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Marine Chemistry 83 (2003) 103–120

Review article

The biogeochemistry of the river and shelf ecosystem

of the Arctic Ocean: a review

Thorsten Dittmar*, Gerhard Kattner

Alfred-Wegener-Institut fur Polar- und Meeresforschung, Am Handelshafen 12, 27570 Bremerhaven, Germany

Received 28 September 2002; received in revised form 15 February 2003; accepted 21 March 2003

Abstract

The Arctic Ocean is, on a volume basis, the ocean with the highest terrestrial input in terms of freshwater and organic matter.

The drainage areas of the Arctic contain more than half of the organic carbon stored globally in soils and are extremely sensitive

to climate change. These changes may considerably influence the huge continental flux of water and organic and inorganic

constituents to the Arctic Ocean. Because of the immediate global concerns we here review the current knowledge about the

biogeochemistry of the Arctic river and shelf ecosystem. Organic matter concentrations in the Arctic rivers are among the

highest reported in world’s rivers. Dissolved organic carbon (DOC) reaches concentrations of up to 1000 AM C. The total

amount of DOC discharged by rivers into the Arctic Ocean is 18–26� 1012 g C year� 1 and similar to that of the Amazon. The

discharge of particulate organic carbon is much lower with 4–6� 1012 g C year� 1. Nitrogen and phosphorus are principally

discharged as organic compounds. The concentrations of inorganic nutrients are among the lowest worldwide (inorganic

nitrogen: 0–20 AM; phosphate: 0–0.8 AM), with the exception of silicate in some rivers (0.5–110 AM).

Freshly produced organic matter is labile and its turnover rates are high in the Arctic Ocean. Riverine organic matter, in

contrast, is soil-derived and refractory. It seems to behave biogeochemically stable in the estuaries and shelves and therefore

does not substantially support the productivity of the Arctic Ocean. Suspended organic matter from the rivers principally settles

in the estuaries and on the shelves, hence the terrigenous signature in the sediment decreases with distance from the coast.

However, a fraction of terrigenous suspended matter escapes the shelves and is present in considerable amounts even in

sediments of the central Arctic Ocean. Terrigenous dissolved organic matter, on the other hand, behaves primarily

conservatively in the Arctic Ocean. There are practically no removal mechanisms in the estuaries and shelves. The molecular

composition of dissolved organic matter can largely be explained as a mixture of refractory marine and terrigenous compounds.

Therefore, the Arctic river discharge plays an important role as a contemporary sink in the global carbon cycle. The few

available data on the biogeochemistry of the Russian rivers indicate that the proportion of taiga and tundra in the drainage areas

has no considerable influence on the concentration and chemical composition of dissolved organic matter, with the exception of

lignin-derived phenols, which can be used as chemotaxonomic tracers. It can therefore be speculated that changes in vegetation

due to climate warming may not considerably influence the composition of dissolved organic matter discharged to the Arctic

Ocean. The discharge of inorganic nutrients, however, may already have increased in the last decades, as indicated by long-term

increases in winter water discharge and the seasonality of nutrient concentrations. For a reliable assessment of future changes

0304-4203/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0304-4203(03)00105-1

* Corresponding author. Present address: School of Oceanography, University of Washington, Box 355351, Seattle, WA 98195-5351, USA.

Tel.: +1-206-221-6748; fax: +1-206-543-6073.

E-mail address: [email protected] (T. Dittmar).

T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120104

long-term and seasonal data of nutrient and organic matter discharge, as well as more detailed biogeochemical information is

urgently needed.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Arctic Ocean; Arctic rivers; Dissolved organic matter; Particulate organic matter; Nutrients; Amino acids; Carbohydrates; Lignin

phenols

1. Introduction Ocean. In Section 3 we focus on natural organic

The continental fluxes of nutrients and organic

matter have important impacts on marine ecosystems.

In combination with freshwater discharge and the

resulting stratification they can be crucial determinants

of the productivity in coastal areas, especially in the

estuaries of large rivers. Furthermore, continent-ocean

fluxes are a principal source for nutrients to the world

oceans, whose productivity is controlled by the avail-

ability of a few elements. A major fraction of terrig-

enous nutrients is bound in organic molecules and

available to most primary producers only after bacte-

rial mineralization (Cornell et al., 1995 and references

therein). The Arctic Ocean is, on a volume basis, the

ocean with the highest terrestrial input in terms of

freshwater and organic matter. About 10% of the

global river discharge enters the Arctic Ocean which

itself only comprises 1% of the global ocean volume

(Opsahl et al., 1999 and references therein). Due to this

large influx of freshwater, the Arctic Ocean is well

stratified with a distinctive surface layer of reduced

salinity. The drainage areas of the Arctic contain more

than half of the organic carbon stored globally in soils

(Dixon et al., 1994) and are particularly sensitive to

climate change, especially permafrost regions. Climate

change has already altered the enormous continental

flux of water to the Arctic Ocean (Peterson et al.,

2002), and may also affect the fluxes of organic

compounds and inorganic nutrients, influencing ocean

water circulation and element cycles on a global scale.

