi fy the

magnitude of TC expo rt from boreal landscape, characterize the spatia l-tempo ral

a lteratio n of carbo n co mposit io n of the TC exported (DOC, DI C, POC, PI C, C0 2 and

CH4), and explo re the regional TC expo1t pattern in Quebec; 3) to upsca le the

reg ional studies on riverine carbo n expo rt to the glo bal sca le so as to cla ri fy the major

natural and anthro pogenic drivers on g lo bal riverine carbo n to the oceans thro ugh a

meta-analys is. T he reg io nal studies in the thesis are based on 127 ri ve r catchments

(83 and 44 in temperate and bo rea l reg ions, respeçtively) in Quebec for which the

Page 32: Regional and global patterns in carbon export from land to water

1 2

Aquatic group at Université du Québec à Mo ntréal had made direct measurements

over the co urse of past 10 years, and whic h were used to explore the regio na l TC

export patterns . In addition, a g lo ba l dataset of publi shed data co mpris ing 566 rivers

was assembled to revis it the g lo ba l TC expo rt patterns in the context of g lo ba l

warming and re-estimate the g lo ba l ri ve rine carbo n budget fi·o m the land to the ocean

and to explo re natura l and anthropogenic drive rs on g lobal riverine carbo n expo tt at

the g loba l sca le.

The three sub-objecti ves of this thesis have been presented as the three chapters,

which are separate ly written in the fo rm of scientific artic le:

Chapter 1 The re lative influence of topography and land caver on inorganic and

organic carbon expo rt from catchments in so uthern Quebec, Canada.

Chapter 2 Magnitude and compos ition of ca rbo n expo rted fi·o m bo rea l catchments to

river systems in northern Quebec, Canada.

Chapter 3 A g lo ba l ana lys is of riverin e carbo n expo rt to the oceans.

T hro ugh these regio na l and g loba l studies, the expo tt of carbo n from watersheds to

aquatic ecosystems w ill be better understood and so rne benefici a i bases fo r the further

study on g lobal carbon cyc le and c limate change could be provid ed .

0.5 Genera l scopes and approaches

This thesis a ims at explo ring the carbo n export fro m watersheds to aquatic

ecosystems and integrating river systems into reg io na l and g lo ba l carbo n cyc les or

budgets. T he sco pe of the studies is foc used on carbo n export fro m land to river

systems, its main c limatic, hydro logie, geo morphic and bio log ical drivers, and the

potent ia l influence of hUJna n activities at the sca les va rying fro m a catchment to the

Page 33: Regional and global patterns in carbon export from land to water

globe. Therefo re, a series of mixed approaches ( e.g. co llecting data, cha racteriz ing

catchments, making mode ls) are applied to ex plo re the regional spatia l-temporal

patterns of carbon ex port from watersheds to aquatic ecosystem in the natural

environment so as to upsca le this study from a catchm ent to the g lobe.

0.5.1 Regional riverin e carbon export from northern catchments

ln this thes is, 1 used the data of 127 rivers and streams in Quebec, ofwhich 83 and 44

are respective ly in temperate and borea l regions (see Figure 2), that had been

intens ive ly sampled, respective ly, fo r 1 to 3 years. For each sampled system, a series

of phys ica l (temperature, discharge, pC0 2 etc.) and chemica l (pH va lue,

concent rations of DOC, DI C and nutrients, and flu xes of C0 2 and C H4) were

measured or calculated, so as to link them to ri verine carbon exported fi·om the

landscapes. Topographie (e.g. slope, e levation, shape and area of a catchrnent) and

land use/cover (e.g. fo rest% , vegetation% , pasture%, wetland%, lake%) variables

were extracted from the dig itized maps using A reG IS 10 and used to characterize

each catchment. Then r iverine carbon exported tfom each catchment was calcu lated

as the product of discharge and DOC and DIC co ncentrations. T he evas io n of C0 2

and CH4 fro m surface waters was adjusted to the flu xes from the entire catchment

area . Fina lly , TC export ffo m watersheds to rivers was estimated from the sum of

these components, and 1 ex plo red the relative contribution of the various C

co mponents (DOC, DIC, C0 2) to TC ex port, and its drivers using a series sing le or

multiple linear regress ion mode ls.

Page 34: Regional and global patterns in carbon export from land to water

l 4

Af·J•II ( ; 1

lm\•nriLI1":'

JB

r

Ot-:owo c•

QUCCEC

~'J~rvc- hur•.rve .4 J:>t!\.ll, 'irJC'.i

~·~,~(]

""" \

• •

f'RlN,"" [ Cr. ~~~:~~en oLWAI!L l'l14'JO

~ r tl ir.,<c$'1

Figure 2 T he research areas for the reg ional studies on riverine carbon expo rt (ET :

Eastern Townships; JB : James Bay; AB: Abitibi . JB and AB, two sub-regio ns of this

study area, are c lose ly ne ighbo red in no rthern lowland of Quebec)

0 .5.2 A meta-ana lys is up-sca ling to g lo ba l riverine carbo n expo ri to the oceans

1 carried out the meta-analys is of publi shed data on river C co ncentration and expo rt

that were co llected fro m publi cations and technica l reports publi shed mostly after

year 2000, covering 566 rivers wo rldw id e, dra ining 74% of glo bal exoreic area. The

catchments fo r w hic.h 1 co llected data are shawn in the map (F igure. 3 & Appendix

A). The data on catchment area and mult i-year average river discharge were taken

mostly from Meybeck and Ragu ( 1996) (www.unep.org), while others based on the

specifi e references. A li the river mo uth coordinates have been identified w ith Google

Maps. River length and e levat io n in watersheds were mostly co llected from

Page 35: Regional and global patterns in carbon export from land to water

.1

Figure 3 The total sam pied catchment area (in brown) is 56% of the global terrestrial area, covering 74% of the global exorheic area (excluding Antarctica)

1 5

http ://www. waterfootprint.org . So il organic carbo n, percent land cove r/use and

percent carbonate in the catchments were extracted respectively fi·om the g lo ba l land

use/cover and geo logica l map using ArcG IS 1 O. We only inc luded studies tha t had

fo llowed ri verine DOC, DI C, POC or PI C concent ratio ns and expo rts fo r at least one

full annua l cyc le. Sorne big rivers studied by d ifferent groups, in which case we used

the average of the river carbo n concentrat io ns or exports reported in the va rious

studies. Multiple linear regress io n model is used to explore the re lat io nships of

nvenne DOC, DI C, POC and PIC exports to env ironmental va riables and human

activ ities . T he carbo n export mode ls we made are used to estimate the riverine carbon

export of the other watersheds to the oceans and the mi ss ing C data of the 566 ri vers,

and predict the future trend of g lo bal ri verine carbon expo rt fi·om land to sea in the

co ntext of g loba l warming and under the pressure of cropland expansion.

Page 36: Regional and global patterns in carbon export from land to water

1 6

0.5.3 Stati stica l ana lys is

Simple and multiple linear regressio ns and covariance ana lyses (AN COV A) were

used to identi fy the re latio nship s between carbo n expo rt/concentratio n and

environmenta l variables and test s ig niti ca nt diffe rences in the reg io na l and g loba l

patterns of riverine carbon export. Data were log 1 0 transfo rmed so metimes so as to

satisfy the co nditions of ho moscedastic ity and normality. In a li the stat istical ana lyses,

the thresho ld for signitica nce is P<0.05 . Statistical ana lyses were carried out on JMP

9.3 (SAS in stitute) .

N.B. Refe rences cited in the introductio n a re presented at the end of the thes is.

Page 37: Regional and global patterns in carbon export from land to water

C HA PTER 1

T HE RELATIV E IN FL UENCE OF TOPOGRAPHY AND LAND COYER ON

INORG ANI C AND ORG ANl C CA RBON EX PORT FROM CATCHM ENTS IN

SO UTH ERN Q UE BEC, CANADA

Mingfeng Li , Paul A. de l Giorg io, A lice H. Pa rkes and Yves T. Pra irie

l 7

Published in Journal of Geophysical Research: Biogeosciences, (2 01 5), 120,

doi: 10. 1 00212015JG003073.

Département des sc iences bio logiques, Université du Québec à Mo ntréa l, C. P. 8888,

suce. Centre-vill e , Mo ntréa l, Q uébec, H3C 3P8 Canada

N.B. References cited in this chapter are presented at the end of the thesis.

I. I Abstract

Expo rt of carbo n (C) fro m wate rsheds represents a key co mponent of local and

regio na l C budgets. We ex plo red the magnitude, variability and drivers of inorganic,

organic and tota l C expo rt fro m 83 temperate catchments in so uthern Qu ébec, Canada.

The average dissolved inorganic (DI C), disso lved organic (DOC) and tota l C (TC)

expor1s from these catchments were 4 .6, 5. 1 and I 0.2 g m-2 yr-1, respective ly.

Multiple regressio n models, us ing a co mbination of topographica l va riables

(catchment area, shape and slo pe), a long w ith land-caver va riab les (%vegetation,

%wetland , % lake and building density) , expla ined 34%, 62% a nd 53% of the

variability in the DI C, DOC, and TC expe rts, respective ly. An examinatio n of

va riance partitio ning in the models revea led that topography is s lightly more

impo rtant than land co ver in expla ining the variance in DIC ex port ( 19% vs 15%),

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1 8

whereas land caver is much more important than topography in determining DOC

export (44% vs 18%). lnterestingly, %vegetation had a negative etfect on DIC export

but a positive effect on DOC export, suggesting that a change in land caver that

reduces vegetation (e.g. deforestation) would lead to modest decreases in TC export,

but large increases in the DIC/DOC export ratio. We conclude that topography and

land caver together determine DIC, DOC and TC exports. While topography is stat ic,

land caver can be altered, which will determine the quantity, form and by extension

the fate of C expo rted from these catchments. Fina lly, ann ual differences in export

values that are related to temperature and precipitation suggest that climate change

a lso have an impact on C export.

Key words: riverine carbon expo rt; DOC; DIC; topography; land caver; carbon cyc le

1.2 Introduction

The export of materials from land to fluvial networks and eventua lly to the ~cean has

been a major focus of research for decades (Likens and Bormann, 1974; Dillon and

Malot, 2005; Hassler and Bauer, 20 13). Not only are these land-derived materials

transported and transformed during transport, they also influ ence the functioning of

the receiving aquatic eco systems (Cole and Caraco, 2001; Aufdenkampe et al. , 20 Il).

More recently, latera l inputs of C from watersheds have been recognized as important

not just to in land and coastal waters, but a lso to our understanding of the terrestrial C

budget as we il (Cole et al., 200 7; Ballin et al., 2009; Buffam et al. , 201 1; Stets and

Striegl, 20 12; Dornblaser and Striegl, 20 15). Most of the d issa lved and parti cu la te

organic C expo rted from watersheds originates from terrestrial prirnary production

(Kardjilov et al. , 2006 ; Wilkinson et al., 20 13; Ga/y et al., 20 15). Sirnilar ly, most of

the DIC is ultirnately of biological origin because bicarbonate and carbonate ions are

Page 39: Regional and global patterns in carbon export from land to water

1 9

derived from the interaction between respiratory so i! co2 and so i! minera is th ro ugh

the process of chemica l weathering (Liu et al., 2000; Zhang et al., 2009; Tank et al.,

20 12; Wang et al., 20 12). Regardless of its orig in , C expo•t ultimate ly represents a

loss of terrestriEtl primary production that needs to be acco unted fo r in regional C

budgets. How much C is !ost fro m watersheds, in what fo rm, and w hen these expo rts

occu r, a re issues of majo r biogeochemica l interest .

The fo rm in w hich C is exported is of c ritica l impo rtance in determining its fa te. It

large ly dictates the extent to whic h the C w ill e ither be reta ined in the local aquatic

system, re leased to the atmosphere, stored in sediments, or transpo rted downstream,

because di fferent fonns are not regulated by the same bio logica l, chemica l, and

phys ical processes. For exa mple, a significant portio n of the DOC entering aquatic

systems is transfo rmed by micro-organisms, such that it is e ither inco rporated into

bio mass or respired as an energy source (Tranvik, 1992; Ne.ff and Asner, 2001 ). DOC

is a lso affected by photo-chemica l processes that may minera lize into C0 2 (Lapierre

et al. , 201 3), render it more susceptible to micro bia l processes, or even tlocculate in to

POC (von Wachenfeldt et al., 2008, 2009). In contrast, the io nie fract io n of DIC (i.e.

C0 32- and HC0 3-) is like ly to be have in a mo re co nservat ive manner (Zhai et al. ,

2007; MacPherson et al., 2008), whereas the disso lved C02 fractio n will be largely

!ost to the atmosphere, w ith sorne be ing ass imilated during photosynthesis (Striegl et

al., 20 12; Wall in et al., 20 13) . Because DOC and DJ C are processed differently in

aquatic systems, the two C spec ies w ill impact C budgets in di ffe rent ways and thu s

sho uld be examined indi vidua lly. lt fo llows fro m this that the expo rt of these two

general fo rms of C (DI C and DOC) w ill like ly not be driven by the same fac tors .

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2 0

A review of the literature shows that DOC export is at !east partly dependent on

aspects of catchment topography, su ch as si ope (Eckhardt and Moore , 1990; D'Arcy

and Carignan, 1997; Hazlett et al. , 2008), area (Mu/ho/land, 1997; France et al.,

2000; Agren et al. , 2007), or e levation (Johnson et al. 2000; Hazlett and Foster,

2002). However, land caver changes, such as deforestation, a lso have an influence on

DOC expo r1 (Meyer and Tate , 1983; Carignan et al., 2000; McLaughlin and Phillips,

2006; France et al. , 1996 ; Wilson and Xenopoulos, 2008) . Although wetlands are

wide ly regarded as sources of DOC, wet land loss due to human activities can have

varying effects on DOC export, depending on land use and management practices

(Royer and David, 2005; Armstrong et al., 201 0; Stanley et al. , 20 12). External

forcing, such as hydrology (Eckhardt and Moore, 1990; D 'Arcy and Carignan, 1997)

and c limate (Freeman et al., 200 1; Raymond and Ho , 2007; Raike et al. , 2012;

Lepisto et al., 2014) also strongly modulate DOC export.

Dissolved inorganic carbon export, on the ether hand , is influenced by catchment

geology, in particular by the presence of carbonate deposits in the catchment (Liu et

al. , 2000; Zhang et al. , 2009; Tank et al. , 20 12). Sorne studies have noted that

topographical position and basin e levat ion have a marked effect on concentration and

export of DIC from watersheds (Soranno et al., 1999; Kling et al., 2000; Finlay et al. ,

20 1 0). Others have shawn that changes in land caver affect DIC export, for example,

through logging, farming, pasturing or urbanization (Daniel et al. , 2002; Raymond

and Cole , 2003 ; Baker et al., 2008; Barnes and Raymond, 2009; Regnier et al., 20 13).

Topographical position and land caver likely interact with geology, and collectively

determine the degree of weathering of the underlying rocks, the principal source of

carbonate and bicarbo nate ions. This interaction between topography and land caver

underscores the need for an integrated approach .

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2 l

Most studies to date have explored DlC and DOC export separately (Hope et al.,

1994; Wall in et al. , 20 1 0) , and a lthough there is considerab le insight to be ga ined

with this form-specific approach, it nevertheless yields a rather fragmented view of

the magnitude and regulation of total C export from watersheds. Since the relative

influence of topography and land cover may be different for D!C and DOC export,

chan ges in land cover may lead to shi fts not only in total C export, but a lso in the

DlC/DOC export ratio . Here we exp lore topographie and land cover predictors of

DIC, DOC, and TC exports in a set of 83 diverse catchm ents, located in the temperate

landscape of southern Québec. T he main objectives of this research were three-fold:

(1) to identify the relative importance of topography and land caver on DlC, DOC,

and TC export from temperate watersheds, (2) to exp lore the effect of potential land

cover changes on the DIC/DOC export ratio, and (3) to compare DlC and DOC

exports across 3 co nsecutive years ofvaryi ng hydrologie regimes.

1.3 Materials and methods

1.3.1 Study area

Est imat ing carbon export from a large number of catchments over severa! years

requires a cons id erable samplin g effort and necessar ily invo lves a compromise

between capturing the temporal (within streams) and spat ia l (amo ng streams)

components of variabi lity. As our focus centered on identifying the landscape drivers

most c losely associated with export rates, we opted to maximize landscape variability

while ensuring a sufficient temporal coverage to obta in robust estimates of annua l

export of the various carbo n form s. We therefore se lected 83 catchments in southern

Québec, Canada, abo ut 100 km east of Montreal (45° 12 ' 17''N - 45°49 '22"N,

71 °49 ' 34" W - 72°39 ' 50" W), ranging in area from 0. 13 to 520 km2 (Table 1 ). The

Page 42: Regional and global patterns in carbon export from land to water

2 2

streams and rivers draining these catchments were sampled in 2004 and 2005 , and a

subset (32) were also sampled in 2003. The rivers sampled range from first arder

streams to fo urth arder ri vers. Vegetation in the watersheds is characterized by mixed

temperate fo rest, dominated by native sugar maple trees, mixed with basswood, red

oak, eastern white pine, eastern hemlock, and ye llow birch. Land use va ried greatly

among catchments, so rn e bein g largely forested, others dominated by agriculture or

pasturelands (Table 1 ). Geo logicall y, the study area is located in the transition region

between the Humber and Dunnage Zones of the Appa lachian Uplands striking

northeastwards; bas rolling topography, controlled by a series of we ll-developed

fa ults and fo lds, is underlain by carbonate-rich and non-ca lcareous sil iceous

sedimentary rocks, imbedded with mudstone and sandstone, and is dotted with

outcrops of metamorph ic and igneous rocks (Tremblay and St-Julien, 1990; Robinson

and Fyson, 1976; Paradis and Lavoie, 1996). The geo logy is thus quite diverse across

the 83 catchments, with the dominant rock type being sedimentary in 56 catchments,

vo lcanic in 18, and intrusive in the remaining 9. The surface depos its in the reg ion

consist mostly of glac ial ti ll and so rne glac io- lacustrine fin e sediment (Prairie et al.,

2002), such that the dominant general formation is till in 19 of the catchments studied,

mud in 6, although rock is the dominant formation in the majority of the catchments

in this study (58). a ils are mainly humo-fe rric podzo lic and dystric bruniso lic, with a

loamy to sandy loam texture and moderate to good internai drainage, such that the

dominant sai l arder is podzo lic in 49 catchments and bruniso lic in 23 catchments.

Gleyso lic so ils dominate in 10 catchments and only 1 catchment is dominated by

yo unger regoso lic so ils. Mean annual prec ipitation in the region is about 1000 mm, of

which 500-600 mm runs off (Natural Resources Canada, 2009), and mean daily

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2 3

temperature in July is about l 8°C , while in January it is about -10°C (Environment

Canada, 1981-2010) .

1.3.2 Sampling , ana lyses and ca lculatio ns

The 83 sites were vis ited 4-6 times each in 2004 and 2005, at a bo ut 5-week intervals

during the ice-free period between March and N ovember (tota ling around 400 s ite

vis its per year) , and a subset of 32 sites were vis ited an add it io na l 6-7 times in 2003,

at abo ut 4-week in te rva ls between March and October (totaling around 200 site vis its) .

At each ofthese s ites, water samples were co llected and fil tered in s itu using 0.45 J.lm

syringe filters and transported to the lab in 40 mL g lass via ls w ith s ili co ne septa

(1-C HE M). DJC and DOC co ncentrations were determined fo llowin g ac idification

and oxid atio n w ith phosphoric ac id and sodium persul fa te, respective ly, using a

TOC IOJO tota l carbo n ana lyze r, equipped w ith an infrared C02 detector (01

Analytical, 2% precis io n of2 replicates per vial , 3% accuracy at 5mg L-1 standard) .

The carbon export (g m-2 yr-1) at any g iven s ite is defin ed as the produ ct of discharge

and C concentrati o n per unit catchment area, and it is the refo re essentia l to determine

the first two co mpo nents accurate ly. We determined discharge (m3 s-1) at each site fo r

each sampling date as the product of the measured stream cross-sectio nal area and

water ve locity (sampled at 0 .6 x stream depth at severa! stat io ns across the stream

w id th using the two-dimens iona l FlowTracker aco ust ic Doppler ve locimeter, SonTek).

