diurnal variations in carbon isotope composition of dissolved inorganic carbon (dic) in a freshwater...

Mariusz Orion-Jêdrysek1, Marta Kurasiewicz1, Adriana Trojanowska1,Dominika Lewicka1, Agata Omilanowska1, Adam Ka³u¿ny1,

Katarzyna Izydorczyk2, Wojciech Drzewicki1, Maciej Zalewski2,3

AbstractDissolved Inorganic Carbon (DIC) is a common inorganic component of freshwatersand the main source of carbon for primary producers. The pilot studies on dailychanges of the carbon stable isotopic ratio of dissolved inorganic carbon ( 13C(DIC))were conducted during the clear water phase in shallow, eutrophic, freshwater damreservoir. Both, DIC concentration and 13C(DIC) demonstrated pattern of dailychanges with higher variability in the lacustrine part of the reservoir than in riverinezone following higher biological activity. Results of the first studies on diurnal varia-tions in the 13C(DIC), suggest that 1/ sediments can be an important sink for dis-solved inorganic carbon due to microbial reduction of CO2 from the water column tomethane and other organic compounds, 2/ large amplitude of diurnal variations ofDIC concentration and 13C(DIC) values are due to daily fluctuations of phytoplank-ton photosynthetic activity - typical for eutrophic system, 3/ Many carbon cyclingmodels based on seasonal observations of DIC may be biased with large error resultedfrom DIC diurnal variations, which amplitude appears to be comparable to those com-monly interpreted as seasonal and spatial fluctuations.Key words: dissolved inorganic carbon, carbon stable isotopes, daily changes, damreservoir.

1. Introduction

Carbon is a crucial element in biologicalprocesses in freshwaters. Dissolved inorganiccarbon (DIC is composed of CO2, HCO3

- andCO3

2-), among other freshwater ionic compo-

nents, is usually the dominant one. Carbon isotopecomposition of DIC ( 13C(DIC)) depends onboth, carbon origin and its biogeochemical path-ways. DIC in lakes is derived from varioussources, including atmospheric CO2, oxidizedcarbon compounds from microbial mineralization

Vol. 6No 1-4, 53-592006

Diurnal variations in carbon isotope composition of dissolved inorganic carbon (DIC)

in a freshwater dam reservoir

Ecohydrologyfor Implementationof the European Water FrameworkDirective

1 Laboratory of Isotope Geology and Geoecology, Department of Applied Geology,University of Wroc³aw, Cybulskiego 30, 50-205 Wroc³aw, Poland,

e-mail: [email protected] International Centre for Ecology Polish Academy of Sciences,

3, Tylna Str., £ódŸ, Poland3 Department of Applied Ecology, University of £ódŸ, 12/16, Banacha Str., £ódŸ, Poland

M. Orion-Jêdrysek et al.

of organic matter and runoff from the watershed aswell as groundwater inflow, where in each stage ofcarbon cycling kinetic fractionation of carbon iso-topes occurs (Herczeg, Fairbanks 1987; Coffin etal. 1994; Zhang et al. 1995; Aucour et al. 1999;Myrbo, Shapley 2006). Preference in uptake of12C by primary producers leads to increase ofstable isotopic values of DIC ( 13C(DIC)) in theeuphotic zone (Quay et al. 1986; Goericke et al.1994; Myrbo, Shapley 2006). Decomposition ofsinking organic matter decreases 13C(DIC)values on greater depths of lakes (Ogrinc et al.2002; Myrbo, Shapley 2006). These two processesare recognized as major pathways and drivingforces of stable isotopic composition of DIC inlake waters (Myrbo, Shapley 2006). Carbon stableisotopic data appear to be especially rich source ofinformation on derivation of carbon compounds,processes of their transformations and migrationswithin abiotic and biotic pool and exchangebetween of them as well as can be a basis for iso-tope mass balance of freshwater ecosystems. DICconcentrations, even supported by standard meas-urements of temperature, pH, Eh, are not able togive such complex information on direction andtrends of changes of environmental conditions, bythemselves.

Recognition of DIC origin and its isotopeevolution in lakes and reservoirs, is surprisinglypoor and still arise many questions, from whichshort term (hours) dynamics is of special interestas a key to understand processes such as eutrophi-cation, emission of greenhouse gassess from natu-ral sources, organic matter burial.