It is therefore of immediate interest to find answers to

the most urgent questions regarding the biogeochem-

istry of the Arctic ecosystem. In this review, we present

the available information on the continental discharge

to the Arctic Ocean, its biogeochemistry and processes

in estuarine areas and shelf regions.

In Section 2 we discuss quantitative aspects of

nutrient and organic matter discharge to the Arctic

matter and address the following questions: (i) Does

riverine organic matter represent a contemporary sink

in the global element cycle, i.e. is it labile or refractory

to microbial or abiotic degradation in the ocean? (ii)

Are we able to establish a chemical fingerprint of the

terrestrial source in the ocean, and evaluate thus the

effect of possible watershed changes? (iii) Does the

continental flux of organic matter impact the oceans

on a global scale, or do the estuaries and shelves

represent an effective sink?

2. Discharge of nutrients and organic matter

The Arctic rivers (Fig. 1) become free of ice in early

summer and discharge >90% of the annual delivery to

the Arctic Ocean from May to July (Fig. 2). Nutrient

concentrations in the rivers generally reach a minimum

during freshet in summer and increase gradually to a

maximum in early spring (Cauwet and Sidorov, 1996;

Holmes et al., 2000). The exception is ammonium,

which generally does not exhibit seasonal trends. In

contrast, organic carbon concentrations increase par-

allel to water discharge with maximum values in

summer. The reason for these patterns is the mixture

of different water masses (Cauwet and Sidorov, 1996).

In wintertime the rivers are fed by ground water with

relatively high nutrient and low organic matter con-

centrations. Melting water from snow and river ice is

poor in oxidized nutrients, but contains ammonium.

The input of these waters during spring and summer

decreases the concentrations of oxidized nutrients in

the rivers, but has no considerable influence on am-

monium. Melting water from snow percolates taiga

and tundra soils before entering the rivers, and is

therefore strongly enriched with organic compounds.

These processes likely do not vary considerably in

the drainage basins. Detailed biogeochemical studies,

Fig. 1. Rivers entering the Arctic Ocean. The widths of the arrows is proportional to the discharge of dissolved organic carbon (DOC), which is

given in 1012 g C year� 1 (for references see Table 1). For the sum of discharges, small rivers are considered with estimates of Opsahl et al.

(1999) and estimates based on average DOC and POC concentrations (Lobbes et al., 2000). Ice edge data from AMAP (1997).

T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120 105

which have been performed for the Lena River along

2000 km from Yakutsk to the river delta (Lara et al.,

1998), show little variations in nutrient and dissolved

organic matter concentrations and no consistent

trends. This is in accordance with Cochran et al.

(2000) who observed constant dissolved organic

carbon (DOC) concentrations in the main stem of

the Ob River. These features must be taken into

account for estimates of future variations due to

climate change. An increase in winter precipitation

Fig. 2. Seasonal variations of water discharge (Q), total organic

carbon concentration (TOC=DOC+POC) and silicate concentra-

tion in the Lena delta (modified after Cauwet and Sidorov, 1996).


may lead to higher runoff maxima in summer, which

in turn would lead to an increase of organic matter

discharge. A general increase of annual mean tem-

perature, on the other hand, may enhance ground

water flow and mineralization of soil organic matter

(e.g. Weller et al., 1995). A higher nutrient discharge

by the rivers may be the consequence. According to

Savelieva et al. (2000), the winter discharge of the

great Siberian rivers has already increased signifi-

cantly during the last four decades, probably because

of enhanced underground water supply caused by

elevated soil temperature. Summer runoff, on the

other hand, did not exhibit significant trends in the

last decades. It can therefore be speculated that the

riverine nutrient supply to the Arctic Ocean may

already have increased.

Despite pronounced seasonal patterns, organic

matter concentrations in the Arctic rivers are gener-

ally among the highest, and nutrient concentrations

among the lowest reported in world’s rivers. DOC

concentrations of the rivers are between 230 and

1000 AM C (Romankevich and Artemyev, 1985;

Martin et al., 1993; Lara et al., 1998; Opsahl et al.,

1999; Lobbes et al., 2000; Kohler et al., 2003). DOC

values exceed particulate organic carbon (POC) by

far (Cauwet and Sidorov, 1996; Lobbes et al., 2000),

with the exception of the Mackenzie River, which is

exceptionally rich in suspended matter and has com-

parable concentrations of DOC and POC (Telang et

al., 1991; Macdonald et al., 1998). Dissolved and

particulate organic nitrogen (DON, PON) concentra-

tions, on the other hand, are similar (each 12 AM on

average for 12 Russian rivers; Lobbes et al., 2000).