These po int measurements are however, inadequate to capture the seasonal var iatio n

in discharge, and because the vast majo rity of these rivers are not gauged it was

necessary to deve lop a lternat ive approaches to reconstruct the fu ll annual discharge

pattern for each river. We deve lo ped an empirica l ca librat io n that wo uld a llow us to

estimate the discharge for any given river at any given poi nt in tim e, that is based on

Page 44: Regional and global patterns in carbon export from land to water

2 4

Table 1. Stream and catch ment characteristics of the 83 study sites. Statistics for discharge and water chemistry were determined by first averaging ali measured values from 2004 and 2005 for each of the 83 streams, theo calculating the minimum, maximum, mean and standard deviation of these values (n=83). Statistics for topography, land cover, geology and soi! were obtained from digital elevation models, and maps of topography, land cover, rock type, surficial deposits, and soi! order, using GIS (n=83).

Varia bl e Min M ea n (SD) M :ax

Strea m c h aracter i st i cs

Dischar·ge (m 3 - 1 ) 0 .002 1 0.48 ( 1.0) 6. 1

Dl concentrat io n (mg L"') 1. 5 8.0 (4. 1) 28

DO con centrat io n (mg L -1) 1.9 7.7 (4.0) 19

pH 6 .0 7.2 (0.38) 8.1

A lka l in ity (J.leq L - 1) 80 530 (280) 1800

TN con centrat io n ( m g L · ') 0.14 OA6 (0. 19) 1 . 1

TP concentration (1-lg L - 1) 4. 1 26 ( 19) 1 10

Catchment topograp h y

Catchme nt area (km 2 ) 0 . 13 28 (79) 520

Average e levation (m) 150 3 10(57) 430

Average s lope ( ) 1.2 5. 1 (2 .8) 12

B 1 1.2 1. 6 (0.22) 2.4

Catchm ent la nd cover

% vegetation 42 83 ( 15) 100

%forest 27 77 ( 18) 100

% pasture 0 2 1 ( 19) 73

o/o wetla nd 0 1.1 (1.9) 9

o/o lake 0 3 .9 (5 .9) 25

B uildings perkm 2 0 9.9( 12) 56

Ca t c hm e nt geo logy a nd so il

o/o intrusive 0 1 1 (27) 100

% sed ime ntary 0 66 (4 1) 100

% volcanic 0 23 (37) 100

o/o rock 0 69 (4 1) 100

o/o ti ll 0 23 (38) 100

% mud 0 7 . 7 (24) 100

% bruniso l ic 0 22 (23) 9 1

o/o g leyso l ic 0 15 (22) 83

o/o organic 0 2.4 (4. 1) 23

o/o podzolic 0 42 (30) 100

o/o regoso 1 i c 0 0.95 (2.5) 17

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------------------

2 5

the relationship between our point discharge measurements and discharge data from a

continuous gauging station located in one of our study watersheds, Trois-Lacs (TR)

(hyd rolog ic station 030101 : 45°4 7'30"N, 71 °58 '5" W operated by the Centre

d'expertise hydrique du Québec. The gauging station reports an error of ±5% when in

the stage-discharge relationship ). Our 612 instantaneous discharge measurements

divided by 1 58 the corresponding catch ment a reas were expressed as runoff (mm d-1 ),

and regressed against daily runoff at the TR gauging station, along with other

site-specifie attributes that modulate local discharge. Fo r this reg ion, the best

predictive model of daily runoff at any given site included elevation and catchment,

in addition to the measured daily runoff at the TR station:

log1oSEM = -0.629 + 0.892* 1ogSm + 0.00 188*E + 0.150*1ogJOAo ( 1)

where SEM is the estimated runoff at a given site (mm d·1), STR is the measured va lue

at the T R gauging station, E is the elevation of the sampling si te (m), and Ao is the

total catchment area upstream of the sampling site (km2). These est imates of daily

discharge generated by the empirical model correlated we il with our in stantaneous

discharge measurements, explaining 81 % of the variability (R2=0.8 1, p<O.OOO 1,

n=6 12). We used this relationship to extrapolate discharge to the entire year, including

winter months, for which we had no samples. White the re lationships that we built

between concentration and discharge were based on measurements taken during the

ice-free period, we have no reason to beli eve that these relationships wo uld not hold

for flows under ice-cover. At the gauged site, where discharge was monitored

year-round , the range of discharges recorded during the sampling season

encompassed the range of discharges seen in winter. Furthermore, on the spec ifie

dates when discharge was measured at va rious sites and compared to the gauged

discharge at Trois-Lacs on those same dates, the gauged discharges cover nearly the

Page 46: Regional and global patterns in carbon export from land to water

2 6

full range of discharges seen throughout the year. This allowed us to derive annual

export and to compare our results with the literature, which overwhelmingly reports

annual expott .

Daily C expott was calculated as the product of daily discharge, estimated as

described above, and DIC and DOC concentrations measured at each site, divided by

catchment area . Applying an average DIC or DOC concentration derived from the 7

to Il point measurements assumes that discharge and concentration are independent,

wh ich is not al ways the case ( Wallin et al., 20 1 0; Birgand et al., 20 Il ). We tested this

ass umption by exploring the relationship between measured DIC and DOC

concentrat ion and measured discharge for each of our 83 sites using the data from ali

years combined. Fo r DI C, significant (p<O. OS) negative (dilution) relationships were

fou nd for 31 sites (p<O.OS), and no sites showed a positive (concentration)

relationship . For DOC, a significant dilution effect was found for only 2 sites

(p<O.OS), whereas 5 sites showed a significant concentration effect. For sites with

significant co rrelation between concentration and discharge, we used the

co rresponding site-spec ifie regression to estimate daily concentrat ion from daily

discharge. For sites with no signifi cant relationship between discharge and DOC or

DIC concentration, we applied the average concentration with the estimated daily

discharge in our calculation of DOC or DI C expott . An nuai DIC and DOC experts (g

m·2 yr·1) were then calculated as the sum of daily export va lues.

Particulate organic carbon (POC) export from a catchment was not measured but

rather estimated assuming a POC to DOC ratio of 0.1 , typica l fo r lotie systems in the

temperate forest (Schlesinger and Melack, 1981 ; Hope et al., 1994). Particulate

inorganic carbon (PIC) export was not included in TC export because prev ious

studies have shown that it acco unted fo r a very small fract ion of inorganic C (Aucour

Page 47: Regional and global patterns in carbon export from land to water

2 7

et al., 1999) . T hus, in this study, TC expot1 was defin ed as the sum of DI C, DOC and

POC exports.

1. 3.3 Catchment to pography and land caver

The varia bles used to characterize the 83 catchments are listed in Table 1. Values fo r

topography and land caver were extracted from 1 :50,000 dig ita l topographie maps

(Natural Resources Canada, 2006) as we il as 1 :5 0,000 and 1 :250,000 land caver

maps (Natural Resources Canada, 1999) . Geo logica l data (surfic ia l geo logy and

surfic ia l materia ls) were obta ined fro m 1 :5 ,000,000 dig ita l maps (Natural Resources

Canada , 1995) and so i! data tram an ama lgamatio n of 4 sma lle r reg io nal ma ps

rang ing in sca le fro m 1 :20000 to 1: 126720 (!RDA, 2006). Statistics were extracted

fro m the ma ps us ing ArcMap 1 0 (ESRl). Here, average si o pe (0) was derived fro m the

digita l e levatio n ma del w ith 1 Om x 1 Om reso lution. Bas in shape index (BSI ), a

measure of watershed roundn ess, is defin ed as the ratio of the perimeter of the

catchment to that of a c irc le w ith the same area (Miller, 1953) :

BSl = P/(2-Y(n*(Ao))) (2)

where P and Ao are the catchment perimeter and catchment area, respectively.

Geo logica l va riables are ex pressed as a percent of tota l catchment area (Ao).

However, land caver and so it va riables are expressed as a percent of tota l catchment

area (Ao) minu s the area of the catchment covered by waterbodies (Aw), leav in g only

the terrestrial catchme nt area (Ao-Aw). We used two di ffe rent ma p laye rs of different

categorical reso lution to characterize the land caver pro perties of our catchments. In

the fir st land co ver c lass ification, the land scape was broad ly defin ed as vegetated,

un-vegetated, and water (BNDT, Natura l Reso urces Canada). The percent vegetated

derived from this laye r is a broad catego ry that inc ludes wooded areas and shrubland s,

Page 48: Regional and global patterns in carbon export from land to water

2 8

but excludes pastures and agricultura l land s, and wet lands. The non-vegetated land

inc ludes pastures and agricultura l lands, as weil as bare rock (which is rare in our

landscape ), and therefore these two categories roughly correspond to " natura l" versus

" managed" land scapes . We further characterized the land scape usin g another

landcover layer that prov id ed a finer c lass ificatio n (Canada Land lnvento ry, Natural

Reso urces Canada, http://sis.agr.gc.ca/cansis/publica tions/maps/ index. html), and we

derived percent forest, pasture, wetlands and mines fo r each of our catchment. T he

areas co nsidered as forest inc luded the zo nes on land caver maps c lass ifi ed as

" product ive woodland", " no n-productive woodland", and "outdoor recreat io n", which

co nsisted of fo rested parks in these catchm ents. T he areas cons idered as pasture were

the zo nes on land caver maps c lass ified as " improved pasture and fo rage crops" and

" unimproved pastu re and range land" . To calculate percent wet land s, regio ns on land

caver maps that were coded as " swamp, marsh or bog" were merged w ith "wetland s".

Land caver categorized as cropland or urban was not present in the stud ied

catchments. Building dens ity is expressed as the number of buildings per square

kilo meter of te rrestria l catchment area. Ali the above land caver categories are

expressed as % of the terrestria l area in each catchment, whereas percent la kes is

the water area over the tota l catchment area.

1.3.4 Statistica l analyses

A princ ipal compo nent analys is was perfo rmed on the variables describing

topography, land ca ver, geo logy and sa il in Table 1 to explo re the multiple

re lationships among variab les. T hey were then offered fo r inc lu sio n in multiple linear

regressio n models predict ing D!C, DOC and TC export. T he models were buil t using

a mixed step-w ise se lectio n process, w ith p<0.05 as the conditio n fo r inc luding a

var iab le in the madel. Bath %fo rest and %pastu re from land caver maps were

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2 9

excluded fro m these analyses as they were strong ly corre lated w ith the broader

category of %vegetation from to pographie maps (%forest: positive, R2=0 .73, n=83,

p<O.OOO 1; % pasture : negative, R2=0.72, n= 83, p<O.OOO 1 ), and were therefo re

co nsidered redundant. The inter-annu al va riability in DI C and DOC expo rt was

examined fo r a subset of 32 catchme nts usin g a one-way ana lys is of variance and the

Tukey- Kramer post-hoc test to ftnd significa nt di ffe rences amo ng three sequentia l

years (p<0.05) . Exports for the 32 sites were centered by express ing the expo rt fro m

each site in a given year as the di ffe rence re lative to that site's average expo rt over

the 3 years (2003, 2004, and 2005). This procedure a llowed us to examin e more

robustly inter-annua l diffe rences for streams w ith very di ffe rent average ex port.

1.4 Results

1.4.1 Carbo n expo rt

We observed a w ide range m export rates of both DI C and DOC across the 83

catchments studied . DIC export was 4.6 g m·2 yr·1 (average of 2004 and 2005 va lu es),

and ranged an o rd er of magnitude, fro m 1. 1 to Il g m·2 yr·1 (Fig ure 1 ) . Simila rly,

DOC export averaged 5. 1 g m·2 yr·1 over the same period, and ranged fro m 1. 1 to 13 g

m·2 yr·1 (Fig ure 1 ). As a result, TC expo rt averaged 10 g m·2 yr·1, and ranged fro m 2.5

to 18 g m·2 yr·1• Co mbining the uncerta inty of both di scharge and co ncentratio n

estimates, e rror propagatio n ca lculatio ns suggest that the ex port va lues have an

associated erro r o f abo ut 25%.

Page 50: Regional and global patterns in carbon export from land to water

3 0

14

12

•,_ 10 >-

8 .9 t 0 ~ 6 (])

0 0 4 0

2 •

• 0

• • • •

• •

• • • • ~-- -... . - .

• . · .. ' ·f-• • • • •• • 1 . ...... . .. .- . . . . . .., .

.. •

••

2 4 6 8

DIC export (g m-2 y(

1)

10 12

Figure 1. DOC export versus DIC export for the 83 catchments . Exports are

expressed as the average of 2004 and 2005 measurements in g of C pe r square

meter of total catchment area per year.

Page 51: Regional and global patterns in carbon export from land to water

14 ~ c 0

12

• 0 0

0 ~ 10 ~

>- / e B "! 0 E

8 • -9 • t 0 0.. x 0 Q) 6 • c

0 0 -e co u

4 0

• 2 0 •

~ • 0

0.1 10 100

catchment area (km2)

Figure 2. Carbon exported as DIC (solid squares) and DOC (open circles) in g m·2 yr" ' for the 83 catchments , average of 2004 and 2005 measurements, as a fun ction of total catch ment area . Significant correlations with catchment area are shawn for DIC export (thin line}, DOC export (thick line) and TC (dashed line , points not shawn).

3 1

Despite the similar range and magnitude of Dl C and DOC exports, there was no

sig nifica nt corre lation between the export of these two C spec ies. T he re lati ve

co ntributio n of the two disso lved constituents to TC expo rt thus varied cons iderably

among the catchments, w ith expo rt from some sites be ing overwhelm ing ly dominated

by inorganic C, and others by its organic co unterpart. T he rat io of DI C to DOC expo rt

ranged 20-fo ld , from 0 . 19 and 3.9, averag ing 1.1 , w ith 56 of the 83 catchments fa lling

in the range between 0.5 and 2.0. ln additio n, the range and magnitude of DI C and

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3 2

DOC concentrations (in mg L· ') across the 83 s ites were s imilar (average of 2004 and

2005 va lues, Table 1 ), yet the re was no re lationship between co ncentratio ns of these

inorganic and o rganic co mponents for the region.

Overa ll , there was a s ignifi cant pos it ive spatia l scale effect on C expo rt (Fig ure 2),

such that DIC, DOC and TC exports inc reased w ith catchment s ize (log10(DICexport)

= 0.54 + 0. 11 *log10(Ao), R2=0. 19, p<O. OOOl , n=83; log,o(DOCexpo rt) = 0.56 +

0.12* 1og ,o(Ao), R2=0.1 6, p=0.0002, n= 83; log10(TCexport) = 0.90 + O. ll *logiO(Ao),

R2=0.33, p<O.OOOI , n= 83).

1.4.2 Factors influenci ng carbon expo1t

A principa l compo nent ana lys is of DIC, DOC and TC exports (average of 2004 and

2005 va lues) as weil as topographie and land cover variables demonstrates the large

degree of unco upling between DIC and DOC export, as these two variables· are

orthogonal to each othe r on the summ ary plot of the fir st 2 compo nents (F ig ure 3).

T he fir st two co mponents expla ined mo re tha n 50% of the variance in the data, w ith

co mpo nent 1 a ligning strong ly w ith topographica l va riables, such as catchment area,

s lope and e levation (34%), and component 2 a lig ning mo re with land cover variables,

su ch as %vegetatio n and %wetlands ( 18% ).

Page 53: Regional and global patterns in carbon export from land to water

1.0

0.5

N 0.0 +-' c Q.) c 0 o._

E 8 -0 .5

%vegetation

TC export

buildings km·•

-1 .0 +---..------,-----r----;----.----.-----.----, -1 .0 -0.5 0.0

Component 1 (33.8 %)

0.5 1.0

3 3

Figure 3. Principal co mpo nent analys is of DIC, DOC and TC expe rts (average of

2004 and 2005 va lues) and key to pographie and land-cover va riables for the 83

catchments.

T he positio n of DIC and DOC exports at 45° to the axes revea ls tha t the export of

e ither C co mpo nent is re lated to a co mbinat io n of the topographica l va riables of

com ponent 1 and the land cover va riables of co mpo nent 2 . Not surpris ing ly, TC

expo rt was intermediate between DIC and DOC expo rts. T he multiple linear

regress io n mode ls presented in Table 2 th us incorporate a combinat ion of topography

and land cover variables, and expla in 34%, 62% and 53% of the variance in DIC,

DOC and TC expo rts, respect ively. Both DI C and DOC expo rts were posit ive ly

Page 54: Regional and global patterns in carbon export from land to water

3 4

related to total catchment area (as shawn in Figure 2) . BSl and %vegetation both had

a significant negative effect on DIC export, but a sign ifi cant positive effect on DOC

export. ln addition, building density (a measure of human in fluence) was positively

related to DIC export, whereas wet lands were positively related to DOC export.

Fina lly, the presence of lakes in the catchment had a negative effect on DOC export

but none on DJC export. Variables describing geo logy (as e ither genera l rock

formation or surface material type) and so it type did not contribu te significant ly to

predicting C export. As mentioned in Section 2.4, %fo rest and % pasture were not

offered in the step-wise madel-building process, because of their strong correlation

with %vegetation .

Table 2. Multiple linear regression models predicting DIC, DOC, and TC export in g

m-2 yr1 (n=83 for each). Estimates of coefficients and corresponding p values are

given for ali variables offered during the step-wise selection process. Variables were

included in the mode! if p<0.05 (in bold) and the corresponding R2 values are shown.

DIC DOC TC Parame ter

Estimatc p value Estimatc P value Estima tc

Catchmenl area (lcm2)" 1.069 <0.0001 0.672 0.0323 1.412

Average elevation (rn) 0.7584 0.2181 Topography

Average slope (0) 0.2418 - 0.271 0.0041 -0.266

BSJ -2.088 0.0201 2.705 0.0068

%vegetation - 0.038 0.0017 0.061 0.0003

Land cover %wetland 0.8931 0.530 <0.0001 0.690

%lake 0.3783 -0.139 0.0002

Buildings per km2 0.036 0.0116 0.4498

Intercept 9.914 <0.0001 -3.423 0.1072 9.731

p value

0.0002

0.6454

0.0066

0.8220

0.5486

<0.0001

0.0601

0.8632

<0.0001

Rz 0.34 0.62 0.53

* Ca!chment area is log JO lransformod.

Page 55: Regional and global patterns in carbon export from land to water

DIC Export

8.8%

- % welland [:=::J % vegetation

- build ings km-2

!:=::J % lake -BSI c=J Catchment area ~Slope

- unexplainedJ

"12%

3 5

DOC Export

6.2% 3.7%

Figure 4. Variance partit ioning in the multiple linear regression models of DIC and DOC export , showing the percentage of variability explained by each compone nt variable and the remaining variability, unexplained by the models.

Figure 4. Variance pa rtitio nin g in the multiple linear regress ion mode ls of DI C and DOC

export, showing the pe rcentage of va riabili ty expla ined by each compo nent varia ble and the

remaining va ria bil ity, unex pla ined by the mode ls.

An examina tio n of the sums of squares assoc iated to each va ri a ble in the multiple

regress io n mode ls a llows us to determine the re lati ve influence of topogra phica l and

land cover varia bles on C expo rt (Figure 4). T he topographica l variables (catchment

area and BSI) were s lig htly mo re important than the land cover varia bles

(%vegetatio n and building density) in predicting DI C expo rt, w ith topogra phy and

land cover expla ining 19% and 15% of the va riability, respecti ve ly. ln co ntrast, land

cover varia bles (%vegetatio n, % wetland and % lake) were more impo rtant than the

topographica l va ria bles (catchment area, BSl and slo pe) in predicting DOC export,

land cover and topography explaining 44% and only 18% of the variability in the

DOC mode !, respective ly. ln terms of TC expo rt, the topographica l var ia bles

(catchment area and s lo pe) and land cover (%wetland) expla ined roughly the same

amount of va riatio n (24% versus 29%, respective ly) .