The 13C value in lacustrine DIC is con-trolled mainly by: variations of the photosynthe-sis-respiration rates of primary producers,ecosystem productivity, burial of organic matterand carbonates, CO2 exchange between water andatmosphere as well as oxidation of methane andother organic compounds (McKenzie 1982;Turner et al. 1983; Fritz 1984; Quay et al. 1986;Herczeg, Fairbanks 1987; Andrews et al. 1993;Jêdrysek 1995, 1999, 2005ab; Scholle, Arthur1980; Dean, Stuiver 1993; Wachniew, Ró¿añski1997). Photosynthetic removal of CO2 from sur-face waters results in significant enrichment of theremaining DIC in 13C isotope. The exchange ofCO2 between the lake and the atmosphere usuallyresults in enrichment of DIC in 13C. During sub-sequent oxidation of methane or/and organicmatter, 13C-depleted CO2 is released into the lakewaters. However, 13C in bubble methane fromfreshwater sediments shows significant diurnalvariations, probably due to diurnal variations of

13C(DIC) or/and changes in organic substrates(Jêdrysek 1995, 1999). Thus, for qualitativedescription of carbon dynamics in freshwaterecosystem, diurnal observations of 13C(DIC)values have been undertaken in this project.

Authors of this paper believe that information ondiurnal changes of DIC observed in the SulejówReservoir, its tributaries and water outflowingfrom the reservoir, may explain interrelationshipbetween DIC fluctuations, planktonic communi-ties abundance and development with implicationsto eutrophication process and cyanobacterialblooms development. The aim of the studies wasto recognize pathways of abiotic/biotic C transfor-mation and DIC exchange between water columnand sediments, particularly to examine if DIC isreleased from sediments or DIC sinks into sedi-ments. As indicated by Ecohydrology concept(Zalewski, et al. 1997), stability of hydrologicalconditions is a major factor affecting abiotic andbiotic pathways of biogeochemical cycles inwaterbodies. Thus differentiation of hydrologicalconditions along dam reservoir was taken underconsideration as an element that may modifyintensity of abiotic and botic transformations ofDIC, even in a short time scale

2. Materials and methods

Study area and sampling

Research was conducted on shallow (max.depth 8 m), lowland Sulejów Reservoir located inmiddle course of Pilica River in central Poland(Fig. 1). At full capacity the reservoir has an areaof 22 km2, a mean depth of 3.3 m, a volume of 75x 106 m3 (Wagner, Zalewski 2000). The reservoiris classified as eutrophic with periodically accru-ing strong cyanobacterial blooms (total phyto-plankton biomass >186 mg dm-3; chlorophyll-aconcentrations >60 μg dm-3) (Wagner, Zalewski2000; Izydorczyk et al. 2007 in press). Bicarbon-ates concentrations (HCO3

-2) in the SulejówReservoir usually oscillate between 121 and 43mg dm-3, with an average of 87 mg dm-3 (Tro-janowska 2004).

Mean water retention time in reservoir isabout 30 days (Wagner, Zalewski 2000). Hydro-dynamics of the reservoir, depends on two maintributaries: Pilica (average discharge 24 m3 s-1)and Luci¹¿a rivers (average discharge 3 m3 s-1)and plays a key role in control of suspended mattertransport as well as density and activity of plank-tonic communities. Sulejów Reservoir is suppliedin merely 4% by 7 small streams and groundwa-ters from direct catchment. (Zalewski et al. 2000).

Weather conditions during sampling werechangeable: from sunny, dry and calm to rainy andwindy on very early morning and late afternoon ofthe second day of sampling. Average air tempera-ture was 13.7oC, with maximum achieving 17.0oCand minimum around 6.2oC, recorded duringrainy and windy morning of 17th May 2005.During two episodes of heavy rain, daily sum of

54

precipitation was noted around 16 mm. Prevailingwinds were of Western and North-Western direc-tions, with significantly increasing speed (max. to12 m s-1) during rain, which caused mixing ofwater column due to high weaving (25-30 cm).Meteorological data by Institute of Meteorologyand Water Management (IMGW).

Sampling

Water samples were collected during theclear water phase in two sampling stations: 1/ Zarzêcin, located in the riverine zone of thereservoir and 2/ Tresta situated in its lacustrinezone (Fig. 1). Samples were taken each four hoursfrom 16th - to 17th May 2005 from surface waterand twice a day (2:00 am and 2:00 pm) fromwater column at depths 3 m and 6 m (just abovethe water/sediment interface). Sample was imme-diately closed in vacuum-tight ampoules contain-ing bactericide (HgCl2) and left at 4oC for furtherlaboratory examination of DIC concentration and

13C(DIC) analysis. Simultaneously temperature,pH, O2, conductivity measurements as well astotal phosphorus (TP) and total nitrogen (TN)concentrations analysis were carried out in allwater samples.