Nitrogenous nutrients and phosphate concentrations

were found from close to 0 up to about 20 and 0.8

AM, respectively. Nitrate concentrations are as low as

ammonium in summer, while wintertime nitrate con-

centrations generally exceed that of ammonium (Cau-

wet and Sidorov, 1996). Silicate levels are higher

with concentrations ranging from 0.5 to 110 AM(Telang et al., 1991; Letolle et al., 1993; Cauwet

and Sidorov, 1996; Gordeev et al., 1996; Lara et al.,

1998; Kattner et al., 1999; Nothig and Kattner, 1999;

Lobbes et al., 2000). Average concentrations in 12

Russian rivers entering the Arctic Ocean are 2.5 AMfor inorganic nitrogen, 0.25 AM for phosphate, and

23 AM for silicate (Lobbes et al., 2000). During the

Soviet era, the Russian rivers and shelves were

among the most extensively monitored on earth.

The extensive long-term data sets of phosphate,

silicate and nitrogenous nutrients produced between

1948 and 2000 at about 20,000 stations by the

Soviet/Russian water quality monitoring network

(OGSNK/GSN) have recently been compiled in a

hydrochemical atlas of the Arctic Ocean (Timokhov,

2002). The river data have been reviewed and eval-

uated by Holmes et al. (2000, 2001). Unfortunately,

many of these records are not reliable, and ammoni-

um concentrations are particularly erroneous and

uncorrectable. Because of the lack of comprehensive

seasonal or long-term data, discharge estimates for

nutrients and organic matter are at present rough

and vary widely. It is therefore not yet possible to

establish a baseline of riverine fluxes to the Arctic

Ocean against which to judge future changes (Holmes

et al., 2000).

For the rivers with the largest water discharge, Ob,

Yenisey, Lena, and Mackenzie, current flux estimates

range from 2.8–70� 109 g N year� 1 for inorganic

nitrogen (Table 1). Some of the differences may be

due to seasonal dynamics. Based on annual data,

Cauwet and Sidorov (1996) estimated for the Lena

River fluxes of 42–46� 109 g N year� 1. Based on

September data only for the Lena River (Lara et al.,

1998), 3.4� 109 g N year� 1 can be estimated. Annual

phosphate fluxes range from 1.5–23.5� 109 g P

year� 1 (Table 1). The value of 23.5� 109 g P year� 1

for the Ob River as calculated by Holmes et al. (2000)

may be too high because of the low reliability of the

historic data. Also silicate fluxes range widely be-

Table 1

Annual discharges of water, dissolved and particulate organic matter (DOC, DON, POC, PON), and inorganic nutrients (dissolved inorganic

nitrogen DIN=Nitrate +Nitrite +Ammonium, Silicate, Phosphate) for the rivers entering the Arctic Ocean

River Watershed Discharge

area (km3)Water

(km3�year� 1)

DOC-C

(1012 g�year� 1)

POC-C

(1012 g�year� 1)

DON-N

(109 g�year� 1)

PON-N

(109 g�year� 1)

DIN-N

(109 g�year� 1)

Silicate-Si

(109 g�year� 1)

Phosphate-P

(109 g�year� 1)

Yenisey 2440 562–577 4.1–4.9 0.17 82 17 2.8–70 200–1223 6.0–6.9

Lena 2430 524–533 3.4–4.7 0.47 80–245 54 3.4–46 890–1640 3.5–6.5

Ob 2950 404–419 3.1–3.2 0.31–0.6* 66* 28*–54* 20–40 311 7.9–23.5

Mackenzie 1680 249–333 1.3 1.8–2.1 27* 160*–190* 23.6* 470* 1.5

Pechora 312 135 2.1* – 44* – 7.1* – 4.2

Northern Dvina 348 106 1.7* – 35* – 6.7* – 2.0

Kolyma 526 71–98 0.46–0.7* 0.31 16 34 2.5* – 0.76

Indigirka 305 50 0.24–0.4* 0.17 8.4 24 0.18–2.3* 0.7 0.11–0.35

Taz 100 33 – – – – 0.75* – 2.8

Olenek 198 32 0.32 0.03 7.9 2.5 0.20–0.78* 21 0.03–0.23

Yana 224 31–32 0.09 0.05 2.9 4.8 1.2*–1.7 61 0.08–0.36

Pur 95 28 – – – – 0.74 – 3.0

Mezen 56 20 0.25 0.04 4.5 3.2 0.71*–1.3 10 0.27–0.44

Onega 56 16 – – – – 0.99* – 0.15

Nadym 48 15 – – – – 0.55* – 2.0

Anabar 79 13 – – – – 0.09* – 0.03

References:

Watershed area: R-ArcticNet database.