Page 56: Regional and global patterns in carbon export from land to water

3 6

The positive re latio nship of catchment s ize w ith both ore and DOC expo rts resulted

in an overa ll pos itive effect of catchment size on TC expo rt (Table 2) . ln contrast, the

oppos ing effects of BSI and %vegetation on DIC and DOC expo rt cance led each

othe r out and as a result, these variables had no overa ll impact on TC expo rt. The

pos itive effect of %wetland and the negat ive effect of s lope o n DOC export were

strong eno ugh to influence overa ll TC export, despite the ir lack of influence on DIC

export.

1.4.3 Inter-annual va riatio n in carbon export

The precedin g results were based on the average C expo rt fo r 83 bas in s in 2004 and

2005 ; howeve r, we a lso examined inter-annual va riation in C expott fro m 2003 to

2005 fo r a subset of 32 bas ins . Co ntinuo us measurements fro m 13 weathe r statio ns in

the study area revea l that 2005 was the warmest and wettest year, w ith 1 oc hig her

respective ly (ANOVA, R2 = 0.59, p<O.OOO 1, n = 96, Tukey- Kramer p<O.OOO 1)

(Fig ure 5). DOC expo rts were a lso s ig nificantly hig he r in 2005 than in 2004 and 2003,

w ith average DOC exports fo r the 32 bas ins of 6.1 , 5.0, and 5.2 g m·2 yr·1 in 2005,

2004, and 2003, respective ly (A NOVA, R2 = 0.3 1, p<O.OOO 1, n = 96, Tukey-Kramer

p<O.OOO 1) (F ig ure 5). As a consequence, TC exports were also s ig nificantly highe r in

2005 than in 2004 and in 2003, w ith average T C exports fo r the 32 bas ins of 13.6,

1 0.7, and 11.0 g m·2 yr· 1 in 2005, 2004, and 2003, respective ly (ANOVA, R2 = 0 .67,

p<O.OOO 1, n = 96, Tukey-Kramer p<O.OOO 1 ). T here was no inter-annua l di ffe rence

between exports in 2003 and 2004 for any C species .

Page 57: Regional and global patterns in carbon export from land to water

3 7

Table 3. Loca l climate, gauged da ily discharge, as weil as discharge and DIC and

DOC concentrations measured in situ at the 32 sites in 2003, 2004 and 2005.

2003 2004 2005

Annual mean air temperature ("C) a 5.0 5.1 6.0

Annual precipitation (mm) a 1245 1026 1316

Mean (SD) of mean daily discharges at 13.4 (1 8.7) 12.7 (15 .9) 15.6 (23 .9) Trois-Lacs (m3 s·') b

Mean (SD) of mean daily discharges al 0.57 (0.77) 0.62 (0.8 1) 0.68 (0.97) Waterloo (m3 s-') c

Mean (SD) measured in situ discharge (m3 s·•) NA 0.30 (0.59) 0.59 (1.4)

Mean (SD) DIC concentration (mg L·') 8.7 (4.0) 8.9 (4.7) 9.8 (5.4)

Mean (SD) DOC concentration (mg L-1) 8.3 (3 .7) 8.2 (4.0) 8.7 (4.7)

a Data from http://cl imate. weatheroffi ce.gc.ca

b Data from https://www.cehq. go uv. qc.ca/sui vihydro/graphique.asp?NoStation=030 101

c Data from https://www.cehq. go uv.qc.ca/sui vi hydro/graphique.as p?NoStation=030343

1 .5 Discussion

The C exports and DI C/DOC export ratios that we measured for these 83 bas ins in

so uthern Québec are weil within the range ofvalues found in the literature. Our range

of DOC export (1 .1 to 13 g m·2 yr-1) is in very good agreement with that estimated by

Eckhardt and Moore ( 1990) in roughly the sa me area ( 1 to 18 g m·2 yr·1 ). The average

DOC export of 5.1 g m·2 yr·1 co rresponds to the mid-range of DOC exports reported

for Atlantic Canada (1.6 to 12.4 g m·2 yr-1) (Clair et al., 1994), fo r fo rested landscapes

in southeastern Canada (0.9 to 13.7 g m·2 yr-1) (Creed et al., 2008), or for forested

watersheds in other temperate regions of North America (0.3 to 4 1.7 g m·2 yr-1) (Hope

Page 58: Regional and global patterns in carbon export from land to water

3 8

et al., 1994), and very similar to the average DOC export of the 6 g m-2 yr-1 reported

for wet temperate regions by Meybeck (1993). Similarly, our DIC export range of 1.1

to Il g m-2 yr- 1, and average of 4.6 g m-2 yr-1, were weil within the range of riverine

exports found in Europe (0.5 to 67.8 g m-2 yr-1) (Hope et al. , 1994), although they

~ere slightly higher than those fou nd in Atlantic Canada (0.04 to 4.19 g m-2 yr- 1,

average 0.71 g m-2 yr- 1) (Clair et al. , 1994) and were much higher than those found in

central Ontario (0.81 to 1.69 g m-2 yr- 1, average 1.12 g m-2 yr-1) (Dillon and Molot,

1997). As for TC export, our range of 2.5 to 18 g m-2 yr- 1 and average of 10 g m-2 yr-1

agree weil with TC export from north Atlantic rivers in the United States (3.7 to 15 g

m-2 yr- 1, average of 7.2 g m-2 yr-1) (Stets and Striegl, 2012), but is lower than TC

exports from European rivers at similar latitudes (e.g. TC exports for the Adige,

Danube and Po Rivers, which were 16.7, 12.4 and 30.1 g m-2 yr- 1, respectively)

(UNEF, 2003) .

The decoupling between DIC and DOC exports that we observed in our systems (Fig

1) has also been observed in other regions, leading to variations in DJC/DOC export

ratios bath within and across regions . In this regard , our DIC/DOC export ratio varied

widely across catchments (from 0.2 to 3.9), and the overall mean of 1.14 was much

higher than published DIC/DOC ratios in Atlantic Canada (0.0 1 to 0.86, average 0.13)

(Clair et al., 1994) and central Ontario (0.13 to 0.32, average 0.27) (Dillon and Molot,

1997). This inter-regional difference is largely attributable to differences in the

amount of wetlands and carbonate rocks, which relate to the production of sail DOC

and DIC, respectively. Atlantic Canada and central Ontario are lithologically

dominated by volcanic and granitic rocks, respectively, whereas most of our study

area, is underlain by carbonaceous sedimentary rocks (Paradis and Lavoie, 1996),

which contribute more DIC by weathering. ln addit ion, most of the catchments

Page 59: Regional and global patterns in carbon export from land to water

3 9

studied in Atlantic Canada, are located on islands, where the soils are poorly

developed, thereby producing less DIC from soit respiration. Furthermore, the study

area in central Ontario has more wetlands (up to 25%) than ours (up to 9%), which

contribute more DOC and further lower the DJC/DOC ratio.

We designed this study to maximize spatial coverage and environmenta l gradients,

wh ile st ill capturing at least so me of the seasona l var iabi 1 ity in riverine d ischarge and

C concentration. Discharge is without doubt the most variable of these two

components, but we were able to reconstruct the annual discharge pattern by relating

our point measurements to a cont inuous discharge record in one of study streams

(n>600 point measurements). This approach is effective to capture both the total

runoff fi·om each st ream and the main features of the annua l hydrographs. With

regard to the temporal var iab ility in C concentrations, we examined the

mean-variance relationship for DIC and DOC concentrations, by plotting the variance

of ali co ncentrat io n measurements at a given site in a given year, V, versus the mean

concentration for that s ite in that year, X. Combining the 32 sites samp led in 2003

with the 83 sites samp led in 2004 and 2005 , there were a total of 198 site-years for

which we could compare the variance to the mean. Applying the resulting

mean-variance equations (Vo,c=0.032*Xo,c249 , R2=0.51 , p<O.OOO 1, n= 198;

Yooc=O.O 19*Xooc2·62, R2=0.67, p<O.OOO 1, n= 198) following Cattaneo and Prairie

( 1995) a llowed us to determine that no more than 4 samp les per year were req uired to

obtai n a mean concentration for a given site with a precision of 20%. As we visited

each s ite 4-7 times per year, the mean concentration calculated for any site shou ld

have an error of 20% or less , thereby confirming the adequacy of our sampling

strategy. Carbon concentrations are less temporally variable than nutrients such as N

and P (Moatar and Meybeck, 2007; Birgand et al. , 20 Il ), and a si milar precision was

Page 60: Regional and global patterns in carbon export from land to water

4 0

obtained by Birgand et al. (2011 ) fo r tota l disso lved carbon sampled mo nthly in a

fo rested catchment. With our experimenta l des ign, we found mo re va riability in

carbo n expo rt amo ng sites than w ithin a year at a s ing le s ite, which a llowed us to

explore the drivers of carbon export across catchments of di ffe ring topographica l and

land cover character ist ics.

1.5. 1 Drivers ofTerrestrial Carbon Export to Aquatic Systems

While topography and land cover together expla ined only 34% of the va riability m

DIC expo rt, they expla ined 62% of the varia bi! ity in DOC expo rt, c learly illu strating

that DI C and DOC export are co ntro lled by diffe rent biogeochemica l processes.

Furthermore, whereas land cover and to pography were equa lly impo rtant in

determining DI C ex po rt (expla ining 15% and 19% of the va riability, respective ly),

land cover was c learly a stronge r driver of DOC expo rt than was topography

(expla ining 44% and 18% of the va riabil ity, respect ive ly). These results suppo rt our

hypothes is that a combinatio n of underlyin g topographical fea tu res and potentia lly

more dynamic land cover features are in vo lved in determining the va rio us fo rrns of C

exported .

Despite these di fferences, there was one dri ve r that was co mmo n to a li fo rms of C

export, whether DIC, DOC, or TC: catchment area was pos itive ly re lated to a li fo rms

of C expo rt, e ither a lone (F igure 2), o r in co mbinat io n w ith other etfects in multiple

linear regressio n models, exp la ini ng 15%, 4% and 16% of the va riab il ity in DIC,

DOC, and TC export, re pect ive ly (Table 2, Fig ure 4) . T his is in contrast to the

finding of Agren et al. (2007) that sma ll headwater catchments expo rt the most

terrestria l DOC, in a co mpari so n of 15 sub-catchments in Sweden rang ing in s ize

fro m 0.03 to 22 km2 • lt is di ffi cult to expla in why catchment area sho uld play a ro le in

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4 l

how much C is exp01ted per sq uare kilo meter. We fo und no sig ni fica nt re lat io nship

between carbon concentratio n (D!C, DOC, or TC) and catchment area (p>0.05, n=83,

usin g the average of 2004 and 2005 co ncentrations for each site and using e ither

terrestr ial or tota l catchment area). T his is inco nsistent w ith the posit ive re lationship

with DOC co ncentration reported by Jnamdar and Mitchell (2006) and the negat ive

re lat io nship w ith DOC concentratio n reported by Wolock et al. ( 1997). T h us, the

ultimate driver is like ly hydro logy, and in this regard , we fi nd a relat ive ly strong

positive re lat io nship between catchm ent s ize and runoff (parameters), and a lso w ith

catchment e levat io n. T he reaso ns unde rly ing this pos it ive re lart io nship are not c lear,

but could be re lated to shifts in la nd cover patterns w ith catchment s ize. ln pa rticular,

the re was a trend fo r larger catchments to have less fo rest and vegetat ion cover and

highe r proportion of agricultura l land s, and it has been suggested that runoff actu a lly

inc reases with defo restation and human-induced landscape a lternat io ns (A llan, 2004;

Maetens et al., 20 12).

1.5. 1.1 Drivers of DI C Expo tt

A fte r catchment a rea, %vegetatio n in the watershed was the seco nd most important

facto r determ ining D!C export, w ith less vegetated bas in s export in g mo re DI C. This

agrees with prev ious work that has shown that defo restat io n or co nvers ion of natura l

vegetat ion into pasture inc reases DI C expo rt fro m the landscape, eithe r when

co mparing DIC export across bas in s (Baker et al. , 2008; Rantakari and Kortelainen,

2008; Regnier et al. , 20 13) or when fo llow ing DI C export within a bas in as its land

co ver changes over ti me (Raymond and Cole, 2003 ; Yan et al., 20 13). T here are

severa! processes that can exp la in the observed pattern between %vegetat io n and DI C

ex port. ln o ur study regio n, unvegeta ted areas often co rrespo nded to pasture lands

which, in co mpar ison w ith forest so ils, tend to have hig he r so it respi rat ion rates

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4 2

(Smith and Johnson, 2004; Kellman et al. 2007), leading to elevated soil C02 and

greater weathering potential (Likens, 201 0; Bayon et al. , 2012) . Previous studies have

shown that replacing forest species with forage or farm crops results in a decrease in

the soil C/N ratio , leading to a more rapid mineralization of soil organic matter and

litterfall , thus increasing groundwater DIC and soil C02 (Marland et al. , 2004;

Hedley et al. , 2009). Moreover, deforestation, due to agriculture or pasture, can

intensify weathering (Likens, 201 0; Bayon et al., 20 12), th us releasin g more

bicarbonate and carbonate ions into river water. Therefore reducing the vegetation

coverage and/or shifting land uses to agricultural or residential may increase DIC

export by increasing so il respiration rates and weathering.

In addition, C geochemistry and water chemistry in river systems are dependent

largely on lithological variability m carbonate/silicate-dominated terrains

(Amiotte-Suchet et al. , 2003 ; Zhang et al. , 2009). Our study area is located in the

transition region between the Humber and Dunnage Zones, underlain by

carbonate-rich and non-calcareous s il iceo us sedimentary rocks and ma fic volcanic

rocks associated marine sed im ents, respectively. Particularly, in the Humber zone

there are the world 's largest asbestos mine, and severa! talc mines (Castonguay and

Tremblay, i003), both of which are hydrous magnesium silicates that are often

associated with carbonates and easily hydrolyzed to release HCOJ·. The fact that 9 of

the 10 catchments with DI C export of more than 6.6 g m·2 yr· 1 are in the Humber

Zone (except the stream outflowing Lake Nick), further highlights the importance of

carbonate and silicate rocks in controlling riverine DIC export from the catchment.

DIC export also increased with building density. Buildings and their residents are not

point sources of DIC but higher building density usually results in land clearing and

road construction, causing an anthropogenic increase in erosion and therefore DIC

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4 3

export from so ils. This pos itive effect is strong ly suppo rted by previo us studies

(Daniel et al. , 2002; Bornes and Raymond, 2009; Zeng et al., 201 1 ).

Bas in shape index, BSI , a lso played a s ig nificant role in co ntro lling DlC export. The

greater the departure fro m a c ircular bas in (BSl> 1.0), the less DI C that is exported .

T hi s negative re latio nship may be due to hydro logica l pathways be in g more

co nvo luted in bas in s w ith mo re complex shapes. Fo r example, c ircular catchments are

mo re prone to fl ooding than e longated ones (Waugh, 1995 ; Rasool et al., 20 Il ),

leading to hig her erosio n and flu shing out ofva rio us fo nns of terrestria l DIC.

1.5. 1.2 Drivers of DOC Export

The two variables that expla ined most of the va riatio n in DOC expo rt were % lake and

% wetland . Bas in s co nta ining mo re lakes expo rted less DOC, w hich highli ghts the

ro le of lakes as s inks of terrestria ll y-derived organic matter (Larson et al., 2007) .

Temperate and borea l lakes accumulate large amo unts of terrestria l C in the ir

sedim ents (Fer/and et al. 20 12; Tranvik et al. 2009), and a lso deco mpose and emit a

portio n of this te rrestria l DOC as C0 2 and CH4 (Larson et al., 2007; Dinsmo re et al.,

20 13). A ltho ugh the catchments in this study did no t co nta in many wetlands

(max imum 9% coverage) , wetlands still played a role in sha pin g DOC expo rt , as has

been reported fo r othe r reg io ns (Eckhardt and Moore, 1990 ; Dillon and Malot, 1997;

Huntington and Aiken, 20 13). T hese two land co ver va ria bles are susceptible to

anthro pogenic and c lim ate change through dra inage, damming, and changes in the

hydro logie reg ime . T herefo re changes to the amo u nt and extent of wetlands and lakes

in a watershed w ill affect two impo rtant so urces and s inks of DOC and thu s the

moveme nt of te rrestria l C into the aquatic system and the atmosphere.

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We fo und that DOC export was a lso co rre lated w ith %vegetation and BSI, but the

direction of the corre lation was opposite of that fo r DI C export, hig hli ghting the

independent nature of DOC and DIC expo rts (F igures 1 and 3) . Altho ugh the

presence of vegetation lowered DI C expo rt, it increased DOC expo rt in these bas ins,

which agrees with previo us work (Meyer and Tate, 1983; France et al., 1996) . T he

positive re lationship between DOC expo r1 and %vegetatio n refl ects the fact that most

riverine DOC is ultimate ly deri ved from land vegetatio n (via direct litter input and

leaching) and so il s (v ia micro bia l activ ity, root exudation, leaching and eros io n of

organic matter) (Spitzy and Leenheer, 199 1 ). T he in fl uence of BSI on DOC export

was oppos ite to that on DI C expo r1, w ith e longated mo re complex hig h BSI bas ins

exporting more DOC than ro und, less complex low BSI bas ins. Simil a rly, Pacifie et

al. (20 1 0) showed that a mo re e lo ngated bas in oft:en has a highe r ratio of ri parian to

upland area and can export mo re DOC to river systems. As mentio ned above, that

larger catchments expo rted more C, but in this reg ion, la rger watersheds were a lso

characteri zed as hav ing a lower average slo pe (R2=0. 17, p=O.OOO 1, n= 83) and lower

average e levatio n (R2=0.08, p=0.0080, n=83), a ltho ugh these co rre lations are weak.

Once entered into a multiple regress io n with catchment area as a factor, e levation had

no effect on C expo rt; however, watersheds w ith a fl atter average slo pe expo rted

more DOC and TC, independent of the effect of catchment size. Gentler slopes

fac ilitate wetland fo rmation, leading to mo re DOC productio n and ex po rt (D 'A rcy

and Carignan, 1997), and a Iso have lo nger wate r res idence times, a llowing more time

for so il DOC to leach into so il water and ne ighboring waterways (Hazlett et al., 2008;

deCatanzaro and Chow-Fraser, 20 Il ).

1.5.1.3 Drivers of TC Export

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We were able to expla in 53% of the variatio n in tota l C export using two

topographica l variables, catchme nt area and slope, and one land cover variable ,

%wetland . T he pos it ive effect of catchment area on both DI C and DOC exports

results in a s imil ar influence on TC export, like ly thro ugh its effect on d ischarge, as

discussed above. T he negat ive effect of slo pe on DOC was strong eno ugh to result in

a negat ive effect on TC expo rt, expla ining 8% of its va riability, desp ite the lack of a

re lat ionshi p between slo pe and DI C export. Similarly, the effect of inc reas ing TC

expo rt w ith inc reasing wetland coverage arose so le ly because of the important ro te of

wet land s in co ntro lling DOC export, as wetland s did not play a ro te in DlC expo rt.

This land cover featu re of the catchment ex pla ined 29% of the va riability in TC

expo rt. ln co ntrast, a ltho ugh BS I and %vegetatio n played impo rtant ro tes in both DlC

and DOC exports, they acted in o ppos ite directions on the two C spec ies, and as a

result, had no overa ll impact on TC export. As we observed fo r DI C and DOC export,

TC export is influenced by a co mbinat ion of topographie and land cove r effects,

expla in ing 24% and 29% of its varia bility, respect ive ly. Because the drivers of DI C

export and DOC export are qui te di ffe rent, the model of TC export prov ides a

simplified summ ary of what influences the move ment of terrestria l C to aquatic

sys tems, white hiding the complex ity of what influences the movement of indi vidua l

C spec ies.