Measurements and isotope analysis

Temperature, pH, conductivity and O2 weremeasured with Multi 340i/SET system usingSenTix 41-3 and OxiCal-SL electrodes (WTWWissenschaftlich - Technische Werkstatten). Totalphosphorus (TP) was determined using standardcolorimetric method according to Golterman, etal. (1978) with accuracy to ±2 μg dm-3. TotalNitrogen concentrations were measured usingspectrophotometric HACH TNT test kit (methodNo.10071) with precision of 95% (HACH, 1997).

The entire DIC was removed from the waterand quantitatively collected in the gaseous form ofCO2 (e.g. Graber, Aharon 1991; Atekwana, Krish-namurthy 1998). The obtained CO2 was cryogeni-cally purified (in vacuum about 1*10-3 Torr) usingliquid nitrogen and dry-ice ethanol mixture. Thecarbon isotope ratio was analyzed with the Finni-gan Mat CH7 mass spectrometer (modified detec-tion system) and DeltaE. The carbon isotopecomposition was expressed as 13C value relativeto PDB international standard. The analytical errorof the obtained results was ±0.15‰. Concentra-tion of DIC was determined by mean of massspectrometric measurements. This was accom-plished by introduction to the mass spectrometerthe entire CO2 obtained from decomposition ofDIC under acid condition. The voltage on theFaraday cup of the mass 44 corresponded to theamount of CO2 introduced to the mass spectrome-ter. Earlier calibration enabled calculation of DICconcentration in analyzed waters. The analyticalprecision was about ±0.045 mMol CO2 dm-3.

3. Results

The pH values varied from 6.92 to 7.71 inTresta and in a very narrow range from 7.50 to7.72 in Zarzêcin. Conductivity value were fluctu-ating between 308 μS cm-1 and 372 μS cm-1 inTresta and between 307 μS cm-1 and 310 μS cm-1

in Zarzêcin. Total phosphorus (TP) concentration in water

varied from 114.06 μg dm-3 to 194.06 ug dm-3 inTresta and from 144.65 μg dm-3 to 227.00 μg dm-3

in Zarzêcin. Total nitrogen concentration was fluc-tuating between 1000 μg dm-3 and 2000 μg dm-3

in Tresta and between 1200 μg dm-3 and 2300 μgdm-3 in Zarzêcin.

The results of diurnal observations of DICand 13C(DIC), indicate in general larger varia-tions in Tresta station than on Zarzêcin (Table I,Fig. 2, 3). DIC concentrations observed in Zarzêcinwere fluctuating negligible, from 2.05 to 2.19mMol dm-3 (standard deviation - SD=0.05). TheDIC concentration in Tresta showed greater varia-tions from 1.68 to 2.09 mMol dm-3 (SD=0.14).Simultaneously, variations in 13C(DIC) valuewere much higher also in Tresta than in Zarzêcin(form -10.87 to -14.72 (SD=1.39) and from-11.09to -13.88 (SD=0.82), respectively). Although,model of diurnal changes of two studied parame-ters were differing between the both riverine part(Zarzêcin) of the reservoir and the lacustrine(Tresta) one, trends in variations of DIC concentra-tion and 13C(DIC) value were parallel in each sta-tion.

In the riverine Zarzêcin station, two maxi-mum 13C(DIC) values were noted at 6:00 pm and2:00 pm. The second one, due to atmospheric pre-

Diurnal variations in isotope composition of DIC in dam reservoir 55

Fig. 1. Location of the sampling stations on the SulejówReservoir: 1 - Zarzêcin riverine part, 2 - Tresta lacus-trine part.


cipitation was also corresponding to intimateincrease of DIC concentration.

In the lacustrine (Tresta) sampling stationthree maxima of DIC concentrations and 13C(DIC)enrichment were observed: at 10:00 pm, 6:00 amand 6:00 pm. Lower DIC concentrations andcarbon isotopic ratios were observed at 2:00 amand 2:00 pm.

4. Discussion

Samples from Tresta show higher concen-trations and much wider distribution of meas-ured values (Table I Figs 2, 3). Differences invalues and strength of their diurnal fluctuationsof measured parameters are probably due todistinct hydrological dynamics which, as pos-tulated by Staskraba (1999), is a key variable inecology and limnology of reservoirs. HigherDIC concentrations noted in Zarzêcin are dueto observed lower abundance and activity ofphytoplankton. This is probably due to moreriverine character of Zarzecin sampling station,where the reservoir hydrodynamics is con-trolled by tributaries. Tresta station appears tobe more lacustrine system regulated by theentire Sulejów lake catchment and internalprocesses with more stable hydrological condi-tions. However fluctuations of DIC concentra-tions and 13C(DIC) occurring on both sites in

parallel suggest important role of photosynthesis-respiration rate of planktonic, and benthic organ-isms in carbon cycling between water andsediments on both sites with much stronger inter-actions in lacustrine site.