Water discharge: GEMS (2001), Lobbes et al. (2000).

DOC: Telang et al. (1991), Macdonald et al. (1998), Opsahl et al. (1999), Lobbes et al. (2000), Kohler et al. (2003).

POC: Telang et al. (1991), Macdonald et al. (1998), Lobbes et al. (2000).

DON: Cauwet and Sidorov (1996), Lobbes et al. (2000).

PON: Lobbes et al. (2000).

DIN, Phosphate, Silicate: Cauwet and Sidorov (1996), Holmes et al. (2000, 2001), and calculated using data from Lara et al. (1998), Nothig and

Kattner (1999), Lobbes et al. (2000).

Data marked with an asterisk (*) are estimates:

*DOC: sum of Pechora and Northern Dvina is 3.8, and sum of Kolyma and Indigirka is 1.1 (Opsahl et al., 1999), individual values were

estimated from water discharge.

*POC: estimated using data of Fernandes and Sicre (2000).

*DON, *PON: estimated from DOC and POC data and average C/N-ratios (Lobbes et al., 2000).

*DIN: only nitrate (Holmes et al., 2000), data from the same authors for Ob and Yenisey were completed with ammonium data from Holmes

et al. (2001).

*Silicate: estimated using data from GEMS (2001).


tween 200 and 1640� 109 g Si year� 1 for the major

rivers. The sum of the nutrient discharges of all Arctic

rivers together is only 20–40% of the Amazon fluxes

and even lower than the Mississippi fluxes alone

(Holmes et al., 2000). The exception is silicate which

is discharged in similar amounts by each of the major

Arctic rivers and the Mississippi (calculated from

GEMS, 2001).

The biotic activities in the northern parts of the

rivers and in the ocean are directly related to the water

discharge dynamics of the rivers. During a long

period of the year, low irradiance and the ice cover

limits primary production. However, it is questionable

how far primary productivity in the estuaries and on

the shelves is enhanced through nutrient supply by

riverine discharge, because of the low nutrient con-

centrations in the Arctic rivers and the corresponding

low fluxes to the ocean. The stable stratification of the

water column in the estuaries and shelves throughout

the year further hampers primary production. A

patchy distribution of phytoplankton growth or even

blooms and superficial nutrient depletion can be


observed at the ice edge when melting begins in June

(Cauwet and Sidorov, 1996; Kattner et al., 1999), but

nutrients regenerated in the shallow bottom water

cannot reach the photic layer because of the stable

stratification (Cauwet and Sidorov, 1996). Vertical-

mixing energy is low on Arctic shelves (Weingartner

et al., 1999), and storm-driven mixing events are

episodic and restricted to the ice-free period of the

year. Therefore, nutrient concentrations in the photic

zone of the shelves are generally low (e.g. Cauwet

and Sidorov, 1996; Kattner et al., 1999) and compa-

rable to those in the central Arctic Ocean (e.g.

Anderson et al., 1994; Wheeler et al., 1997). Silicate

behaves non-conservatively and exhibits low concen-

trations in the surface water of the Kara and Laptev

Seas despite considerable riverine inputs. This indi-

cates diatom growth and the availability of regener-

ated nutrients within the photic layer, which are not

derived from bottom waters because of the stable

stratification.

A source for regenerated nutrients may be the large

amount of organic matter discharged by the Arctic

rivers to the ocean (Fig. 1). The Yenisey, Lena and Ob

rivers have the highest DOC discharge with 3.1–

4.9� 1012 g C year� 1 (Table 1). About 80% of the

total organic carbon flux occurs in form of DOC. The

Arctic rivers discharge a total of 18–26� 1012 g DOC

year� 1, which is similar to the flux of the Amazon

(19� 1012 g DOC year� 1; Degens et al., 1991). This

amount is comparable to the flux of dissolved inor-

ganic carbon (Olsson and Anderson, 1997). The DON

flux of Yenisey, Lena and Ob range between 66 and

245� 109 g N year� 1. An additional 17–54� 109 g

N year� 1 is discharged in particulate form (PON) by

each river. Nitrogen is principally discharged in or-

ganic compounds. Similarly, the dissolved organic

phosphorous discharge is 13� 109 g P year� 1 for

the Lena river (Cauwet and Sidorov, 1996), compared

to 3.5–6.5� 109 g P year� 1 of phosphate. The

chemical identity of these organic compounds deter-

mines the impact of the enormous flux of organically

bound nutrients on the marine ecosystem. Labile

compounds are rapidly available for primary pro-

ducers via heterotrophic mineralization in contrast to

recalcitrant compounds. Because of the extraordinary

importance of organic matter in the Arctic Ocean, we

focus on the biogeochemistry of these compounds in

the following section.