Despite the re be ing ev idence in the literature fo r the effect of so it ty pe and geo logy

on C expo rt, these fac tors did not emerge as s ig ni fica nt drivers in our watersheds. We

used ma ps of geo logy and so it to div id e the catchments, based on area l percentages

(Table 1 ), in to 3 mutua lly exc lu sive categories of rock type ( in trusive, sedim entary

and vo lcanic) as we il as 3 mutua lly exclu sive categories of surfic ia l deposits (rock,

t ill and mud) and 5 mutua lly exc lus ive categories of so it (bruniso lic, g leyso lic,

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regoso lic, podzo lic and organic). We fo und one potentia l mode! of DTC expo rt that

inco rporated the percent coverage of sedim entary rocks and rocky surfic ia l depos its

but it is unclear why DI C expo rt would dec line w ith increas ing presence of

sedimentary rocks and rocky surfic ia l depos its. ln additio n to expla ining only a

marg ina l 4% mo re of the variabili ty in DI C export tha n the mode! in Table 2, it

required removing BSI as an effect and remov in g one outlie r s ite, and so this mode!

was co nsidered less appropriate fo r these catchments. There were no potentia l mode ls

for DOC or TC expo rt that inco rporated geo logy or so il. ln summary, we did not find

that geo logy or so il type played very important ro les in co ntro lling OTC, DOC and TC

ex port from the landscape.

1 .5.2 Influence of Land Cover on the Forms of Carbon Expo rted

A lthough vegetatio n coverage did not play a s ig nifica nt ro le in tota l C expo rt, it did

have an impact on the actua l nature of this export . We used the models in Table 2 to

project tota l C expo rt and its partition into DI C and DOC expo rts, under scenarios of

changing land use in term s of %vegetat io n, fo r two of o ur bas ins (Fig ure 6). ln the

case of the bas in that dra in s in to Lac d ' Argent, which currently has 92% vegetatio n

coverage, reducing the vegetatio n coverage to 40%, fo r example due to agricul ture or

urbaniza tion, wo uld result in an Il % decrease in C expo rt fro m the bas in (as

DI C+DOC) (our test using DIC+DOC, in stead of DIC+ I .1 *DOC, as TC expo rt

showed the same result a ltho ugh the coeffic ients are s li ghtly diffe rent), but a 3-fo ld

inc rease in the DI C/DOC expo rt ratio, fro m 0.8 to 2.2. The reductio n in vegetatio n

coverage would thus cause a shi ft in this bas in from a system that expo rts most of its

terrestria l Cas DOC, to a bas in that exports mostly DI C. Co nversely, for an inflow of

Roxton Pond, which current ly dra in s a watershed that is 57% vegetated, increas ing

the vegetatio n coverage to 100% would result in only a 9% inc rease in C export,

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----------------------------------------------------------------------------------------------

4 7

while the DIC/DOC export ratio wou ld be reduced to half, from 1. 1 to 0.5 (F igure 6).

ln this case the watershed wou ld shi ft from exporting equal amounts of D1C and

DOC to exporting mainly DOC. Land use change that modifies vegetation coverage,

such as deforestation or reforestation, wou ld therefore have a modest effect on total C

export, but wou ld greatly alter the form of C exported. Although deforestation is

widely regarded as one of the most common anthropogenica lly-dr iven land cover

changes, especially in developing countries (Nagendra, 2007) , many of the temperate

regions in Eastern North America and Western Eu rope have been undergoing

reforestation due to a decline in agriculture (Rudel, 1998; !PCC, 20 13). As DOC and

ore are processed differently in aquatic systems, changes in the form of terrestrial c

exported will lead to changes in the fate of this C , w ith DOC being mo re likely to be

mineralized and released to the atmosphere as C02 and CH4 than DIC, w hich may be

transported downstream in a more conservative manner. To summarize, reductions in

vegetation coverage wi ll shift the C export to favor the inorganic rather than the

organic forms of C, potentially leading to the terrestrial C being transported further

downstream, rather th an being re leased to the atmosphere through bio logical

processes in the aquatic system.

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4 8

11 .0 11 .0 A B

..-- 10.5 10 .5 · ..... >-N 10.0 10 .0 'E g t::

9.5 9.5

0 0.. x Q)

9.0 9 .0

u 8.5 8 .5

0 20 40 60 80 100 0 20 40 60 80 100 20 20

c D

t:: 15 15 g_ x Q)

10 10 u 0 0 ü 5 5 0

0 0 0 20 40 60 80 100 0 40 60 80 100

% vegetation % vegetation

Figure 6. Carbo n expo rt as the sum of DIC export and DOC export in g m-2 yr-1 and the

DI C/DOC expo rt ratio ve rsus %vegetation fo r two examp le watersheds, an inflow to

Lac d ' Argent (pane ls A and C) and an in fl ow to Roxton Pond (pane ls B and D) . The

current vegetatio n coverage of the catchment is ind icated by an "X" in each pane l.

1.5.3 Inter-annual Var iation in Carbo n Export

The diffe rences in C expo rt observed in 32 bas ins over 3 consecutive years were

like ly driven by inter-annual va riations in temperature and prec ipitat io n, causing

in ter-annua l va riations in stream discharge (Table 3). Expo rt of both DI C and DOC

was about 25% hig her in 2005 (at 6.8 and 6 .1 g m-2 yr-1, respective ly) re lat ive to 2003

and 2004 exports (Figure 5) . In term s of temperature and precipitation, 2005 was a

wann , wet year and 2004 a dry yea r, as co mpared to a re lative ly average 2003. T his

resu lted in the mean dai ly discharge at the Tro is-Lacs gauged s ite be ing highe r and

more variable in 2005 than in the two preceding years (ANOVA p=0.0063 , n= I 096,

2003=A, 2004= AB, 2005= 8). Sim ilarly, at the Waterloo gauged site, mean dai ly

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4 9

discharge was hig her and mo re va riable in 2005 (A NOVA p=0.0004, n= 1096,

2003=A, 2004=8 , 2005=8 ) . Discharge was therefo re sig nifi cantly higher in 2005

than in 2003, and this like ly drove the diffe rences in C expo rt, a pattern that has been

prev io usly repo1ted (Dillon and Malot, 2005; Dinsmore et al., 201 3). As C export is

the product of discharge and C concentration per unit catchment area, we a lso

examined inte r-annua l variability in concentratio n. Average DJC and DOC

co ncentratio ns were highest in 2005 (Table 3), yet there were no signifi cant

diffe rences in co ncentration among years. 1 n other words, va riatio n in C

co ncentratio n across the 32 sites was more important than va riat io n across the 3 years.

lnteresting ly, at the sca le of the individual site, we found a negative effect of

discharge on DI C concentratio n w ithin 3 1 of our 83 sites (dilution effects, outlined in

Sectio n 2 .2), yet at the regio na l sca le, the highest discharge year (2005) was

associated to the highest average DI C co ncentrations, and the hig hest average DI C

ex port fro m a li s ites co mbined. We suggest that trans ient increases in runoff and

discharge w ithin a catchment may not necessarily lead to increased DTC re lease rro m

so ils, and this may expla in the local dilution effects that we so metimes observed.

However, a systematic increase in overa ll prec ipitatio n and temperature, and the

assoc iated sustained increased in runoff and discharge, may act to increase overa ll

DLC and DOC export on an annual scale.

Acknowledgemen ts

Ali the data used in this study are ava il able fo r co llaborati ve use; contact Alice Parkes

(parkes.a lice@ uqam.ca) fo r in formatio n regarding access and handlin g. T his study

was jo intly funded by the Nationa l Science and Eng ineerin g Research Counc il of

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5 0

Canada (NSERC) Strategie Projects Grant Program (SPG) and the Program for aiding

developmental and environmental research (PARDE), awarded to Y. Prairie, as wei l

as the NSERC Hydro-Québec lndustrial Research Chair in Carbon Biogeochemistry

in Boreal Aquat ic Systems (CarBBAS Chair), held by P. del Giorgio, and by a

scholarship from the Fonds québécois de la recherche sur la nature et les technologies

(FQRNT), given to M. Li . Thanks to Jean-François Lap ierre, Marie-Eve Ferland,

François Guillemette, Dominic Vachon and Martin Berggreri for their va lu able

statistical and academie assistance.

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CHAPTER Il

MAGNITUD E AND COMPOSITION OF CA RB ON EX PORTED FROM BOREAL

CATC HM ENTS TO RIVE R SYSTEMS IN NORT HE RN QU ÉBEC, CANA DA

Mingfeng Li, Yves T. Prairie, and Paul A. de l G io rg io

Being preparedfor Global Biogeochemical Cycles

Gro upe de Recherche Interuniversitaire en Limno logie et en Enviro nnement

Aquatique, Département des sc iences bio logiques, Université du Québec à Mo ntréa l,

C. P. 8888, suce. Centre Ville, Mo ntréal, Québec, H3 C 3P8 Canada

N.B. Refe rences c ited in this cha pte r are presented at the end of the thesis.

2. 1 Abstract

Rivers play a major ro le in regio nal and g lo ba l carbo n (C) cyc ling by channelin g and

processing la rge amount of C that is de rived fro m land . T he form of the C exported

fro m watersheds to flu via l systems determines the biogeochemi ca l ro le and fate of

this C, yet few studies have simultaneo usly assessed the majo r fo nns of C ex ported

fro m land to rivers. Here we have quantified ri ver-mediated ex port of disso lved

organic and ino rganic C (DOC and DIC), as we il as the integrated aquatic emiss ions

of both C02 and CH4, fro m 44 bo rea l catchments that range w idely in size,

topography and land cover. Our results show that DOC and DIC expo rts averaged

9. 1± .3 and 2 .8± 1.9 g c m-2 (catchment) yr-1, respect ive ly, whereas tota l aquat ic co2

and C H4 emiss io ns averaged 3. 1±3.6 and 0.1 ±0. 1 g C yr·1 per m2 catchment,

respect ive ly. The resulting tota l C (TC) export was seasona lly very variable, driven

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5 2

mostly by the annua l runoff cyc le , and ave raged 15 .6±5.3 g C m·2 yr·1• DOC

dominated on ave rage this TC export over the annua l cyc le (59%), but aquatic C0 2

emiss ions were a majo r co mponent of TC expo rt in a li catchments (average 20%).

Our results confi rm that DOC and DIC exports are mostly driven by runoff but

further regulated by fu ndamenta lly di ffe rent env ironmenta l fac to rs, and that wetlands

are a majo r so urce of DOC expo rted to rivers, but further demo nstrate that lakes

within the catchment are a strong DOC s in k, such that the net expo rt of DOC results

fro m the ba lance between them. C02 emiss ions replace DOC expo rt as the ma in

latera l C export w ith increasing vvate r coverage in the catchment. T he annua! TC

exported via rivers is w ithin the range of net ecosystem productio n (NEP) that has

been estimated fo r these boreal landscapes, but it is it still unc lear w hat co mponents

of this to ta l ri ver ine export, if any, may be inc luded in these N EP estimates.

Regardless, this river-med iated lateral loss of C has the potentia l to fund amenta lly

a lter our perception of the ro le of these bo rea l land scapes as so urces or s inks of

atmospheric co2.

Key words Riverine carbo n; DOC; DI C; greenhouse gases ; bo rea l catchm ents;

carbon cyc le.

2.2 Introductio n

Rive rs are a fundamenta l compo nent of the g lo ba l C cyc le, processing C that

o rig inates in the terrestri a l biosphere, transporting it to the oceans and returning it to

the atmosphere. lt is now recognized that a s izable fractio n of watershed terrestria l

net primary producti vity (NPP) is channe led to ri vers, but whereas there is a

re lat ively strong consensus that the tota l amo unt of C de livered to the oceans by

rive rs wor ldw ide is in the range of 0.9 Pg C y- 1 (Meybeck, 1982 ; Cole et al., 2007);

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5 3

the re is still much uncerta inty regarding how much C is actua lly exported fro m land

to rivers. lt is now recognized that the re is a s ignificant but va riable amo unt of

processing of ino rganic and espec ia lly organic C of te rrestria l o rig in during trans it

within river ne tworks (Cole et al., 2007), such that the amo unts of C reaching the

ocean do not necessarily reflect the C that was orig in a lly exported fi·om land . ln fact ,

the estimates of C export fro m land to rive rs have systematically increased over the

past decade, to current estimates that are in the order 2.5 to 3. 1 Pg C annua lly

(Tranvik et al., 2009; Battin et al., 2009), and so have the estim ates of the portion of

this c that is buried in aquatic sedim ents, and returned to the atmosphere as c o 2. lt is

c lear that there is still much uncerta inty concernin g not only the magnitud e but a lso

the regulatio n of these compo nents of the g lo ba l C cyc le.

T he fo rm of the C exported from watersheds to flu vial systems is key not only. to the

fun ctio ning of these aquatic aquatic ecosystems, but to the actua l biogeochemica l fa te

of this C in the land scape. Fo r exa mple, whereas D!C expo rted fro m so ils tends to

trans it thro ugh aquatic networks w ith re lative ly little a lteratio n, DOC tend s to be

transfo rmed and degraded thro ugh bio logica l and photochemi ca l processes, fue ling in

s itu metabo lism in the rece iving systems, and further fu e ling C0 2 and CH4 emiss io ns

from these ecosystems. So il-deri ved C0 2 and C H4, on the o the r hand , will tend to

flu x o ut to the atm osphere, a lthough a portio n w ill a lso be transpo rted downstream

togethe r with the DOC and DIC. Over the past decade, there has been inc reas ing

interest and research seeking to establi sh the magnitude and the regulatio n of each of

these compo nents of river C dynami cs. For example, studies have identi fied the ma in

fac tors driving DOC export to ri vers, inc lud ing regional prec ipitatio n (Clair et al.,

1994; Dinsmore et al. , 201 3), runoff (Brinson, 1976; Raymond et al., 2007), land use

(Barnes and Raymond, 2009; Regnier et al., 201 3), catchme nt s lo pe (Eckhardt and

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5 4

Moore, 1990; Dosskey and Bertsch, 1994; Hazlett et al. , 2008; Li et al., 20 15), sail

C:N ratio (Aitkenhead and McDowell , 2000), pH (Brooks et al. , 1999), and the

density of lake and wetland systems in the catchments (Koprivnjak and Moore, 1992;

Dillon and Malot, 1997; Fer/and et al. , 20 12).

Dissolved inorganic C (DlC), on the other hand , appears to be strongly influenced by

catchment geology, in particular by the presence of carbonate deposits in the

catchment (Liu et al., 2000; Zhang et al. , 2009; Tank et al. , 20 12), and topographical

position and basin e levation (Soranno et al. , 1999; Kling et al. , 2000; Finlay et al.,

20 i 0; Li et ai., 20 i 5). Others have shown that land co ver change affects DIC export.

For example, the reduction of natural vegetation due to logging, farming , pasturing or

urbanizing are ali believed to increase riverine DIC export (Daniel et al. , 2002;

Raymond and Cole, 2003 ; Baker et al., 2008; Barnes and Raymond, 2009; Regnier et

al., 20 13 ; Li et al. , 2015). Further, it has been known for decades that most streams

and rivers are supersaturated with C02 (Kiing et al., 1991 ; Cole et al. , 1994), and CH4

(Billett and Moore, 2008; Bastviken et al. , 2011; Campeau et al., 2014), but the

evasion of C02 and CH4 from river systems ha.s only recently been recognized as an

important regional source of atmospheric greenhouse gases (GHG) (Butman &

Raymond, 2011 ; Bastviken et al., 2011; Raymond et al. , 2013 ; Campeau et al. , 2014),

and the number of studies focusing on river C gas evasion has increased

exponentially in recent years.

One of the main patterns to emerge from this collective body of work is the fact that

the various forms of riverine C (DIC and DOC export, C02 and CH4 transport and

emission) are regulated very differently from each other, and that therefore the

contrais of total C export from land to ri vers, and the fa te of this C are more complex

Page 75: Regional and global patterns in carbon export from land to water

5 5

than what were o rig ina lly thought. One of the ma in cha llenges that still remains in

arder to reso lve thi s co mplex ity is tha t the data on C export is still fragmented and

lacking integratio n, s ince most studies to date have focused on a spec ifie C species

(Mu/ho/land and Watts, 1982; Hope et al., 1994; Alvarez-Cobelas et al. , 20 12;

Hossler and Bauer, 201 3), and in a limited range ofwatershed types. Very few studies

to date have simultaneo usly qu antified ali the ma in compo nents that make up the tota l

poo l of C that is expo rted from la nd to rivers (Bille tt and Moore, 2008; Polsenaere et

al., 20 13; Striegl et al., 20 12; Abri! et al., 2000), and fewer still have do ne thi s ac ross

a range of enviro nmenta l, geographie and c lim atic gradients (Butman and Raymond,

20 Il ; Stets and Striegl, 20 12).

ln this study we have explic itly assessed the magnitude, composition and regulation

of tota l C ex po rt from a w ide range of catchm ents that are located in the bo rea l reg ion

of Québec. The borea l biome represents one of the largest C poo l on the earth , where

a large proportio n of a li terrestria l organic C is stored in so ils and in peat lands (Molot

and Dillon, 1996). lt is a lso a landscape w ith among the highest dens ity of surface

waters in the world , and where freshwaters are mo re like ly to play a key ro le in te rm s

of regiona l C processing and transport. The spec ifie objectives of this study were: 1)

to quantify the dynamics of DOC, DI C, C0 2 and CH4 across a w ide range of rivers in

the boreal regio n of Québec; 2) to reconstruct the magnitude of tota l C export fro m

these borea l watersheds; and 3) to explo re how the magnitud e and compos itio n of this

tota l C expo rt to rivers varies across catchments and a lo ng enviro nmenta l, geograph ie

and c lima tic gradients.

2. 3 Mate ria ls and methods

2 .3. 1 Study area

Page 76: Regional and global patterns in carbon export from land to water

5 6

Our general study area is located in the lowland reg io n of northe rn Québec, Canada,

w ithin the No r1hern C lay Be lt, which was created by lacustrine deposits from the

proglac ia l lakes Barlow and Ojibway. T he rivers we sampled and the ir respective

catchments are located in two dist inct sub-regio ns: South Abitibi (47-48°N, 78- 79°W)

and James Bay (48-49°N, 78- 79°W). Geo logica lly, this area is located in the Abitibi

sub-province of Canadian Shie ld. T he bedrock is composed of granitoid (50%),

vo lcanic (40%), and sedim entary ( 1 0%) rocks fo rmed ca. 2.7 billio n years ago and

covered by a thin layer of so il (Asselin et a l. 2006). The James Bay regio n is located

w ithin the black spruce - feath r mos bioc !imat ic domain, and is characterized by

extensive peat bogs and fens, whereas South Abitibi is w ithin the ba lsam fir (A bies

balsamea L. Mill.) - paper birch (Betula papyrifera Marsh) bioclimatic doma in , and

has much Jess coverage of peat bogs (Bergeron et a l. , 2004). Fro m south to north

across the study area, mean an nua i temperature va ries from 0 to 1. 7 °C, and mean

annual precipitatio n from 880 to 975 mm (Asse lin et a l. 2006). Beaver dams are

ubiquito us thro ugho ut South Abitibi , especia lly alo ng 151 to 3'd order streams, wh ile

they are pract ica lly absent in James Bay, which is dominated by peatlands. Further

details of the s ites and of the sampling regime can be fo u nd in Cam peau et al. , (20 14) .

T he C0 2 and C H4 dynamics and flu xes to the atmosphere in these bo real rivers have

been previo us ly reported by Campeau et a l. (20 14) and Campeau and de l Gio rgio

(20 14). Here we combine these previo us results on gas dynamics with additio na l data

on DOC and DIC concentrations and river discharge to assess the magnitude of C

ex port fro m these bo rea l catchments, and in particula r to derive est imates of tota l

watershed C export that inc lude not only DI C and DOC transpo rt by rivers, but a lso C

gas emiss ions by both rivers and lakes w ithin these catchments.

Page 77: Regional and global patterns in carbon export from land to water

5 7

2.3.2 Sampling and chemical analyses

In this study we use data collected from 44 sites, 30 located in South Abitibi and 14

located in James Bay, ranging from Strahler Order 1 to 7. Thirteen of these rivers

were visited 8 times each from May 2010 to May 20 Il , at around 5 week intervals,

20 were visited three times between May and October 20 1 0, and 11 were visited once

in mid-summer 2010. Water was sampled approximately 10 cm below the water

surface and filtered in situ using 0.45 ~-tm syringe filters and then sealed, refrigerated

and transported to the lab in 40 mL glass vials (1-CHEM) for chemical analyses.