The majority of carbon studies in lakesaddress the problem such as the effects of chemi-

56

concentration of DIC

[mMol CO 2 dm-3] δ13C(DIC)

center of the reservoir center of the reservoir sample

name

sampling

stations

time

[hours]

surface depth 3m

depth 6m

(water/

sediment

interface)

surface depth 3m

depth 6m

(water/

sediment

interface)

S1T/1 18:00 1.96 n.a n.a -11.65 n.a n.a

S1T/2 22:00 2.07 n.a n.a -10.81 n.a n.a

S1T/3 02:00 1.68 2.01 1.83 -14.72 -13.25 -13.48

S1T/4 06:00 2.09 n.a n.a -11.46 n.a n.a

S1T/5 10:00 1.96 n.a n.a -12.73 n.a n.a

S1T/6 14:00 1.82 2.74 1.96 -13.17 n.a -11.32

S1T/7

Tresta

18:00 1.99 n.a n.a -11.09 n.a n.a

S1Z/1 18:00 2.07 n.a n.a -11.57 n.a n.a

S1Z/2 22:00 2.09 n.a n.a -13.51 n.a n.a

S1Z/3 02:00 n.a. n.a n.a n.a n.a n.a

S1Z/4 06:00 2.05 n.a n.a -13.88 n.a n.a

S1Z/5 10:00 2.19 n.a n.a -13.24 n.a n.a

S1Z/6 14:00 2.11 n.a n.a -12.48 n.a n.a

S1Z/7

Zarzecin

18:00 2.13 n.a n.a -13.01 n.a n.a

Table I. Results of daily studies of DIC and 13C(DIC) in waters of the Sulejów Reservoir, May,16th-17th, 2005.

18:00 22:00 2:00 6:00 10:00 14:00 18:0018:00 22:00 2:00 6:00 10:00 14:00 18:0014:0014:0014:0014:00

time [hours]

1.0

2.0

3.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

DIC

[m

Mo

l/l]

s ampling s tationsTres ta - suraface

Tres ta - depth 3m

Tres ta - depth 6m (water/sediment inte rface)

Zarzęcin - suraface

Fig. 2. Diurnal variations of DIC in water of the SulejówReservoir, May 16th-17th, 2005.

n.a. - not analysed

cally enhanced absorption of CO2 from the atmos-phere on 13C(DIC) in a lakewater (Herczeg, Fair-banks 1987). On the other hand, keeping in mindthat our sampling has been carried out after thewinter and just after diatomic bloom, during"clean water" period, the role of microbialprocesses within the sediments and sediment/water interaction should be considered as poten-tially dominant. Thus, rise of respiration rate ofplanktonic communities and reduction of DIC bybenthic heterotrophic bacteria may explain theobserved 13C depletion of DIC during the night.Lower DIC concentrations and carbon isotopicratios noted at 2:00 am and 2:00 pm were probablyresulted from microbial reduction of CO2 from thewater column to methane and other compounds asindicated by Jêdrysek (1995, 1999, 2005ab).

Organic reach sediments are usually the mostrich source of inorganic carbon dissolved in thewater column due to diagenetic decomposition oforganic matter and subsequent diffusion of DICaffecting 13C(DIC) (Herczeg 1988). Thereforevariations in 13C(DIC) may lead to equivocalconclusions concerning mechanisms of carboncycling in reservoir system. High productivity ofeutrophic ecosystems causes increased depositionand burial of organic matter, which results inincrease in 13C in DIC (Scholle, Arthur 1980;Dean, Stuiver 1993). There are numerous exam-ples of models describing evolution of DIC ingroundwater or lakewater systems (Wigley 1975;Reardon, Fritz 1978; Salomons, Mook 1986;Quay et al. 1986; McConnaughy et al. 1994;Wachniew, Ró¿añski 1997) that combine massbalance calculations with a Rayleigh distillationmodel or/and with net organic carbon production

rate, chemical budget approach etc. to predictboth chemical and isotopic evolution of asystem. However, none of them describe iso-topic evolution of DIC in lakes with respect toits diurnal variations. Nevertheless, it seemscrucial as diurnal variations in DIC and C iso-tope budget can strongly influence carboncycling models based on isotope analysis ofsamples collected from lakes.