3. The biogeochemistry of natural organic matter

3.1. Organic matter in the Arctic rivers

Despite the huge continental flux of organic matter

to the Arctic Ocean, few studies have so far been

performed on the chemical composition of natural

organic matter in the Arctic rivers. The available

information on dissolved organic matter is confined

to the Russian rivers. Geochemical data of the Cana-

dian Mackenzie River are restricted to sediments and

suspended solids of its estuary and shelf region. Arctic

rivers undergo a pronounced seasonal cycle in terms

of water runoff and organic matter concentration.

Most studies were performed during or after freshet

in summer. They therefore represent a major part of

the continental flux of organic matter into the Arctic

Ocean (Cauwet and Sidorov, 1996).

Autochthonous production in the rivers is low

(Cauwet and Sidorov, 1996; Sorokin and Sorokin,

1996), and therefore a considerable in situ production

of aquatic organic matter is not expected. Lobbes et al.

(2000) determined DOC/DON-ratios of 29–69 in

various Russian rivers indicating a predominantly

terrestrial source. The values for the Mackenzie River

are within the lower range of these values (Telang et

al., 1991). Lara et al. (1998) attributed high DOC/

DON ratios of the ‘black’ Lena River waters to a

predominance of soil-derived organic material, anal-

ogous to the tropical Amazon River (Hedges et al.,

1994). Positive correlations between silicate, DOC

and DON were also indicative for soil-origin (Lara

et al., 1998). The main fraction (60–70%) of DOC in

the Siberian rivers Yenisey, Olenek and Moroyyakha

exhibit characteristic features of terrigenous humics,

in terms of aromaticity and molecular mass, as evident

from gel-permeation chromatography (Dittmar and

Kattner, 2003). Nominal molecular masses (number

averaged Mn) were 870–1050 g�mol� 1, and molar

UV-absorption coefficients (254 nm) of 72–78

m2�mol� 1 indicated high aromaticity. The remainder

was composed mainly of subunits of humics. A very

small fraction ( < 5% of DOC) could be attributed to

diagenetically fresh biomacromolecules, primarily

polysaccharides and peptides. Molecular analyses of

dissolved organic matter in 12 Russian rivers entering

the Arctic Ocean confirm these results (Engbrodt,

2001; Dittmar et al., 2001a). Dissolved carbohydrates


and amino acids comprise the largest fraction of

dissolved organic matter that can be characterized

on molecular level. The yield of total dissolved

hydrolyzable neutral sugars (THNS) and amino acids

(TDAA) in the Russian rivers is low (2–6% of DOC;

Figs. 3 and 4), and comparable to other major world

rivers (Ittekkot et al., 1982; Hedges et al., 1994, 2000;

Benner and Opsahl, 2001). The concentration of free

amino acids is negligible with only about 2% of

TDAA determined for the Lena river (Lara et al.,

1998). Most TDAA, which contributes about 40% to

total DON and < 1% to DOC, is probably of soil

origin and largely incorporated into the complex

structure of humic substances. This is indicated by a

positive correlation between TDAA and lignin con-

centrations (Fig. 5; Dittmar et al., 2001a). Dissolved

lignin is released during the degradation of debris in

the soils of taiga and tundra, and probably bound into

humic substances. The elevated acid to aldehyde ratio

of the vanillyl phenol family ((Ad/Al)v = 1.2 on av-

erage) is evidence for an advanced diagenetic state of

lignin (Lobbes et al., 2000). A fraction of TDAA,

however, is probably not strongly associated to ter-

rigenous humics. The concentration of this fraction of

TDAA is approximately 2 AM and can be computed

from the relationship between lignin and TDAA at a

lignin concentration of 0 (Fig. 5). In contrast to the

amino acids, the incorporation of sugars or other

organic compounds into humic substances is highly

variable between the rivers. Neither DOC, DON nor

Fig. 3. Concentrations of dissolved and particulate organic carbon (D

carbohydrates (TCHO or THNS) and amino acids (TDAA, PAA). Average

the Arctic Ocean, the Siberian shelf, and surface waters of the Laptev Sea.

deep-sea (Amundsen Basin) are given for comparison. Data from Dittmar

hydrolyzable sugars exhibit any correlation with lig-

nin, as tested with data of Lobbes et al. (2000),

Engbrodt (2001) and Dittmar et al. (2001a).