Concentrations of DIC and DOC were determined following acidification and

oxidation with phosphoric acid and sodium persulfate, respectively, using a TOCI010

total carbon analyzer, equipped with an infrared C02 detector (OI Analytical, 2%

precision of2 replicates per vial, 3% accuracy at 5 mg L-1 standard).

2.3.3 Discharge and DOC and DIC flux calculations

Carbon expo11 of each stream was calculated as the product of the water loading and

the water C concentration (mg L-1), and therefore it is essential to accurately

determine both components. During the study period, we had pressure sensors

(True-Tracks) installed in four additional rivers and streams for continuous

monitoring of river water levet. For each of these streams we developed an empirical

discharge/water levet relationship based on the channel morphometry, which we used

to estimate the daily discharge of each of these 4 rivers. ln addition, the daily

discharges of Harricana and Kinojévis rivers were obtained directly from the

government-operated continuous gauging stations (hydrologie stations: 043012 and

080101) (http://www.mddelcc.gouv.qc.ca). Based these 6 sam pied streams and rivers,

we obtained an average daily runoff (mm d- 1) curve for the entire water year (Figure

Page 78: Regional and global patterns in carbon export from land to water

5 8

1 a), and this average regional curve was then used as the local runoff pattern to

estimate daily discharge of a li the sampled rivers based on their respective watershed

areas. There was good overall agreement between the discharge est imated this way,

and the actual measured discharge in our point samp ling across a li of our rivers

(R2=0.84, n= 113, p<O.OOOJ) (Figure 1 b).

We measured DOC and DIC concentrations at each samp ling date, and these

concentrations need to be extrapo lated in time in arder to calculate da.ily export.

Previous studies have shawn that there may be both concentrat ion and dilution effects

reiated to shifts in discharge (Li et al. , 20 15). We explored this possibility by

assessing the concentration versus discharge relationships of ali the individual

streams using ANCOYA, and testing the significance of the resulting s lopes. The

results show that the daily runoff had a significant overall negative (di lution) effect

on DJC concentration (R2=0.62, RMSE=0.2481 , n= 165 , p<O.OOOl), and a sign ificant

positive (concentration) effect on DOC concentration (R2= 0.68, RMSE=0.151, n= 169,

p<O.OOOl). We used the resulting s lopes and the river-specifie DOC or DIC intercept

offset to correct for discharge-related shifts in DOC and DIC concentration for each

river. Daily C export was then calculated as the product of daily runoff and daily C

(DOC and DIC) concentration, and annual DIC and DOC export was calculated as

the sum of daily export values for the whole water year. River POC concentrations

were not measured and therefore POC flux had to be approximated, which we did by

assuming POC roughly equal to 5% TOC for boreal forest eco-region (Ivarsson and

Janssen 1994; Hope et al. 1994). PIC flux was not included in the TC exported from

the catchments because previous studies have shawn that it accounts for a very sma ll

fraction of inorganic C (Aucour et al. 1999; Hassler and Bau er 20 13).

Page 79: Regional and global patterns in carbon export from land to water

c, 6 %i Dat

e (J

une

1, 2

010

to M

ay 3

1, 2

011)

0.1

0.1 5

9

10

100

1000

Mod

ele

d 0

(L s

·')

Fig

ure

1 a.

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l da

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(m

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·') f

or t

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ampl

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year

(20

1 0)

, w

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rs (

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), a

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(fro

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aily

disc

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vers

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the

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ear

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t, 20

10 t

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ay 3

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(R2 =

0.8

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=ll

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p<O.

OOO

1 ).

1000

0

Page 80: Regional and global patterns in carbon export from land to water

6 0

2.3.4 Aquatic gas emissions

During the study period, C02 and CH4 concentrations (ppm) and fluxes (mmol m·2 d·1)

across the air-water interface were measured in ali the rivers (as reported in Campeau

et al. (2014) and Campeau and del Giorgio (2014)). C02 and CH4 concentrations

were determined in situ using the headspace technique, and fluxes (mmol C m·2 d· 1)

across the air-water interface were determined using the floating chamber technique,

as described and reported in Camp eau and del Giorgio (20 14). Measurements were

not made from December to March, when most of these rivers are frozen . The C

fluxes of December 201 0 were assumed to be 1/2 of those measured in November

201 0, because most ri vers and streams were ice-covered for approximately half of the

month, and we assumed zero fluxes for January to March. We a lso used the average

C02 and CH4 fluxes measured in over 40 lakes in the same study region and reported

by Rasilo et al. (20 15) to estimate the average monthly C02 and CH4 emission from

lakes across the study catchments. Measurements were available for May, June, July

and October 2010, so we linearly extrapolated the measured fluxes to August,

September, November and December, and further used the average lake C02 and CH4

evasion measured in May 2010 to estimate fluxes for April and May 2011. Average

monthly lake C02 and CH4 emissions within each catchment were estimated by

multiplying the average measured gas fluxes by the total lake area within the

catchment; likewise, average monthly total stream gas emissions were calculated as

the average monthly C02 and CH4 fluxes multiplied by the total stream area within

catchment. Tota l aquatic C02 and CH4 emissions are the sum of the estimated lake

and stream emissions within each catchment.

2 .3.5 Watershed properties

Page 81: Regional and global patterns in carbon export from land to water

6 1

For each sampled site, the catchment area was delineated from digital maps with a

resolution of 1 :50,000 scale (available at Natural Resources Canada (National

Topographie Data Base) and average catchment slope, lake area as weil as stream

length of each river order within this catchment on the maps were calculated using

ArcGIS 9.3. The stream area for different stream order within the catchment was

calculated separately as the product of total stream length and the average stream

width of the corresponding stream order, the latter derived from the measured width

for 328 across notthern Quebec; total stream surface in the catchment is the sum of

the total areas for each stream order existing within the catchment.

2.3.6 Statistical Analyses

ln most cases, data input for modeling were log 1 0 transformed for normality. A series

of simple linear regressions were performed to explore the correlations between

variables. Significance is determined at p<0.05 and results reported as non-significant

have p values >0.05 . We used JMP 9.3 (SAS institute) to do the statistical analyses.

2.4 Results

2.4.1 Riverine DOC and DIC exports

The rivers sampled in this study, as weil as their respective catchments, span much of

the range in physical, hydrological and chemical characteristics found in the Boreal

region of Québec, summarized in Table 1. The estimated annual DOC export varied

by one order of magnitude across watersheds, fi·om 1.8 to 21.4 g C m·2 yr·1, averaging

9.1 g C m·2 yr·1 (Table 2). The annual DlC export was systematically lower than

that of DOC, averaging 2.8 g C m·2 y- 1, also ranging an order of magnitude across

watersheds (from 0.5 to 7.7 g C m·2 yr·1, Table 2).

Page 82: Regional and global patterns in carbon export from land to water

6 2

Table 1 The physical , hydrological and chemical characteristics of the 44 catchments

and the rivers sampled.

Variable Max Min Mean (SD)

Catchment arca (km2 ) 6013 .2 0 .03 401 .3(1261.2)

Catchment average slope (") 5.9 0.3 2 .0( 1.5)

Total stream length (km) 10207.7 0 . 1 694.3(2210.8)

Forest coverage (%) 100 0 72.0(41.1)

Wetland coverage (%) 100 0 27.0(42. 1)

Lake coverage (%) 10.0 0 1.4(2 .6)

DOC concentration (mg L ' 1) 35 .2 8.7 16.9(6 .2)

DIC concentration (mg L-1) 32.6 1.9 11.3(7 .3)

Total P concentration (J.tg L· 1) 170.5 2 .2 35.4(26.7)

Total N concentration (mg L ·1) 1.6 0.2 0.6(0.2)

pH value 9 .9 5.0 7.1(0.6)

COz evasion flux (mmol m·2 d· ') 727.6 -1.6 126.3(156.8)

en. evasion flux (mmol m -2 d-1) 214.5 0.03 7.5(27 .2)

Average CHL a (1-lg L ' 1) 39.0 0 . 1 3 .2(4.7)

DOC and DIC export were both significantly related, albeit in opposite directions, to

average catchment slope (logDOCexp = 2.2 -0.5* log-slope, R2=0.69, n=44, p<O.OOO 1;

logDICexp = 0.61 + 0.45*1og-slope, R2=0.31 , n=44, p<O.OOOl) and wetlands%

(logDOCexp = 2.7+0.21 *log-wetland%, R2=0.71, n=21 , p<O.OOOl; logDICexp = -0.03

- 0.19*1og-wetland%, R2=0.37, n=21 , p=0.0003). It is clear that DOC and DIC

export fluxes are regulated very differently in these catchments, and interestingly,

Page 83: Regional and global patterns in carbon export from land to water

there was a strong negative relationship between both (logDICexp

0.67*logDOCexp, R2 = 0.25, n = 44, p=0.0005).

6 3

2.13 -

Since DOC and DIC exp01t are both driven by the same discharge, the factors

identifted above appear to be mostly acting the concentrations of DOC and DIC. ln

particular, there was a strong negative relationship between river DOC concentration

and the percentage of water in the catchment (o/owater), the latter mostly driven by the

density of lakes within the catchment (logDOC = 1.41 - 0.16* logo/owater, R2 = 0.24, n

= 25, p=0.01) .

Table 2 The ranges, means and medians of the fluxes of DOC, DlC, POC, C02, CH4

and TC exported annually from the 44 catchments (units : g C m-2 yr-1 on catchment

area basis).

C species DOC DIC POC co2 CH4 TC

Max 21.4 7.7 1.1 14.7 0.4 27.3

Min 1.8 0.5 0.1 0.3 0.01 4.7

Mean (SD) 9.1 (5.3) 2.8 (1.9) 0.5 (0.3) 3.1 (3.6) 0.1 (0.1) 15.6 (5.3)

Median 6.29 2.0 0.4 1.3 0.1 14.8

2.4.2 Watershed aquatic C02 and CH4 emissions

Page 84: Regional and global patterns in carbon export from land to water

6 4

Watershed aquatic gas emissions from ali aquatic surfaces within the watersheds

averaged 3.1 g C m·2 yr-1 for C02 and 0.1 g C m-2 yr-1 for CH4, and ranged two orders

of magnitude across watersheds for both (Table 2) . C02 and CH4 were significantly

positively related to each other (C02em = -0.70 + 31 .4*CH4em, R2=0.88, n=44,

p<O.OOO 1) and bath had significant positive relationships with catchment area (C02:

R2=0.39, n=44, p<O.OOOI; CH4: R2=0. 14, n=44, p=O.OI), total stream length (C02:

R2=0.44, n=44, p<O.OOOI ; CH4: R2=0.18, n=44, p<O.OOOI), stream arder (C02:

R2=0.49, n=44, p<O.OOOl ; CH4: R2=0.24, n=44, p=0.0007). In particular, C02 and

CH4 were both strong ly positive ly correlated to the %water in the catchment (C02em =

1.07 + 137.2*%water, R2=0.97, n=44, p<O.OOOI ; CH4em = 0.07 + 3.65*%water,

R2=0.77, n=44, p<O.OOOI).

2.4.3 Total C export from boreal catchments

Table 2 shows the average and range of total C export (TCexp) from these boreal

catchments, estimated as the sum of DOC, DIC and POC export and C02 and CH4

fluxes. There was a strong seasonal variation in the magnitude of the average regional

TC export (Figure 2a). TCexp peaked in the early spring, and May and April fluxes

accounting for 38% of the annual TCexp, corresponding to a peak in annual discharge

driven mostly by snowmelt (Figure 1 a). There was a secondary peak in export in the

fait , this associated to increased discharge driven by precipitation. Export was lowest

in both mid-summer and mid-winter, both periods of base flow (Figure 2a). The

relative contribution of C form to total C export from boreal catchments also varied

greatly along the annual cycle (Figure 2b) . DOC dominated TCexp in the fait , winter

and spring, whereas the combined C02 and CH4 emissions dominated TCexp during

Page 85: Regional and global patterns in carbon export from land to water

6 5

summer (Figure 2b). The contribution of DIC to TCexp was relatively constant

throughout the annual cycle .

................................................................................................................................... a

100•.

so•. ..

40~.

20~o ··

o•.

b

• CH4 O C02 OPOC .. •roc • ore

Figure 2. Total C expott (a) and contribution of each C form to total C export (b) for

each month in the water year. Total C export is expressed in g C per m2 catchment

area for each month, whereas the latter is expressed as percentage of each C form in

total C export.

The cumulative annual TCexp averaged 15.6 g C m·2 yr·1 (Table 2) across ali

catchments, with most values concentrating within a narrow range of 12 to 17 g C m·2

yr·1 (SE around the mean 0.8 g C m·2 yr-1). There were significant positive

relationships between TC export and catchment slope, the proportion of wetlands and

of water in the catchment, and the three combined resulted in the best predictive

Page 86: Regional and global patterns in carbon export from land to water

6 6

mode! of annual total C expo tt : TCexp = 15.4 - 1.35*Siope + 116.5*%water +

4.03*%wetlands (R2=0.64, n=44, p<0.0001 .

a b ~ . DOC QOntribution -

t: . C02 contribution -0 ..... a. >< <ll:;J "' ü

•mc § . ~. .. !2

• noe _g:il • c:

DPOC .2 :5 . . .o o

OC02 ·= ..... ë 0

OCH4 u

~:1 ..

•• • 1 1

• " " water%

Figure 3. a. The average contribution of the various C species in the annual total C

export from these boreal watersheds; b. The contribution of DOC export and of

aquatic C02 emissions to total C export from these boreal catchments, as a function

of the proportion of water in each catchment (% water). The latter is mostly driven by

the presence of lakes. The catchments of ri vers of arder 1 and zero contained no lakes

and were aggregated as %water = O.

DOC dominated the average annual TCexp (58 .5%), whereas DIC contributed on

average < 18% (Figure 3). Gaseous emissions as C02 and CH4 together accounted for

an average of 20.5% of the annual TC exported from these boreal catchments,

overwhelmingly driven by C02 emissions. The estimated contribution of POC to the

average annual TCexp was small , in the arder of around 3% (Figure 3a). The

contribution of the various C species to the annual TCexp varied systematically across

the studied watersheds. In particu lar, there was strong decline in the contribution of

DOC to TCexp with increasing water surface in the catchment, which was mirrored

Page 87: Regional and global patterns in carbon export from land to water

6 7

roughly by a proportion increase in the contribution of C02 along the same gradient

(Figure 3b).

We further explored whether there were systematic differences in the magnitude and

composition of TCexp between the two distinct sub-regions that we covered in our

study (South Abitibi and James Bay) , which are characterized by very different land

cover, particularly in terms of the extent of peatland cover (data summarized in Table

3). The average DOCexp was 2.4-fold higher in James Bay (15 g m·2 yr-1) compared to

South Abitibi (6.3 g m·2 yr-1), whereas the average DIC export was 2.6-fold higher in

South Abitibi; the average C02 and CH4 in South Abitibi were 2.5 and 3.9 times

higher than those in James Bay, respectively. Interestingly, CH4 was low in James Bay

(< 0.2%), but was a significant component of TCexp in South Abitibi (> 1%). As a

result, not only was the average magnitude of the TCex p different between the two

sub-regions (18.6 g m·2 yr·' versus 14.0 g m·2 yr·1, James Bay and South Abitibi,

respectively) , but also actual composition of the TCexp differed substantially: Whereas

in James Bay DOC overwhelmingly dominated TCexp (80%), and DIC contributed

very little (7%), in South Abitibi the TCexp was more equally distributed between

DOC (45%), DIC (25%), and C gas emissions (27% and 1%, C02 and CH4,

respectively) (Figure 4).

Page 88: Regional and global patterns in carbon export from land to water

6 8

Table 3 The comparisons of physical, chemical and biological factors between James

Bay and South Abitibi during the measurement period (June 1 5', 2010 to May 31 5',

2011 ). The values were annually averaged from the 14 and 30 streams and rivers we

observed in James Bay and South Abitibi , respectively.

Variable James Bay South Abitibi

A vera ge air temperature eq 0.6* 2.7*

Average catchment slope e) 1.3 2.3

Average elevation (rn) 296 304

Forest coverage (%) 20.8 97.5

Peatland coverage (%) 84.3 0.3

Lake coverage (%) 0.5 1.9

DOC concentration (mg L·') 31.3 16.8

DIC concentration (mg L-1) 6.0 12.0

Total P concentration (J.Lg L-1) 24.1 37.8

Total N concentration (mg L-1) 0.6 0.5

pH value 7.1 7.2

Alkalinity (Jleq L·') 330.2 1177.1

COz evasion flux (mmol m·2 d·1) 70.6 89.6

CH1 evasion flux (mmol m·2 d·') 3.1 9.1

Average CHLa (Jlg/L) 1.2 3.4

Page 89: Regional and global patterns in carbon export from land to water

South Abitibi

ODI C

• ooc OPOC O C02 OCH4

6 9

James Bay

Figure 4. Comparison of the average contribution of di ffe rent C fo rm s in the annual

total C export between South Abitibi and James Bay reg ions.

2.5 DISCUSSION

2.5.1. River-mediated DOC, DIC and C0 2 loss from landscapes

The loss of C fro m land to aquat ic ecosystems is increasing ly recognized as a

significant component of the reg ional C budgets (Co le et a l. 2007; Tranvik et a l. 2009;

Raymond et a l. , 20 13). T here is an extensive literature on river-mediated C loss from

terrestrial systems, but re latively few studies to date have g iven a complete acco unt of

riverine disso lved, particulate and gaseo us C exports fo r regional C budgets of borea l

biomes yet. Our study of 44 rivers was explicit ly designed to quanti fy C loss in

diffe rent fo rm s, i. e. DOC, DIC, C02 and CH4, so as to obtain an integrated view of

TC export from the landscape.

The ranges for the export of TC and of the diffe rent C species from these boreal

catchments in Q ué bec lie weil within the range of va lu es in the literatu re. Table 4

Page 90: Regional and global patterns in carbon export from land to water

7 0

Tab

le 4

Com

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of

late

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and

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each

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m·2

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., T

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summarizes published results on lateral (dissolved) and vertical (gaseous) Joss of C

from different landscapes. Our range in DOC export (1.8 to 21.4 g m-2 yr-1) agrees

weil with the range between 2.3 and 14.8 g m-2 yr-1 for boreal watersheds in Finland

(Rantakari et al. 201 0), and our average regional DOC export of9.1 g m-2 yr- 1 is close

to the upper limit of the range between 3.1 and 8.4 g m-2 yr-1 for Northeast Canada

given by Mulholland and Watts (1982), but somewhat higher than the mean value of

6.1 g m-2 yr-1 reported by Huotari et al. (20 13) for boreal watersheds in southern

Finland. On the other hand, our average DIC export of 2.8 g m-2 yr-1, ranging from 0.5

to 7.7 g m-2 yr-1, was significantly higher than that in Sweden (0.3-1.4 g m-2 yr- 1,

averaging O. 7 g m-2 yr-1) (Wall in et al. 20 13) and Fin land (TJC: 0.9-1.4 g m-2 yr-1,

averaging 1.1 g m-2 yr-1) (Rantakari et al. 2010), but lower than that found in the

Yukon River (5.8 g m-2 yr-1) (Table 4). These regional differences in DIC export

most likely reflect local geology (Tank et al. 2012), and to sorne extent, variations in

terrestrial primary production, sin ce a sign ificant portion of the DIC exported actually

originates from chemical weathering of rock that is mostly mediated by respiratory

soil co2.

Our results confirm the key role of wetlands as sources of DOC to ri vers (Houtari et

al. 20 13), but further highlight the rote of surface waters, and particularly lakes, as

important sinks of DOC in the landscape (Gergel et al. 1999; Xenopoulos et al. 2003),

such that the balance between the relative coverage of wetlands and lakes appears to

be one of the key determinants of the net DOC export from these northern watersheds.