The amplitude of diurnal variations of13C(DIC) values achieved 3.85‰ in Tresta

and 2.31‰ in Zarzêcin (Table I). The differencemay result from unequal abundance of plank-tonic organisms on both studied sampling sta-tions: phytoplankton and zooplankton densitiesin Zarzêcin were negligible while in Tresta highzooplankton biomass of big filtrators (Daphniacuculata) was noted (over 20 mg dm-3 of a dryweight) and relatively low phytoplankton bio-mass (below 1.2 mg dm-3 of a fresh weight)(Wojtal, Izydorczyk unpubl.). However, inTresta phytoplankton could have elevated pho-tosynthetic activity, characteristic for youngcells in exponential growth phase but produced

biomass was instantly grazed by filtering zoo-plankton - hence not detected in big amounts, butstill extending fluctuations of isotope composition(Reynolds 1988; Trojanowska et al. 2002). Ele-vated concentrations of 13C in water during day-time were probably resulting from discriminativephotosynthetic assimilation of H12CO3

- by phyto-plankton and 12CO2 by terrestrial plants in sur-rounding forest area that lead to enrichment ofwater in heavy 13C isotope (Sharkey, Berry 1985;Boutton 1991; Goericke et al. 1994). Therefore,diurnal variations in 13C(DIC) value could be alsodriven by atmosphere/water exchange and for asome extend also precipitation. Moreover, sedi-ments in the Tresta station contain more of organicmatter (up to 16%) while in Zarzêcin sedimentscontain more inorganic particles (organic mattercontribution less than 10%) (Kwiatkowska 2002).Although the ebulitive flux of methane fromorganic-rich lacustrine sediments is very low(about 1g per day from 1 cubic meter of the sedi-ment as indicated by Jêdrysek 1997) the DICformed due to oxidation of methane can beextremely depleted in 13C isotope. However, theaverage concentration of DIC in Tresta (1.94mMol dm-3) is somewhat lower than in Zarzêcin(2.10 mMol) while average 13C(DIC) in Zarzêcin(-12.94‰) and Tresta (-12.36‰) are very similar.This suggests that sediments are not the source ofDIC but rather sink of DIC, but methanogenicactivity affects the DIC concentration and

13C(DIC) values in the surface waters (both diur-nal variation in 13C(CH4) value and oxidation ofmethane). This appears to be consistent with previ-ous studies (Jêdrysek 1995, 1999, 2005ab). Conse-quently, it can be concluded the difference


18:00 22:00 2:00 6:00 10:00 14:00 18:0018:00 22:00 2:00 6:00 10:00 14:00 18:0014:0014:0014:0014:00

t i m e [hours]

-20

-19

-18

-17

-16

-15

-14

-13

-12

-11

-10

δ13

C D

IC

[

o /oo ]

sampling stationsTres ta - surface

Tre s ta - depth 3m

Tre s ta - depth 6m (wate r/sedime nt inte rface )

Zarzę cin - surface

Fig. 3. Diurnal variations of 13C(DIC) in water of theSulejów Reservoir, May 16th-17th, 2005.


observed between Tresta and Zarzêcin in diurnalvariations in DIC concentrations and 13C(DIC)values may result from possible higher photosyn-thetic activity of phytoplankton in Tresta station.

Conclusions

1. Sediments can be important sink for dissolvedinorganic carbon due to microbial reduction ofCO2 from the water column to methane andother compounds. This results in increase of the

13C(DIC) at the sediment/water interface andlower DIC concentration than in water at thedepth of 3 m.

2. Large amplitude of diurnal variations of DICconcentration and 13C(DIC) values betweentwo studied stations result probably from diur-nal variations of phytoplankton and terrestrialplants photosynthetic activity, and respiration ofzooplanktoners as well as exchange of CO2between water and atmosphere.

3. Many models of carbon cycling based on sea-sonal observations of DIC may be biased withlarge error resulted from its diurnal variations,which amplitude appears to be comparable tothose commonly considered as seasonal andvertical variations.

Acknowledgements

We would like to acknowledge SebastianRatajski, Tristan Crew and students of Environ-mental Geochemistry and Waste Management atUniversity of Wroc³aw (Ma³gorzata Chwiej,Monika Daszkiewicz, Magdalena Gredka,Krzysztof Idzikowski, Marcin JóŸwik, £ukaszMajtyka, Marzena Michalczyk, Maciej Sêk,Roman Wasik), for their substantial help in sam-pling and laboratory works. The project was sup-ported by Polish Ministry of Education andScience, grant No. 2PO4G04528.

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