Nevertheless, the molecular composition of both

the amino acids and neutral sugars points towards an

advanced diagenetic degree of the dissolved organic

matter in all rivers investigated. During decomposi-

tion of organic matter, the accumulation of cell wall

constituents like glycine and non-protein amino acids

(e.g. g-amino butyric acid) is commonly observed

(Dauwe et al., 1999). Both amino acids are signifi-

cantly enriched in riverine TDAA relative to freshly

produced organic matter indicating an advanced dia-

genesis of DON (Fig. 6; Dittmar et al., 2001a). The

degradation index for protein amino acids calculated

after Dauwe and Middelburg (1998) for TDAA in

Russian rivers is � 1.0 on average (Dittmar et al.,

2001a) also indicating a high level of degradation.

The index typically ranges from + 1 (phytoplankton,

bacteria) to � 1.5 (highly degraded oxic sediments).

The molecular composition of dissolved sugars

(THNS) supports the finding that the dissolved organ-

ic matter is highly degraded. Relative to fresh organic

tissue, glucose in the dissolved organic matter of

Russian rivers is highly reduced exhibiting propor-

tions of only 25–28% of THNS (Fig. 7; Engbrodt,

2001). According to Opsahl and Benner (1999), rivers

can be classified into categories of low or advanced

diagenesis depending on whether they have a high

(approx. 50%) or low (approx. 25%) glucose propor-

OC, POC), and nitrogen (DON, PON), and the proportions of

values and confidence intervals ( p< 0.05) for Russian rivers entering

As reference for marine-derived organic matter, values for the Arctic

et al. (2001a) and Engbrodt (2001).

Fig. 4. Concentrations of dissolved organic carbon (DOC) and nitrogen (DON) in the course of the Lena River, C/N-ratios and the proportions

of dissolved carbohydrates (TCHO) and amino acids (TDAA). Data from Lara et al. (1998).


tion, respectively. These findings are in accordance

with considerable D-amino acid proportions in riverine

TDAA. Bacterially derived D-enantiomers of aspartic

acid, glutamic acid, serine and alanine were all found

in significant amounts (Fig. 6). D-aspartic acid, which

is characteristic for soil humic substances, is the most

abundant D-amino acid in the rivers (21% of total

Fig. 5. Dissolved amino acid (TDAA) versus lignin concentrations

for nine Russian rivers entering the Arctic Ocean. The correlation

for the river samples indicates soil humic substances as a common

source. The TDAA fraction of lignin-free riverine dissolved organic

matter (intercept) is probably not strongly associated to soil-derived

humics and presumably labile. Data from Lobbes et al. (2000) and

Dittmar et al. (2001a). Abbreviations: Indigirka (In), Kolyma (Ko),

Lena (Le), Mezen (Me), Moroyyakha (Mo), Ob (Ob), Olenek (Ol),

Vaskina (Va), Velikaja (Ve), Vizhas (Vi), Yenisey (Ye).

aspartic acid). Bacterioplankton, on the other hand,

would produce higher D-alanine than D-aspartic acid

proportions (McCarthy et al., 1998; Amon et al.,

2001) and therefore plays a lesser role in the forma-

tion of riverine DON in the Arctic.

The striking similarity of organic matter from the

various Russian rivers is in accordance with studies by

Lara et al. (1998) on the biogeochemistry of the Lena

River along its course from Yakutsk to the delta. The

river exhibits no obvious gradients in terms of inor-

ganic nutrient, DOC, DON and carbohydrate concen-

trations and amino acid composition (Fig. 4). Along

its whole course, the biogeochemical characteristics

are similar to the lower part of the other Russian

rivers. Also dissolved organic matter from the Ama-

zon River system (Hedges et al., 1994, 2000), the

Mississippi (Benner and Opsahl, 2001) and other

world rivers (Ittekkot et al., 1982) exhibit an overall

similar amino acid and neutral sugar composition

compared to the Siberian rivers. All data confirm

the general finding that organic matter in the Arctic

rivers is largely composed of recalcitrant soil-derived

material and not material released from algae.

The suspended organic matter, comprising in gen-

eral a very minor fraction of total organic carbon in

the rivers, is constituted primarily of refractory com-

pounds derived from vascular plant detritus, whereas

phytoplankton and living bacterial biomass is negli-

gible in all Arctic rivers (Cauwet and Sidorov, 1996;

Sorokin and Sorokin, 1996). Chlorophyll a/POC ra-

tios are low (Cauwet and Sidorov, 1996), and stable

carbon isotope analyses revealed a predominantly

terrestrial source for POC in the Russian rivers

Fig. 6. Average mol percentages and D-enantiomer fractions of individual dissolved (TDAA) and particulate amino acids (PAA) in 12 Russian

Rivers and corresponding near-shore and shelf areas, with confidence intervals ( p< 0.05). As a reference for marine-derived organic matter,

values for the Arctic deep-sea (Amundsen Basin) are given for comparison. Data from Dittmar et al. (2001a). Abbreviations: glycine (Gly),

aspartic acid (Asp), glutamic acid (Glu), serine (Ser), threonine (Thr), arginine (Arg), alanine (Ala), g-amino butyric acid (GABA), tyrosine

(Tyr), valine (Val), phenylalanine (Phe), isoleucine (Iso) and leucine (Leu).