Our study provides further evidence for a differentiai regulation of DOC and DIC

export from these boreal catchments. Whereas the export of both species is primarily

driven by runoff, as has been reported before (Hou tari et al. 20 13), the re are

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7 2

differences in the drivers that determine their respective concentrations, and in

particular, DOC and DIC exports significantly related to catchment slope and percent

wetland, but in opposite directions. This is consistent with previous findings of our

group (Li et al. , 20 15) and others (Hou tari et al. 20 13), wh ich demonstrated a

differentiai control DOC and DIC export in temperate and boreal watersheds. The

resulting strong negative relationship between DOC and DIC export that we report

here is interesting, because it is to sorne extent compensatory and may contributes to

constraining TC export within a narrower range.

The average emissions of C02 and CH4, on a catchment area basis were 3.1 and 0.1 g

m·2 y 1, respective( y, and both ranged widely (see Table 2). There are only a handful

of reports of total aquatic C02 fluxes scaled to the en tire watershed, which range from

a minimum of 2 g C m·2 y· ' in sub-arctic Sweden (Christensen et al. 2007) to a high 9

g C m·2 y 1 for the Yukon watershed (Striegl et al. 20 12); our own average estima te of

C02 emission (lies weil within this range and close to previous boreal studies

(Jonsson et al. 2007). The CH4 value, on the other hand, is within the upper ranges of

emission reported for other river systems (listed in Table 3), and over 3 times more

than the CH4 emissions from a boreal landscape located only 400 km south of our

study area (Billet and Moore, 2008). One of the main reasons underlying these higher

CH4 is the widespread damming of rivers by beavers, especially in South Abibiti,

which generates aquatic habitats that are particularly conducive to the production and

emission of CH4 (Ford and Naiman 1988). C02 significantiy and positively related to

CH4 in this regions, as had been previously shown by Campeau and del Giorgio

(20 14), who reported a strong positive correlation between the partial pressures of

C02 and CH4 (pC02 and pCH4 ) in surface water in the same study area. This positive

Page 93: Regional and global patterns in carbon export from land to water

7 3

correlation is not surprising since C02 and CH4 are bath strongly driven not only by

the average water surface C fluxes but mostly by the relative coverage of water in the

catchment, which in turn is a function of the catchment size and slope, larger

catchments having lower slopes and generally larger lakes, and thus the positive

relationship of bath with catchment area as weil.

2.5.2 Magnitude and composition oftotal C export

On the who le, the magnitude of TC export from the landscape was characterized by

spring>fall>winter>summer. This seasonal variation in TC tightly followed the

average daily d ischarge curve (Figure 1 ), suggesting that TC export was mostly

driven by the seasonal variation in discharge and therefore runoff. In particular, the

TC export from April to May accounted for 38% of the annual TC export, resulting

from a combination of the degass ing of C02 accumulated under the ice, and also of

high spring discharge and associated loads of DOC and DlC. Previous studies have

also reported runoff as explaining 60 to over 80% of the variation in DOC and DlC

export from temperate and boreal catchments (Âgren et al. 2007; Raymond and Oh

2007; Pumpanen et al. 20 14; Li et al. , 20 15), with concentrations explaining the

remainder of bath the cross-system and seasonal variability in dissolved C export.

On average, the contribution of the various C species to annual TC export was

DOC>C02> DIC>POC>CH4, and over 90% of the TCexp accounted for by the first

three, in agreements with previous studies (Striegl et al. 2012; Wall in et al. 20 13;

Dinsmore et al. 2013 , and see Table 3). On the who le, the gaseous C loss as C02 and

CH4, accounted for an average of 20% of the an nuai TC export in our study area, in

agreement with the values reported for other boreal (Hope et al. 2001 ; Oquist et al.

2009; Koprivnjak et al. 2010; Striegl et al. 2012; Wallin et al. 2013), but signiftcantly

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7 4

higher than the 13% and 6% from temperate watersheds reported by Billett et al.,

(2004) and Abri l et al. , (2000), respectively. Our results, together with these previous

studies, collectively confirm that gaseous C export is a major component of

river-driven C loss from boreal landscapes.

Although DOC dominates the average regional annua l TCexp, its contribution within

individual watersheds declines as a function of increasing water coverage in these

watersheds, reaching a minimum of around 20% at the highest water densities in the

landscape, whereas the contribution of C02 tends to peak at around 65% in these

water rich watersheds. lt is interesting to note that these opposing trends in DOC

export and C02 emissions, as weil as with DIC discussed above, confers a certain

stabi lity to the tota l C export from these boreal catchments, and helps explain why

most of the estimates of TCexp ho ver around a relatively narrow range ( 12 to 17 g C

m·2 y·1), in spite of the large environmental, topographie and morphometric

heterogeneity th at exists among the se catchments. The best predictive model of TCexp

actually retlects the net balance in the regulation of its main components: The

positive re lationship with the proportion of wetlands retlects their strong positive

influence on DOC export, whereas the positive etfect of the proportion of water

suggests that the enhancement of C gas emiss ions w ith increasing water surface is not

offset by the decline in DOC that occurs along the same gradient; the positive effect

of catchment area on TCexp further retlects increases in runotf as a function of

catchment ize, a weil as in total gas emis ion and DIC export. We should note that

in this study POC was not measured and was rather assumed to be a fixed proportion

of DOC, so no actual conclusions can be drawn for this particular C species, but

existing evidence suggests that its contribution to TCexp in these mostly

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7 5

non-agricultural watersheds should be minor (lvarsson and Jansson 1994; Hope et al.

1994).

The interplay between the average contribution of DOC and C02 along a gradient of

water density among catchments occurs to sorne extent within a given catchment in

time. We have shown that there is a clear temporal succession in the relative

contribution of the main C species along an an nuai cycle, with a peak contribution of

DOC during periods of high discharge, and dominance of C02 during summer low

flow periods. The mechanisms underlying this replacement are probably different:

Whereas the replacement of DOC for C02 across catchments likely reflects the

increased photochemical and biological degradation of terrestrially derived DOC that

occurs with increasing water coverage and thus water residence time, the temporal

succession reflects seasona l shifts in discharge and aquatic metabolism (Vachon and

del Giorgio 20 14). The increase in the relative contribution of C02 du ring these low

flow periods results from a combination of declines in the loading of DOC, and

increases in the actual aquatic co2 fluxes, likely driven by temperature- and

light-enhanced DOC degradation within lakes. The seasonal pattern in both C02 and

CH4, with marked peaks in mid-summer, also likely reflects a temperature effect,

consistent with Billett et al. (2004), Koprivnjak et al. (20 1 0) and Kosten et al. (20 1 0)

who reported a strong positive correlation between temperature and co2 evasion .

2.5 .3 Cross-regional differences in the magnitude and composition ofC export

It is interesting to note that there are major differences in both the magnitude and the

actual composition of TCexp between the two sub-regions, James Bay and South

Abitibi , that coexist with our general study area but which are very distinct in terms

of land cover and to sorne extent climate (see table 4 for a summary) . The total C02

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7 6

and CH4 emissions in James Bay were 2.5-fo ld lower those in South Abitibi, to sorne

degree, reflecting the effects of regional temperature and water coverage, whereas

DIC export was substantially higher on average in South Abitibi , possibly related to

differences in both underlying geo logy as we il as in temperature-driven soi t

respiration and the resulting chem ica l rock weathering. The largest difference that we

found between the regions, however, was in the DOC export, which was 2.4-fold

higher in James Bay, driven by a 2-fold higher average river DOC concentration there

(Table 3) . These elevated DOC concentrations in James Bay are likely related to the

much higher peatland coverage (12.6- 1 00% vs . 0-4% of the total ar a) and smaller

catchment slope (1.3 vs 2.3 on average) , and reinforce the importance ofthese factors

in determining DOC concentration and export from land (Eckhardt and Moore, 1990;

Dillon and Molot, 1997).

2.5.4 Implications to terrestrial C budgets

The lateral C export from land to water that we report here must be placed in the

context of the who le C budget of this boreal landscape. Much of the recent empirical

and modeling work has focused on better constraining the net C02 exchange between

various landscapes and the atmosphere (i.e., Net Ecosystem Production, NEP), in

order to determine the role of these eco systems as sources or sinks of atmospheric C.

NEP varies greatly with geographie location and climate, but a lso within a given site

as a function of stand age, fire history and clear cutting (Bergeron et al. 2008;

Goulden et al. 20 Il), and also as a function of inter-annual differences m

precipitation and temperature (Litvak et al. 2003). Moreover, there is evidence of

long term shifts in the net C balance of landscapes with in the boreal biome. For

example, Dunn et al. (2007) reported a decade-long shift in net C uptake in a boreal

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7 7

landscape dominated by black spruce, from being small source of C ( -41 g C m-2 y- 1)

to more recently become a sma ll sink (+21 g C m-2 y- 1). Not surprising ly, there is a

very large range in net C02 exchange reported for boreal forests, from - l 00 to

250 g C m-2 yr- 1 (Ryan et al. 1997; Hyvonen et al. 2007).

ln this regard, very few current models of terrestrial primary production and C

storage include lateral C !osses ( i.e. Hayes et al. 20 12; Regnier et al. 20 13; Wu et al.

2013), and even fewer incorporate aq ueous C emissions, although there is ample

evidence that these fluxes are regionally significant, particularly in boreal landscapes

(Battin et al. 2009; Wallin et al. 2013). A key issue is then to what extent current

approaches to estimating NEP include this TCexp, and if they do, what components of

TCexp are included (Fied ler et al. 2006; Hyvonen et al. 2007). Estimates based on

eddy covariance towers may incorporate sorne of the aquatic C emissions, depending

on the locatio n of the tower, but certainly do not include a li the C components !ost to

the aq uatic network (Kij un et a l. 2006). For example, recent studies have reported

that unexpectedly high lasses of soi l C had to be invoked in order to mass balance the

apparent net C02 uptake with the observed long-term ecosystem C accumu lation, and

these were attributed to increased respiration (Lindroth et al. 2008), but it is likely

that at !east a portion of this soi t C loss resulted from the lateral export soit DOC and

DIC to the aquatic network. T he few previous studies that have explicitly attempted

to integrate the main components of the lateral C export to water a li converge to point

out that TCexp is a significant component of regional C flu xes and consistently within

the arder of magnitude as the NEP in for their respective regions (5% to over 80% of

the local NEP, Buffam et a l. 2011; Field 1er et al. 2006; Christensen et al. 2007;

Jonsson et al. 2007; Striegl et al. 20 12).

----------------------------------------

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7 8

ln this regard, although there is an extensive literature on boreal terrestrial primary

production net C exchange, there are few direct est imates of NEP for this region in

particular. G irardin et al. (2011) report an average NEP ofaround 150 g C m-2 y- 1 for

the southern portion of the Abitibi region of boreal Québec, whereas Bergeron et al.

(2007) report a much lower NEP, in the order of 4 g C m-2 y-1 in the nearby region of

Chibaugamau. Sorne of this difference may be methodological , because the former

was based on estimates of C stocks and tree productivity, whereas the latter is based

on eddie covariance data. This large range in reported NEP for this region also likely

reflects the large degree of heterogeneity that exists in this boreal la ndscape, driven

by local climate, fire history and more recently, forestry. Regardless, the estimated

TCexp in the Abitibi region that we report here is like ly to play a key role in

determining wh at portions of this vast boreal landscape act as either a so urce or a sink

of atmospheric c_ This is particularly true for the James Bay region, which is drier

and colder than the adjacent South Abitibi and thus likely approaches the lower

reported range of NEP, and which is nevertheless characterized by a higher estimated

TCexp (18.6 g m-2 yr-1 vs 14 g m-2 yr-1, James Bay and South Abitibi , respectively).

This implies that the relative contribution ofTCexp to the whole landscape C budget is

likely to be even larger in the James Bay region than it is for Abitibi in general.

Interestingly, these TCexp areîn the same range of estimated fire-driven C02 fluxes to

the atmosphere (Van Bell en et al. 20 1 0), yet whereas the latter are incorporated into

most terrestrial C models for these regions, the former are not. More generally, the

large degree of uncertainty that still characterizes most estimates of net tenestrial C

si nk or sources may be at least in part related to the non-inclusion of this lateral loss

to the aquatic components of the regional C budget.

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7 9

ACKNOWLEDGEMENTS

Ali the data used in this study are available for collaborative use; contact Paul A. del

Giorgio ([email protected]) for information regarding access and handling.

This study was funded by the NSERC Hydra-Québec lndustrial Research Chair in

Carbon Biogeochemistry in Boreal Aquatic Systems (CarBBAS Chairwhich is jointly

funded by the National Science and Engineering Research Council of Canada

(NSERC) and Hydra-Québec. Thanks to Audrey Campeau, who generously shared

her data with us, and ali the current and former members of the CarBBAS team who

participated in the collection and analysis ofthe data used in this study.

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0 00

Page 101: Regional and global patterns in carbon export from land to water

CHA PTER III

A GLOBAL ANALYSIS OF RlVERJNE CARBON EX PO RT TO TH E OCEANS

Mingfe ng Li, Yves T. Pra irie, and Paul A. de l G io rg io

Be ing prepared for Nature Geosciences

Département des sc iences bio logiques, Univers ité du Québec à Mo nt réa l, C P 8888,

Suce Centre Ville, Mo ntréa l, Québec, H3C 3P8 Canada

N.B. References c ited in this chapter are presented at the end of the thesis

3. 1 Abstract

8 1

Rive rine carbo n (C) expo rt to the oceans is a key co mpo nent in the g lo ba l C cyc le,

yet it is still poorly co nstra ined . T his is large ly the resul t of the limi ted data base used

to der ive a g lo bal flu x and because most studies assessed the ino rganic or organic

fractio n separate ly, thus yie lding a fragmentary perspecti ve of C exported to oceans.

Here we rev is it the g lo bal rive rine C expo rt, based on the meta-ana lys is of publi shed

data covering 566 rivers dra ining 74% of the g lo ba l exorheic area. This analys is

yie ld s a new g lo bal annual riverine C expo rt of 0.68±0.05 Pg yr-1, substantia lly lower

than the w ide ly-c ited 0 .9 Pg yr-1• Our results show that, at a g lobal scale, the organic

and in organic co mponents of C export are driven by diffe rent combinat ions of natura l

and human-derived features of the land scape. Beyo nd the expected hydro logie co ntro l

on materia l transpo rt, DOC expo rt was ma inly co ntro lled by natural characteristics

su ch as the presence of wet lands as we il as the organic carbon content of the surface

so i! layer. O n the other hand , the natura l drivers of DJC export were the extents of

carbo nate rocks and water surface in the watersheds. However, a li fonns of carbon

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8 2

were also shawn to be dependent on the extent of croplands within the catchments,

and strongly so for the inorganic fraction. A retrospective analysis suggests that 40%

of the current C delivery to the oceans is associated with agricu lture. Our multiple

regression models demonstrate that cropland expansion may be a primary driver of

future riverine C export not on ly in magnitude but also in its composition. This study

further highlights the differentiai regulation of global inorganic and organic C exports

and can serve as a strong· basis for identifying the implications of future c limate and

human-induced environmental changes on this important component of the global

carbon cycle.

3.2 Introduction

Riverine transport of C to the oceans is the ultimate leachate of terrestrial carbon.

Fo llowing the pioneering assessment of Meybeck and coworkers, it is a significant

component of the global C cycling and quantifying it is essentia l to a better

understanding ofthe Earth 's biogeochemical and climatic systems, particularly in the

context of g loba l change (Lai, 2003; Aufdenkampe et al. , 2011 ; Cole et al., 2007;

Battin et al. 2009). How much and in what form continental C is lost to oceans have

thus been of major biogeochemical interests for severa! decades . Regionally, the main

drivers of carbon export are relatively c lear. The combined etfects of topography,

hydrology, climate and land use/caver change on riverine C export from landscapes

are weil known(D' Arcy and Carignan, 1997; Hazlett et a l. , 2002; DeCatanzaro and

Chow-Fraser, 2011; Freemanet al., 2004) and the importance of the in land aquatic

processing of this terrestrially-derived carbon is an active research area (Cole et al.

2007; Tranvik et al. 2009; Raymond et al. , 20 13). A lthough g lobal estima tes of

riverine C export to the oceans have been periodically re-assessed (Richey et al. , 1980;

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8 3

Meybeck, 1982; Schlünz & Schneider, 2000; Dai et al; 20 12; Seitzinge r et al. ,

20 1 0), the ir acco unting is surprisingly poorly constrained (SchiUnz & Schneider,

2000) because of either di ffe rences in approaches or of the limited riverine C data

compilations. Nevertheless, published estimates tend to converge around 0.9 Pg yr·1,

a va lue that has been widely cited as the global total riverine C flu x (Co le et al. 2007;

Battin et a l. 2009; Tranvik et al. 2009; Bauer et al. , 201 3).

Most prev ious estimates considered only so rne of the four C components (disso lved

and particulate, organic and inorganic carbon species; DOC, POC, DlC and PI C,

respectively), thus yie lding a fragmented perspecti ve of riverine C exported to the

oceans and contributing to the uncertain ty of the estimate of global C expo11ed in

di ffe rent fonns from the terrestrial biosphere. Moreover, these globa l estimates are

generally derived from a few to a few dozen large rivers and extrapolated to the rest

of the globe, thereby ass uming an untested representati veness. Considering that there

are about 740 ri vers draining into the oceans that have individual catchments > 10,000

km 2, arriving at a more ace urate estima te of the global riverine C ex po11 to the oceans

will necessarily require a better or complete geographie coverage but will also benefi t

fro m the ident ifi cation of the main dr ivers determining the export of the various

carbon fonns at the planetary scale.

ln this synthesis, we campi led literature C data of fi e ld measurements from 566 ri vers

worldwid e, draining a total of 74% of the global exorheic area (Antarctica is exc luded

from this study) and combined it with land caver and other catchment info rmation to

identity the large sca le determinants of C expo11. The resul ting models were then

applied to the remaining (unsampled) rivers of the wo rld to better constra in the globa l

riverine export of di fferent C species to the coastal oceans. ln addition, we explored

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8 4

the relative roles of natural and human-altered la ndscape features on the delivery of

various forms of carbon and their potential implications in the context of globa l

enviro nmental change.

3.3 Results and discussions

3.3 .1 Drivers of riverine C export

A review of the literature reveals that riverine C export differs wide ly among ·

individual rivers due to the large variat ion in catchment topography, vegetat ion,

geology, climate and hydrology (Hope et al. 1994; Pacifie, 2009; Tank et al., 20 12).

To assess their relative importance at the globa l scale, we developed multivariate

models to test specifica lly the importance of catchment characteristics, such as land

caver (ama lgamated into broad categories: forests, crop lands, wetlands and water, see

Supp lementa l Information S2), soi l organic carbon, mean annual runoff, mean annua l

temperature, carbonate rock outcrops, and catchment s lope and area. ln add ition, we

examined the potential influence ofsome specifie human a lterat ions to the landscapes,

such as the increased water residence time and river segmentat ion induced by the

creation of large dams (S f3) . Average population density was also a candid ate

var iab le and was assumed to be a good proxy of hu man disturbance. T he models were

developed using the e last ic net variab le select ion procedure combined with BIC

va lidat ion, as implemented in JMP Pro vers ion 12 . Variables retained in the final

models each bring independent information useful to the description and prediction of

carbon export.

Without surprise, we found li ke many regional studies (Hope et al. 1994; Pacifie,

2009; Tank et al. , 20 12) that a li forms of carbon export were ultimately constrained

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8 5

by the amount of water flushing through the catchments (as runoff, mm yr-1).

However, beyond this simple hydrologie contro l, each form of carbon exported was

determined by different sets of landscape features . At this globa l sca le, the

non-hydrological var iations in DOC expo1t was best exp lained by edaphic

characterist ics of their catchments, such as the soi l organic carbon (SOC 0-30cm, kg

C m-2) and the extent of wetlands in the catchment. We were also ab le to detect the

signifi cant influence of two important human alterations to the landscape on DOC

export: the extent of croplands and the presence of large reservoirs within the

catchment (expressed as the additiona l residence time of water, AWRT, within the

catchment; DOC vs A WRT: R2=0.11 , n=442, p<O.OOO 1 ). Last ly, we fou nd that large

catchments tended to export less somewhat carbon per unit area, suggesting that a

greater mineralization and loss of DOC during the transit through the hydro logica l

network. At the globa l scale, the relative importance of the various influences ranked

as fo llows: Hydrology > Soil organic carbon > % Wetlands > %Croplands ::::

Catchment size > A WRT.

Variations in DIC export were also best exp lained by a combinat ion of natural and

human-altered features of the landscape. Like DOC, DIC export is tightly contro lled

by the regiona l hydrology (runoff). However, and not surprisingly, it is also strongly

and positively modulated by the presence of carbonate rocks within its catchment.

Moreover, it is negative ly modulated by the fraction of the landscape covered by

water. The mechanisms behind both of these natural drivers are eas il y understood. ln

the former, the chemical weathering of carbonate rocks by soi l respiratory C02 is a

primary source of alkalinity (i.e. HC03- and C032-) , which constitutes the bulk of DlC

in most cases. Rivers draining catchments with a large proportion underlain by

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8 6

carbonate rocks naturally yield more DI C. Similarly, the negati ve influence of water

coverage likely refl ects the converse mechanism: catchments with a lot of water

surface simple have a sma ller surface over which this chemica l weathering of so i!

minerais can occur. In this respect, inland waters largely act as passive transport pipes

fo r these conservative ions (HC0 3- and COJ2-). Beyo nd these natural contra is

however, our analys is revea led the important influence of the extent of agricultural

activ ities on DIC export. Co llective ly, they explain about 57% of the global variation

in DI C export, including a small but significant positive effect of catchment size.

Their re lative importance fo llows Hyd rology > %Croplands ::::: %Carbonates >

% Water > Catchment size.

Always in much smaller amounts than its disso lved co unterpart, the particulate

fraction of organic carbon (POC) was also a significant multivariate function

(R2=0.4 7) combining hydrology and bath na tura! (% Water) and human-altered

(%Croplands) features of the catch ment landscapes. The inverse relationship between

POC export and water coverage can probably be ascribed to POC deposition and

bu rial in the sediments, and/or the greater POC potent ia l degradation because of

longer res idence time in lakes or reservo irs. Conversely, the positive association

between %Cropland and POC export can result fro m the greater eros ional so i! !osses

of croplands relative to fo rests. Table 1 summarizes the globa l models for the

di fferent fo rms of carbon. For PIC export, the limited number of observat ions (n=36

rivers) precluded the development of a spec ifi e mult ivariate mode!. lnstead, we used

the median ratio of 10% between PIC and DI C export observed in the 36 rivers and

applied it to ali the other catchments. This approach, while necessar ily coarser, best

captured the observed variab ility in PIC export.

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8 7

While the importance ofwetlands and so i! o rganic carbo n co ntent to DOC yie ld is not

new to the lite rature ( Dillon and Ma lot 1997, Aitkenhead and McDowe ll 2000), our

g lo bal mode ! can be so lved (fi g. S l4. 1) to assess how DOC expo rt varies in different

regio ns of the world depending on loca l hydro log ie regimes (runoff, mm yr-1) and so i!

organic carbon content (SOC, kg C m-2) when a li other va riables rema ining equal at

the ir mean va lues. Fo r SOC> 10 kg m-2, the re latio nships are nearly linear, suggesting

DOC expot1 can be viewed as a fir st-ord er reactio n w ith the so i! o rganic C reservo ir

e luted w ith di ffe rent e ffi c iencies depending on the annual runoff. As a genera l rule,

DOC expo rt correspo nd s to a rate va rying narrowly between 0.02 and 0.07% of the

SOC per year per mm of runoff. So lv ing the mode! furthe r fo r units of pure

cropland , this removal rate inc reases only s lightly to 0 .03-0.1 % yr-1 per unit runoff

(mm ) whil e a pure wetland unit wo uld remove abo ut 0.3-0.5% yr-1 per unit runoff

(mm), indicating a much greater removal rate ofwetland organic carbo n.

To our know ledge, our study is the fir st to detect a s ig nificant influence of large

reservo irs (>0. 1 Mm 3 capac ity) on DOC yie ld . Altho ugh large ly explo ratory in sco pe,

our ana lys is sugges ts tha t the lo nger water res id ence time induced by the creatio n of

large reservo irs for a va riety of purposes (fl ood co nt ro l, irr igatio n, hydroe lectric ity)

may favor a mo re complete degradatio n of the DOC, bio logica lly or photochemica lly,

into DI C (Lapie rre et a l. , 201 3 ; Spencer et a l. , 2009) during its transit.

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Table 1 Multiple linear regression models predicting DOC, D!C and POC export in g

m·2 yr· 1. Estimates of coefficients and corresponding p values are given for ali

variables offered during the e lastic net selection process. Variab les were included in

the mode] if p<O.OO 1 and the corresponding R2 values are shown.

DOC export DIC expo11 POC export

Parameter

Estima te p Estima te p Estima te p

Annual runoff (mm) 0.744 <0.0001 0.781 <0.0001 0.899 <0.0001

Catchment area (km2) -0 .070 <0.0001 0.102 0.0003

Soit organic carbon (kg m·2) 0.326 <0.0001

o/ocrop lands 0.022 <0.0001 0.055 <0.0001 0.049 <0.0001

o/owetlands 0.202 <0.0001

o/owater body -0.114 <0.0001 -0.155 <0.0001

%carbonates 0.058 <0.0001

lntercept - 1.863 -2 .117 -2.652

R2 0.70 0.57 0.47

n 427 404 339

Notes: ln the models , C export, annual runoff, catchment area and soit organic carbon

are log 10 transformed , o/ocroplands, o/owater surface and %carbonates are square root

transformed, and o/owetlands is 0 .25-power transformed.

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8 9

3.3.2 A néw estima te of g lo ba l riverine C expo1t to the oceans

The deve lo pment of the statistica ll y robust mode ls at this g lo ba l scale has not only

further cla rifi ed the re lative importance of di ffe rent contro ls on riverine C export to

the oceans, they a lso a llow us to estimate carbon expo rt to the oceans from the rest

(26%) of the g lo bal exo re ic area whe re no measurements of carbo n expo rt have been

found in our data co mpilatio n. T hese represent 5098 watersheds of mo re than 3 km 2.

Co mbining the measured va lu es of the 566 rivers with the mode led estimates fo r the

unmeasured catchments yie ld ed a new estimate of the g lo ba l riverin e C ex po rt to the

oceans. Fig ure 1 shows the g lo ba l distributio n of measured (so lid co lors) and

modeled (hatched) DIC and DOC expo rts (F ig ure 1 a and 1 b, respective ly) .

a Global Distribution of DIC export

,/ :::-. /

/

"'"·- -/·--- j-'§!l-~ ) // /

1 1 1 1

1 1

,.,..,. . ~ ! 1 ~JI'---.f...---l

\

""a'~~ /

~/ / / , ,/"".,.'

,.,,·"

( / .-...-· -~ . . ( , ~# ,-· ,--- r · )l"X'i! ëC":: •''! 97:-::Jt: ,~ -:~'! ,~.~:::-: '! 11:)~

DIC (glm2/yr)

c· .. J oo - so c:::::J oo - 123 - 123-24 5-24 5-452- 452 . 94 9

DI C according to the model (gl m2/yr)

CJ oo-27 CJ 27 - 7.4 [ZJ 7.4-1A9 ~ 149-319 ~319-831

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9 0

b Global Distribution of DIC export

DO C (g/m2Jyr}

D oo-16 ~ 1.6-33 .3.3-58.5.8- 96 .9 .6 - 38 ~ DOC accordlng to the m odel(g/m2Jyr}

Oo.o-2s o 2.s-7.si2Zà7.s-16.9 ~ 169-31 0 RJ31.o -6o .s

Projection lo >dmuthal Datum . World Geodetic System 1984 (WGS 84}

Fig ure 1 Maps of G lo ba l Distributions of D1 C and DOC Exports

Our assessment yie lded a tota l g lo ba l C export of 0 .68±0.05 Pg yr-1, of w hich DOC,

POC, DI C and PI C exports represent 0.18±0.01 , 0.07±0.01 , 0 .39±0.02 and 0.04±0.01

Pg yr-1, respectively. Our new estimate thus co nstitutes a significant downward

revision of the wide ly used glo ba l C expo rt va lue of 0.9 Pg yr-1• For compariso n,

Table 2 compiles our new estimates w ith those previously published and it hig hli g hts

that the largest diffe rences are in the POC and PI C export. Our POC expo rt estimate

of 0.07 Pg yr-1 is much lower than the va lu e of around 0. 18 Pg yr-1 reported in

previous studies (Meybeck, 1982; Galy et a l. , 20 15 ; Ludw id et a l. , 1996) altho ugh

consistent with the va lu e of Garre! et a l. ( 1973). We suggest that the glo ba l POC

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9 1

export may have been overestimated in the past primarily because of limited data.

The largest data co mpilatio n of g lo ba l POC expo rt previo us ly publi shed was based on

only 70 ri ve rs, in compar iso n to the 359 watersheds used in th is study, covering a

tota l of 64% of the g lo ba l exo re ic area. Even our own estimate may be so mewhat

inflated. ln o ur data set, 63 POC ex port va lu es from catchments located in Sweden

and Canada were based on the assumptio n that POC corresponded to 5% of TOC

tho ugh the studies indicated tha t POC is genera lly less tha n 5% of TOC in

concentratio n (C la ir et a l. , 1994 ; C la ir et a l. , 20 13; Laudo n et a l. , 2004). Furthermore,

it is often reported that small mo unta ino us ri vers (bas in a rea: < 10,000 km2 and

headwater e levation: 1000 to 3000 m) have an extreme ly high POC fl ux (Mill iman &

Syvitski , 1992; Lyo ns et a l. , 2002; Ko mada et al. , 2004), espec ia lly in the so uthwest

Pac ifi e. Our study included 23 4 sma ll ri vers, of which 54 ri ve rs are in the southwest

Pac ifi e, only showed a very weak re latio nship between POC expott and e levation

(R2=0.03, n=254, p=0.007), and this va riable was not reta ined in the POC mode!

using the elast ic net variable se lection procedure.

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9 2

Table 2 The compariso ns of our est ima tes of g loba l riverine carbon export to the

oceans with those in previous studies

Rivers DOC DIC POC PIC TOC TIC TC

used

This study 0 .18 0.39 0 .07 0 .04 0.25 0 .43 0 .68 566

Mcybock, 1982 0 .22 0.38 0 . 18 0 . 17 0.4 0 .55 0 .95 27

Mcybock, 1993 0 .20 0 .38 0 . 18 0 . 17 0 .3 8 0 .55 0 .93 60

Mackenzie et al .• 1996 0 .61 0 .72 1.33

Aitkeohead & McDowe11, 2000 0 .36 164

Cuuwet. 2002 0 .25

Dai el al., 2012 0 . 17 11 8

Degens et al.. 1 9 9 1 0 .33

Duce and Duursma. 1977 0 .13

Galy el al .• 2015 0 .20 70

Garre) et al. . 1 973 0 . 13 0 .07 0 .20

Haoda, 1977 0 .30

Harrison et al., 2005 0 . 17 68

Kcmpe, 1979 0 .44 0 .19

Kcmpe, 1985 0 .2 8

Ludwig e l al ., 1996a 0 .2 1 0 .32 0.19 0.40

Ludwig e l al ., 1996b 0 .21 0 . 17 0 .3 8 48

Manloura & Woodward, 1983 0 .78

Michaelis et al , 1986 0 .53

Rciners, 1 973 0 .2 - 1

Richcy el al , 1980 1.00

S chJesinger & M elack, 1981 0 .39 12

Schlunz & Schneider, 2000 0.43 18

Smith & Holhbaugh, 1993 0 .2 0

Spitzy & Iuekkol, 1 99 1 0 .50

Stewart e l al. , 1978 0 .52

Williams, 1971 0 .03

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9 3

For PIC export, significant uncertainty remains given the Iimited data we were able to

compile (only 36 rivers) and their geographie coverage (about 16% of the global

exoreic area). However, we argue that our estimate of the global PIC export of 0.04

Pg yr·1 is nevertheless reasonably constrained from the ratio DlC:TIC observed 111

rivers, (median: 0 .95 ; average: 0.96) suggesting that, on average, PIC represent only

about 5% of the TIC export, lower than the fraction we used to estimate it. Our

resulting estimate can therefore be considered an upper limit but is much lower than

the previous reported value of0.17 Pg yr· 1 (Meybeck, 1982), which was itself derived

by assuming that PIC is a constant I% fraction of total suspended matter (Meybeck,

1982; Huang et al. , 20 12).

3.3.3 Past and future effects ofanthropogenic disturbances on global river C export

Human activity is widely regarded as an important driver of global environmental

change, especially in the past five decades, during which the rapid development of

world 's economies is characterized by the expansions of agriculture, industry and

urbanization . Our models ali include the fraction of the catch ment used as croplands

as a major driver of C export. They can therefore be used to estimate how much of

the current global export may be attributed to agriculture. Recalculating the export of

ali 5664 catchments wh ile imposing 0% cropland results in a global estimate of 0.40

Pg yr·1, suggesting that the cur-rent extent of agriculture worldwide accounts for 0.28

Pg yr· 1 (about 40%) of the present global carbon export to the oceans . According to

our models , the bulk (nearly two thirds) of this carbon associated with agriculture is

inorganic, consistent with the results from the Mississippi basin where the extent of

croplands within various sub-basins was directly related to alkalinity export

(Raymond and Cole 2003) . Projections from the FAO of agricultural requirements to

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9 4

feed the world's growing populatio n (Alexandratos and Bruinsma 20 12) suggest that

cropland s will expand at a rate of abo ut 0.1 % per year over the 2050 ho rizon. T his

va lue is the net result offo recasted reductio n in croplands in develo ped co untries (at a

rate of -0 .14% per year) but a much faster increase in the deve lo ping wo rld (+0.24%

per year), particula rly in Latin America (+0.49% per year). Depending on the carbon

export profil e of agricultura l land abandoned because of severe land degradation and

a lso on the geographical location of this future expansion, our models suggest that

tota l carbon export co uld increase by up to 15 % by 2050. Much greate r changes can

be expected at loca l scales. Taking the Changj iang catchment as an 'example,

if %cropland increases fi·om the cur-rent 34% to 80%, and assumin g other variables

remain co nstant, the tota l C exported to the coasta l ocean by th is river w ill inc rease

by 45%. As a genera l rule, it is the DlC and POC exports that are the most sens itive

carbon fo rms to a convers ion to cropland . T his therefo re implies that g lo bal cropland

expansio n w ill modify riverine C expo rt not only in its magnitude but a lso in its

co mpos itio n. The potentia l co nsequences of such co mpos it iona l changes on the

oceanic carbo n processing are currently unknown but may a lter sig nificantly the

fun ctioning of the coasta l eco systems rece iv ing these carbo n loads.

S imila rly we can co nsid er the potentia l effects of chang ing c limatic regimes on

carbon expo rt. Using the scenario proposed by Cao et a l. (20 1 0) that a doubling in

atmospheric C0 2 would results in a 15% increase in runoff, o ur mode l would suggest

a commensurate increase of about 12% in tota l carbon expo rt. Wh ile this conc lusion

depend s strong ly on the geographie distribution of the predicted increases in runoff,

such predictio ns co uld nevertheless be integrated to our modeling approach to y ie ld

better predictions of c limate induced effects on C expo rt . Regardless of the exact

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9 5

magnitude of chan ges, our study highli ghted that anthropogenic disturbances, past

and future, may have become the most impo rtant factors modify ing the magnitude

and compos ition of ri verin e C export from terrestria l to aquatic ecosys tems,

particula rly in the co ntext of g lo ba l env iro nmenta l change.

3.4 M ETHODS

ln this study, a li the data on C co ncentratio ns and expe rts to the oceans are co llected

fro m jo urnal papers, technica l reports and books published after 2000 . T he co mplete

data co mpila tio n is provid ed in SI-Table 5.1. A li the riverine C concentratio ns and

expe rts were fi e ld observed for at least 1 year w ith a frequency of at !east 3-4 times .

Some big rivers were measured severa! times fo r di ffe rent studies. ln this case, we

use the average of the va lues fo r that river. Ri verine C expo rt, when not provid ed

directly, was calculated as C export (g m-2 yr-1) == C concentration (mg L-1) X

annual discharge (m3)/catchment area (m2). A il the data on river discharge and

catchment a rea are based on Mey beek and Ra gu ( 1996) or the specifie references,

while river mo uth coordinates are corrected by Google .Maps. The catchment

de lineatio n for each river bas in was o bta in ed fro m the Hydro Bas in s product

(http ://www.hydrosheds .org/page/hydro bas ins) matching the river mo uth locations.

The basin po lygons were then used to extract the catchment characteri stics using the

g lo ba l raster fil es fo r annua l runoff(mm yr-1) , catchment s lo pe and area.

The same process was then applied to a li other exo rhe ic catchments dra inin g into the

oceans thereby a llowing the applica tio n of the multiva riate mode ls to a li catchments

that had not been sampled in our database. A li stati stical ana lyses were perfo rmed in

JMP Pro 12. Variable selectio n for the multiva riate regress ion models was carried out

usin g the e last ic net a lgo rithm co upled with the Bayes ia n Information C rite rio n (BI C)

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9 6

validation procedure, as implemented in JMP Pro 12. Although this procedure does

not necessarily rely on probability levels for variable retention, ali the variables

retained in the models were statistical ly significant (p<O.OO 1 in ali cases) .

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9 7

CHAPTERlY

CONCLUSIONS

4.1 Main contributions:

The co mmon theme of this thes is is to explore the natura l and anthro pogenic dri vers

of carbo n expo tt fro m watersheds to aquatic ecosystems, as we il as the re lative

impo rtance of indiv idua l drivers in co ntro l ling the magnitude and co mpos itio n of the

tota l carbo n expo rted in di fferent fo rms to the rece iving waters, through

systematica lly ana lyz in g the carbo n database of 127 rivers and streams in Q uebec that

had been sampled for 1-3 years by the Aquatic group of UQAM, and then shifts the

study from a catchment and reg iona l sca le to the g lo ba l sca le fo r a further ex plo ration

of them based on the g lo ba l carbo n dataset that were poo led and integrated fro m the

prev io us ly-scattered knowledge and data in the literature regarding riverine carbon

export to the oceans. On the bas is of the co mbinat ion of the large amo unts of regio nal

and g lo ba l C data, this thes is has (re)assessed and predicted the magnitude,

co mpositio n and trend of the tota l carbon expotted fro m watersheds to the receiving

waters at di ffe rent spatia l and tempora l scales, and deve lo ped a better understanding

of the ro te and importance of riverine carbon export in regio na l and glo ba l carbon

cyc les, quant itative ly and qua litat ive ly.

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Specifically, the following key points emerge from the different chapters:

4.1.1 T he study on the re lative influence of topography and land cover on inorganic

and organic carbo n export from temperate catchments has s imultaneo usly explo red

DOC and DI C expo rt to river systems, based on a la rge amo unt of regiona l

observatio ns that max imize the spat ia l coverage and env iro nmenta l gradients and that

captures some of the seaso nal variabili ty in river d ischarge and C concentratio n. This

study has c lar ified the ro le and importance of d ifferent ind iv id ua l drivers of DOC and

DI C exports fro m the watersheds, in particular, that topography is more important

than land cover in expla ining the va riance in D!C expo rt, whereas land cover is much

more impo rtant than topography in terrns of DOC export. T his furthe r hig hli ghts

hum an activ it ies that lead to land use/cover change, such as urbanizat io n,

defo restat ion and wet land c learance, can potentia ll y modify the magnitude and

co mposit io n of the tota l C expo rt from watersheds. The explorat ion of the

inter-annual va riation of DI C and DOC expo rt fro m these temperate catchments

demonstrates that the flu ctuations in precipitatio n and temperature due to climate

change or g lobal warming wo uld to some extent mod ify TC e port and DIC/DOC

ratio, because of the di ffe rent responses of DOC and DI C exports to prec ipitation and

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9 9

temperature , w hich could prov id e sorne bases fo r the predictio n of C export and

wate r qua lity changes triggered by regiona l or glo ba l c limate shi fts.