(d13C =� 26.6 on average; Lara et al., 1998; Lobbes

et al., 2000). This terrestrial source is further con-

firmed by lignin data. The carbon-normalized yield of

lignin phenols (Xlignin; Lobbes et al., 2000) is 2.3xon average for POC and therefore almost identical to

the Xlignin value of DOC (2.4x). The POC/PON ratio

is 11 on average for the Russian rivers (Lobbes et al.,

2000) and therefore much lower than the DOC/DON

ratio. POC/PON ratios and Xlignin are low in the rivers

in comparison to fresh vascular plant biomass and

resemble soil organic matter. Fernandes and Sicre

(2000) deduced from relatively constant n-alkane

particle loads, which are around 10 Ag per g sediment

in Ob, Yenisei and the adjacent Kara Sea, a homog-

enous composition of riverine particles and soil ero-

sion as one of the main factors controlling particle

composition. Yunker et al. (1991) made similar obser-

vations for the Mackenzie River. Particulate hydro-

Fig. 7. Average mole percentages of individual dissolved neutral

sugars (THNS) in seven Russian Rivers and corresponding near-

shore areas, with confidence intervals ( p < 0.05). Data from

Engbrodt (2001). Abbreviations: fucose (Fuc), rhamnose (Rha),

arabinose (Ara), galactose (Gal), glucose (Glc), mannose (Man),

xylose (Xyl), fructose (Frc), ribose (Rib).

Fig. 8. Correlation between syringyl-/vanillylphenol ratios (released

from CuO oxidation of dissolved organic matter) and vegetation in

11 Russian Rivers entering the Arctic Ocean. Black dots are data

from Lobbes et al. (2000; considered for the correlation); ranges are

from Opsahl et al. (1999). Vegetation data are estimated from Times

Atlas (1997); 0% of tundra corresponds to 100% of taiga. For

abbreviations see Fig. 5.


lyzable amino acids (PAA) contribute on average

approximately 60% to PON. The pattern of individual

hydrolyzed amino acids and positive degradation

indices indicate a lower diagenetic degree for PON

than for DON (Fig. 6; Dittmar et al., 2001a). Accord-

ingly, the proportions of the D-enantiomers and the

non-protein amino acid, g-amino butyric acid, are

clearly below the values of dissolved organic matter.

The acid to aldehyde ratio of vanillyl phenols ((Ad/

Al)v) in POC, as indicator for oxidative degradation,

is 0.44 on average for Russian rivers (Lobbes et al.,

2000) and 0.62 on average for the Mackenzie River

(Goni et al., 2000) and therefore much lower than the

corresponding value for DOC (1.2). This comparison

reveals that the particulate riverine organic matter is

less degraded than the dissolved one although both are

derived predominantly from refractory compounds of

vascular plant debris.

3.2. Watershed vegetation in relation to riverine

organic matter

The molecular composition of riverine dissolved

organic matter might reflect the prevailing watershed

vegetation since it is primarily constituted of refrac-

tory soil-derived compounds. Taiga is the main veg-

etation of northern and central Eurasia and is

characterized by coniferous woods and bogs. Adja-

cent to the north, treeless permafrost tundra domi-

nates. The largest rivers Lena, Ob, Yenisey, and

Kolyma drain primarily taiga. The smaller rivers, like

Moroyyakha, Vaskina, and Velikaja, drain almost

100% tundra. Using lignin-derived phenols as chemo-

taxonomic tracers, Lobbes et al. (2000) were able to

distinguish between organic matter derived from taiga

and tundra in the Russian rivers. In particular the ratio

of syringyl to vanillyl phenols (S/V) correlates signif-

icantly with the proportion of tundra in the catchment

areas of the rivers (Fig. 8). In contrast to lignin,

neither the molecular composition of hydrolyzable

neutral sugars nor amino acids correlate with the type

of vegetation (Engbrodt, 2001; Dittmar et al., 2001a).