4.1.2 As a fo llow up of the patterns observed in the fi rst chapter and m order to

further understand environmenta l drivers of carbon export to rive r systems m the

bo real bio me, the seco nd chapter foc used on the expo rt of disso lved (DOC and DI C),

and gaseous (C0 2 and C H4), so as to quantify the spatia l and tempo ra l va riations in

the magnitude and co mpos itio n of the to ta l carbo n expo rted fro m a w ide range of

bo rea l watersheds.Total ex po rt peaked in spr ing and var ied w ide ly in its magnitude

and co mpos ition seaso na lly, but was dominated by DOC on an annual bas is . T he

integrated aquat ic C02 emiss io ns were a lso a majo r co nt ributor to the tota l C expo rt

fro m these watersheds, and replaced DOC as the dominant C spec ies in watersheds

with hig h water dens ities.

4.1.3 On the bas is of Chapters 1 and 2 that addressed the natura l and anthro pogenic

drivers of carbon expo rt fro m e ither temperate and borea l watersheds to river systems

and the compositio n and magnitude of tota l carbo n exported in di ffe rent fo rms at a

regio na l scale, the third chapter shi fts from a catchme nt and regional sca le to the

g lo be to explore the natura l and anthro pogenic drivers of g lo ba l riverine carbo n

export to the oceans, and to reassess the g lo ba l ri verine carbo n export rate based on a

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l 0 0

synthesis and meta-analysis of the most extensive river carbon dataset assembled to

date. We produce a new an nuai globa l riverine carbon export rate of 0.68 Pg C yr·1,

24% less than the wide ly-accepted va lu e of0.9 Pg C yr· 1 (e.g. Battin et al. , 2009; Co le

et al. , 2007; Tranvik et al. , 2009). Our revised estimates of g loba l DOC and DlC

export, O. 1 8 and 0.39 Pg C yr· 1, respectively, are sim i !ar to the wide ly-used va lu es of

around 0.2 (e.g. Meybeck, 1982&1993; Ludwig et a l, 1996; Harrison et al. , 2005 ; Dai

et al. , 20 12) and 0 .38 (Meybeck, 1982& 1993) Pg C yr·1, respectively. However, we

narrowed the uncertainties of the previous est imations of POC and PIC exports and

obtained the an nuai global riverine POC and PIC exports of 0.07 and 0.04 Pg C yr· 1,

respectively, using the most extensive POC data coverage to date. T his g loba l study

a lso has further clarified the relative importance of different drivers of riverine carbon

export from terrestrial ecosystems at a g loba l scale. Our multiple regression analyses

have further shown that human activities, especially crop land expansion, may become

a main regulator of the magnitude and compos ition of total riverine carbon export to

the oceans in the future. Our models predict that a 15% increase in runoff, predicted

under a doubling of atmospheric C02 (Cao et al. , 201 0) cou ld lead to an increase of

12% in total carbon export to the oceans, whereas increases in the cropland area w ill

result in proportionately larger shifts in the magnitude and composition of total C

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l 0 1

export. Fo r example , an increase in cro pland coverage in the Changj iang catchment

fro m the current 34% to 80% of the tota l surface would result in an inc rease in the

export of DOC, Dl C and POC by 17%, 48% and 42%, respect ive ly, w ith a resulting

inc rease in tota l C export fro m this catchment of 45%.

4.2 Main innovations

4.2.1 Whereas prev ious studies on watersheds to rivers have focused mainly on

spec ifi e C species and on individua l drive rs of carbo n expo rt, this thesis has

co mparative ly explored the re lative influence of the ensembl e of topography and land

co ver variables on tota l riverine carbon expo rt and its ma in co mpo ne nts, scaling fro m

individua l watersheds to the g lo be. At the reg io na l scale, an examinatio n of va riance

partitio ning in our mode ls revea led that topography is s lig htly mo re impo rtant than

land-cover in explaining the va riance in DIC export ( 19% vs 15%), whereas

land-cover is much mo re important tha n topography in determining DOC expo rt

(44% vs 18%). This di ffe rence between topography and land cover in co ntro lling

DOC and DIC expo rts to rivers wo uld lead to the changes in magnitude and/o r

compositio n of tota l carbo n expo rt beca use of the di ffe rence in anthro pogenic impacts

on the two categories of driv ing fo rces. Acco rding ly, the trend s in tota l carbo n and its

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components exported from watersheds could be predicted based on the degree of

anthropogen ic disturbances.

4.2.2 This thesis has sketched an integrated view of total carbon export from boreal

watersheds to river systems through exploring the spat ia l and tempora l var iations in

total carbon and its components (DOC, DIC, POC, C02 and CH4) exported from the

boreal landscape. This st udy not on ly explored the intra-annual variation in the

magnitude and composit io n of total carbon exported from the boreal watersheds also

compared the tota l carbon between the neighboring sub-reg ions that are characterized

by different landscapes (James Bay is dominated by peatlands, while in Abitibi

beaver damming is ubiquitous a lo ng streams), and thus developed a better

understand ing of the spat ial and temporal variations in carbon export to ri ver systems

at a regional scale.

4.2.3 This thesis reports a re-estimate of globa l riverine carbon expo rt to the oceans

based on the most extensive global C dataset to date. Thi s g lobal study yie ld s a new

global riverine carbon export rate of 0.68±0.05 Pg C yr·1 to the oceans, 24% lower

than the widely-accepted va lu e of 0.9 yr· 1 through narrowing the uncertainties in the

previous estimat ions. Through a series of multiple linear regression ana lyses, the

relative importance of different natural and anthropogenic drivers of riverine carbon

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l 0 3

expo rted in di ffe rent fonns fro m land to sea has been fu rthe r c larified . Especia ll y, we

fo und that %croplands in the watershed like runoff (though runoff can expla in

40-58% variance in riveerine carbon export) is an impo rtant varia ble intluenc ing a li

the different carbon forms (DOC, DIC and POC). ln particular, the current cropland

coverage of the g lo ba l terrestria l area is aro und Il %, hav ing huge potentia l of

cropland expans ion, white the g lo ba l runoff has inc reased less than 4% since 1880

(Labat et a l. , 2004; Da i et a l. , 2009), hig hli g hting huma n activ ities, espec ia lly

cropland expans io n around the world , wo uld become a ve ry important va riable

influencing the g loba l riverine carbon ex po rt to the oceans in the future.

4.3 Important implications

The findin g that to pography is s lig htly more important than land-caver in expla ining

the va riance in DI C export (1 9% vs 15%), wh ereas land-ca ver is much mo re

impo rtant than topography in detennining DOC expo1t (44% vs 18%) fro m the

watershed to rivers impli es that how to effective ly and ratio na lly use and manage land

reso urces wo uld be very important to direct the trend of riverine carbon export fi·om

watersheds to aquatic ecosystems scaling from a catchme nt to the glo be . T he average

of tota l carbon exported fro m the boreal watersheds to the rive r systems ( 15.6 gC m·2

yr·1) is in the a rder of 5 to 1 1% of the reg iona l NPP, but of the sa me magnitude of the

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Net Ecosystem Production that has been estimated for this type of landscape

highlighting the need to integrate carbon export from watersheds to river systems into

terrestrial carbon budgets in order to improve our understanding of landscape C

sources and sinks within the boreal biome. Further, our re-ana lys is of the g lobal

export dataset resulted in a new estimate of annua l g loba l riverine carbon export to

the oceans of 0.68 Pg yr· 1, which is signifi cantly lower than current accepted figure of

0.9 Pg yr·1, of which DOC, POC, DIC and PIC exports are 0 . 18, 0.07, 0.39 and 0.04

Pg yr·1, respectively, imply that it is necessary to reassess the role and importance of

carbon export from watershed to river systems in terrestrial and g lobal carbo n. The

thesis has a lso contributed to narrowing down the uncertainties in the estimations of

other C reservoirs, which is essentia l to better constrain the regiona l and global

carbon budget. T he finding that o/ocropland in the watershed is c losely related to the

riverine carbon export through the globa l ana lysis of riverine carbon export to the

oceans further implies that human activities , espec ia lly crop land expansio n, may be

replacing the natural driving forces as the primary determinants of the magnitude and

composition of both current and future transfers of carbon from land, through the

hydro logie network, and ultimate ly reaching to sea, further indicating the importance

of land use and management in co ntro lling riverine carbon export from terrestrial to

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1 0 5

aquat ic ecosystems m the co ntext of c limate and human-induced envi ronmenta l

changes.

N.B. References c ited in this chapter are presented at the end of the thes is .

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CD

0

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Appendi x A

Supplementary Information For

A Global Analysis of Riverine Carbon Export to the Oceans

Sl -1. List and source of landscape characteristics extracted for each catchment draining into the

oceans

Data Type Citation

Fetke, Balazs M . et al (2000). Global Composite Runoff

Fields Based on

Observed River Discharge and Simulated Water

Runoff Raster Balances . Complex

Systems Research Center, University of New Hampshire .

UNH-GRDC

Composite Runoff Fields vl .O. Available at

http:/ /www.grdc.sr.u nh.edu/.

European Space Agency (ESA)- Climate Change

Initiative (CCl) . The Land Caver CCl Climate Research

Land Caver 2010 Raster Data Package (CRDP). Land Caver Maps- vl.S

(2008-2012 epoch). Available at

http:/ /ma ps . elie. ucl .ac. be/CCI/viewer/down load. php

Harmonized World Soil Database Vl.2 Organic

Soil organic C (Top Raster

Carbon density.

0-30 cm) http :/ /www.fao. org/ so ils-porta 1/soi 1-s u rvey /soi 1-m a ps-a

nd-data bases/ha rmo ni zed-world-soil-d a ta base-v12/ en/

Williams & Ford . World map of carbonate rock

Carbonates Polygon (Shapefile) outcrops v3 .0. SGGES, University of Auckland, New

Zealand . Availab le at

http:/ /web .env.auckland .ac. nz/ou r _research/ka rst/

51 -2. Land Caver categories amalgamation

The land caver classes provided by the ESA CCl land caver product (2010) contains 37 distinct

categories. For the purposes of our carbon export madel development, these were simplified to

only 5 classes using the following amalgamation :

% Forests = Sum of {

Tree caver, broadleaved, deciduous, closed to open {>15%)

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1 3 6

Tree cover, broadleaved, deciduous, open (15-40%)

Tree caver, broadleaved, evergreen, closed to open(> 15%)

Tree caver, flooded, fresh or brakish water

Tree caver, flooded, saline water

Tree caver, mixed leaf type (broadleaved and needleleaved)

Tree caver, needleleaved, deciduous, closed (>40%)

Tree caver, needleleaved, deciduous, closed to open (>15%)

Tree caver, needleleaved, deciduous, open (15-40%)

Tree caver, needleleaved, evergreen, closed (>40%)

Tree caver, needleleaved, evergreen, closed to open (> 15%)

Tree caver, needleleaved, evergreen, open (15-40%)

Tree or shrub caver)

%Croplands = Sum of (

Cropland, irrigated or post-flooding

Cropland, rainfed

Mosaic cropland (>50%) 1 natural vegetation (tree, shrub, herbaceous caver) (<50%))

%Wetlands = Sum of (

Tree caver, flooded, fresh or brakish water

Tree caver, flooded, saline water

Shrub or herbaceous caver, flooded, fresh/saline/brakish water)

%Shrublands= Sum of (

Mosaic herbaceous cover (>50%) / tree and shrub (<50%)" ),

Mosaic natural vegetation (tree, shrub, herbaceous caver) (>50%)/

Mosaic tree and shrub (>50%)/ herbaceous caver (<50%)

Shrub or herbaceous caver, flooded, fresh/saline/brakish water"

Shrubland,

Shrubland deciduous,

Shrubland evergreen,

Sparse herbaceous caver (<15%)

Sparse shrub (<15%)

Sparse vegetation (tree, shrub, herbaceous caver) (<15%)

% Water bodies = Water bodies

cropland (<50%)

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1 3 7

Sl-3. Additional Water Residence Ti me (AWRTl from large freshwater reservoirs

We developed the metric AWRT to act as a proxy for the a mount of additional ti me water stays

within a catch ment because of the added volumes contained in large freshwater reservoirs. This

was accomplished by summing the reservoir water volume capacities of ali reservoirs within

each catchment and dividing by the average annual runoff over the catchment. We used the

geolocated GRanD (G lobal Reservoir and Dam) database available at

http :/ /www.gwsp.org/products/grand-database.html .

Sl-4. Average DOC export= (Runoff. SOC)

The generalized DOC export from a catchment can derived by solving the multivariate models

presented in Table 1 of the main text as a function of only Runoff and SOC and replacing the

other variables with their mean values in our data set. The resulting equation as illustrated as a

family of curves representing the relationship between DOC export and Soil Organic Carbon for

different runoff values. For SOC>10 kg m·2 , the relationships are approximately linear (Fig. Sl-4.1)

suggesting that DOC yield can be roughly considered as a first-order reaction with the SOC

reservoir.

Figure S4.1 . Relationship between DOC export and Soil Organic Carbon (kg C m·2) for different

levels of annual runoff (mm yr·1 )

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~----------------------------------

l 3 8

20

u tl.O

t10 0 0.. >< (1)

u 8 5

0

0 10 20 30 Sail organic carbon (kg C m-2)

Runoff

-1 200 mm yr _

40 50

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l 3 9

SI -S. Complete Data set used in this study.

Ri vers A la Baleine Abitibi Abra Adige Adour A~o

A 1211 0 Agu san Aht ava Akheloos Alafia Alazeya Albany Aliakmon Alsea Alsek Altamaha Amazon Ameca Amgerman Amguema A mur Ana bar Anadry Anderson AndroscollS!in Ankobra A palachicola Appomattox Approuague Argens A maud A mo Ashbun on

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M azaruni Medjerda Medway Mekong Men jiang M erikarv ia Mennent au Merrimack Meuse Mezen Minho Miss iss ipp i Mitchell Mobile MoMmi Moisie Moose Morrumsan Motagua Motala Moulouya Murray Musa Musi Mus quaro Myakka n Dvina Nadym Nal!avali Nan du Napa Narmada Niirpionjoki Narraguagus Narva Nas tapoca Natas hquan Navidad Neches N_egro (A rg) Nehalem Nelson Neman us Nes tos Nethravat i Neuse Neva Ney y ar Niger Ni le Nissan Nottaway Nueces Nykopingsan Nyong Ob Ochlo.ckonee Odra

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1 4 1

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01?f!eChee Ogooue Olenek Olfusa Olomane Omoloy One!?f! Orange Orb Ore Orinoco Oueme Oulujoki Oyapok Paimionjoki Pal ar Pamunkey Panuco Paraiba Do Parana Pamaiba Pars?Ta Pascagoula Patuxent Pawcatuck Pawtuxet Peace Pearl Pechora Pee Dee Peel Peinan Penn er Penobscot Penz hina Perdido Perhonjoki Periyar Petit Baleine .Petit Pinios Pitalven Po Pongoma Ponnaiya Pontax Porvoonjoki Potomac Povuntnituk Pregolya Pro go Pur Purari Puyallup Pyasina Py hlljoki Queets Quinault Quoi ch

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0 20 11

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..,

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Shuya Siak Si ikajoki Simo joki Sirppujoki Si us law Skagjt Skeena Skelleftalv Skieselva Skokomish Slup ia Smith Snohomish Solo Spercheios St Marv's St. Marks St. Marvs Steinhatchee Stikine Strymon Su armee Subamarekha Suriname Susitna Susquehanna Swan-Avon Swam a Ta Chia Chi Taku Tan Shui Tana (Ken) Tap ti Tar Taunton TaviJ!!HlllO Taynwra Taz Teio Tenry u Teshio Tet Tevere Th ames T helon T hiorsa Tocantins Tokachi Tone Tonnerre Tomionioki T rent T rinit e Trinit y Tsengwen T ugur

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tets & St riegl, 20 12 Stets & Strieg), 20 12 Clair et al. , 20 13 Skoulikidis et al. , 1998: UNEP, 2003 Mey beek and Ragu, 1996 Sam1a et al. , 20 12 Mey beek and Ragu, 1996 Mey beek and Ragu, 1996 Stets & Striegl, 20 12; Hoss ler and Bauer, 20 13 Petrone, 20 1 0 T ri p t et al. , 20 13 Pan ,20 12 Ncal, 2007 Pan , 20 12 Bouillon et al. , 2007 Sham1a&Subramanian , 2008; Sann a et al. , 20 12 Davis el al.. 1978: Baies el al.. 2000: Lin et al .. Nixon et al. , 1995 UNEP. 2003 Dolman et al.. 20 12 Cochran et al .. 1996: Gordeev et al. . 1996 Meybeckand Ragu, 1996 Bosser et al. . 20 12 Naganuma&Seki, 1987: Alan1 et al. . 20 12 UNEP. 2003 Mey beek and Ragu, 1996 Arauio et al. , 20 14; Neal et al., 1998 Chouinard and M ilbum, 1995 Mey beek and Ragu, 1996 Lewi and Saunders, 1982. Alam et al. , 20 12; Büsser et al. , 20 12 Büsser et al. , 20 12 1-ludon et al. , 1996 Kulinski and Pempkowiak, 20 Il Tipp inget al. , 1997 1-ludon et al. , 1996 Warnken & Santschi 2004 : Stels & Strieg]_ Pan,2012 Dolman et al. . 20 12

Tunguda Tusket Tyne Uda Ume­Umpqua UruJ!)J ry Uskelan joki Usumacinta Van1sadhara Van taan ioki Var Vell ar Venta Ven tura Vizhas Volta Waiau Waikato Waimakariri Waipaoa Waitaki Waitara Waitotara Wanquan Warnow Watshishou Wenj iao Weser Whangaehu Whanganui Wieorza Winisk Wisla With.lacooclte Wolf Wu Chi Xiaolingl1e Yalu Y ana Yarra Yellow (US) Venisey Y es il irmak Yodo Yukon Zaire Zambezi Z hu jiang

Pokrovsky & Schott, 2002 Clair et al., 1994 Baker & Spencer, 2004 Dolman et al. , 20 12 Kulinski and Pempkowiak, 20 11 Stets & Striegl , 2012 M ey beek and Ragu, 1996

1 4 3

Kulinski and Pempkowiak, 201 1: Raike et al. , 2012 Mey beek and Ragu, 1996 Sarma et al.. 20 12 Raike et al. , 20 12 UNEP. 2003 Sarma et al.. 20 12 K ?avi?? et al., 20 1 1 Stets & Striegl, 20 12 Lobbes et al. .. 2000 Andah et al.. 2003 : Arauio et al. , 20 14 Carey et al .. 2005 Mey beek and Ragu, 1996 Carey et al. . 2005 Gomez et al. , 2003 Mey beek and Ragu, 1996 Carey et al. . 2005 Carey et al. . 2005 Chen&Chen, 1992; T ian et al. , 20 10 F reese et al. . 2007 !-ludon et al. , 1996 T ian et al. , 20 10 M eybeck and Ragu, 1996 Carey et al. , 2005 Carey et al.. 2005 Niemirycz et al. , 2006 Gransko et al. , 2007 Niemirvcz ct al .. 2006: Kulinski and Pemokow iak. Stets & St riegl, 20 12 Stets & StriegJ, 20 12 Pan,20 12 Xia & Z hang, 20 Il Wang et al. , 1998: Wu & Zh ang, 200 1 Gordeev et al .. 1996: Dittmar & K..attner. 2003: Sant os et al. . 20 12 Stets & St riegl. 20 12 Gordeev et al.. L996: Diltmar & K.attner. 2003: Jin et al .. 20 13: Duran and Suicmez.2007: Odemis and Mey beek and Ragu, 1996 St rieg) et al .. 2007 Wang et al., 20 13: Arauio et al., 2014 Kunz, 20 11 Yao et al. , 2007

Page 164: Regional and global patterns in carbon export from land to water
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Page 167: Regional and global patterns in carbon export from land to water

Cag

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Page 168: Regional and global patterns in carbon export from land to water

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Page 169: Regional and global patterns in carbon export from land to water

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Page 174: Regional and global patterns in carbon export from land to water

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Page 176: Regional and global patterns in carbon export from land to water

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Page 181: Regional and global patterns in carbon export from land to water

1 6 1

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