Engbrodt (2001) observed slightly elevated glucose

proportions and THNS yields in small Siberian rivers

that drain exclusively tundra relative to the average

values of all other rivers. This may be due to a lower

diagenetic degree of dissolved organic matter in these

rivers, which exclusively drain permafrost soils. We

performed a principal component analysis (PCA) and

multivariate regression with the data sets from the

Fig. 9. DOC concentrations versus salinity in the eastern Arctic

Ocean. Lena River/Laptev Sea (Cauwet and Sidorov, 1996: white

dots; Kattner et al., 1999: black dots). Ob and Yenisey Rivers/Kara

Sea (Kohler et al., 2003: crosses).


above mentioned authors. Again, only the proportion

of syringyl phenols exhibits systematic variations

according to the type of vegetation, whereas the

variations of all other parameters cannot be explained

by differences in vegetation. Dittmar et al. (2001a)

interpreted the striking similarity of the amino acid

signature of the Russian rivers and other world rivers

with a prevalence of overall diagenetic processes in

soils. These processes probably do not vary between

different ecosystems and account for the formation of

amino acid containing humic compounds all over the

globe. Analogous to amino acids, the pattern of

hydrolyzable neutral sugars dissolved in the Russian

rivers resembles that of other world rivers in different

climate zones (Engbrodt, 2001).

Particulate organic matter in the Arctic rivers may

resemble the precursor material to a higher degree than

dissolved organic matter, since it is also soil-derived

but less degraded than its dissolved counterpart. How-

ever, a significant correlation between the chemical

signature of the particulate organic matter and water-

shed vegetation can, as was observed for dissolved

organic matter, only be established for the S/V ratio of

lignin (Lobbes et al., 2000). The variations in the other

parameters investigated, i.e. PAA and other lignin

phenols, cannot be related to vegetation.

3.3. Dissolved organic matter in the estuaries and

shelf regions

Do the Arctic estuaries function as a barrier to

the huge flux of riverine dissolved organic matter,

or do these terrigenous humic substances pass the

estuarine mixing zone without modifications?

Does phytoplankton contribute significantly to the

dissolved organic matter budget of the estuaries and

shelves? Several authors addressed these questions by

plotting DOC concentration versus salinity, i.e.

compiling mixing diagrams (Fig. 9). Cauwet and

Sidorov (1996, Laptev Sea), Kattner et al. (1999,

Laptev Sea), Dittmar et al. (2001a, Eastern Arctic

Ocean) and Kohler et al. (2003, Kara Sea) reported, in

general agreement, a tight fit of DOC concentrations

with the conservative mixing lines for Russian estu-

aries and shelves. No losses or gains of DOC are

evident. These findings are consistent with laboratory

experiments with dissolved organic matter from Yeni-

sei, which showed no loss after mixing with marine

waters, and only very minor losses due to bacterial

consumption (Kohler et al., 2003). For the Canadian

Mackenzie River, Macdonald et al. (1998) also sus-

pected that riverine DOC simply transits the shelf to

enter the surface pool of the interior ocean.

Single datapoints, however, may significantly dif-

fer from conservative mixing. The possibility that a

minor fraction of terrigenous organic matter is reac-

tive in the Arctic Ocean cannot be ruled out on the

basis of the existing data, but other factors are more

likely to cause deviations from ideal conservative

mixing. One of the reasons for these deviations may

be that freshwater (salinity = 0) is not always a con-

servative tracer for river water in the Arctic Ocean.

Even though the primary source of freshwater to the

Arctic Ocean is terrestrial runoff (Bauch et al., 1995),

melting or freezing may locally lead to increases or

decreases in salinity and thereby to a different behav-

ior of salts and organic solutes. Additionally, autoch-

thonous production can lead to local patches of

increased DOC concentrations. Kattner et al. (1999)

observed silicate and nitrate depletion of surface

waters especially near the receding ice margin. Intense

short-term diatom blooms that significantly reduced

nutrient levels in coastal waters of the Laptev Sea

have also been described by Cauwet and Sidorov


(1996). These reports corroborate with the data on the

chemical composition of dissolved organic matter.

Elevated glucose proportions of >45% of THNS at

some inshore stations of the eastern Arctic Ocean are

evidence for fresh plankton-derived dissolved organic

matter (Engbrodt, 2001). Dittmar et al. (2001a) ana-

lyzed the deviations from conservative mixing of

inorganic nitrogen, DON and amino acids in the

eastern Arctic Ocean. They calculated a 1 mol release

of DON per 5 mol of assimilated nitrogen. The

terrigenous contribution to the DON pool is only

20–30% in the Laptev Sea and therefore less than

half than for DOC (Kattner et al., 1999; Dittmar,

submitted for publication). The amino acid signature

and concentration, however, do not exhibit systematic

deviations from conservative mixing. This is probably

due to fast microbial turnover of labile phytoplankton-

derived compounds. Systematic increases of D-enan-

tiomers in phytoplankton-derived DON and the gen-

eral finding of low TDAA yields in marine DON also

point towards a fast microbial turnover of phytoplank-

ton-derived TDAA in the Arctic Ocean (Dittmar et al.,

2001a). Most of the changes to DON take place in the

Siberian estuaries and the near shore areas, and not in

the Arctic Ocean (Kattner et al., 1999). Amon and

Benner (2003) estimated on the basis of neutral sugar

yields that only about 2% of the DOC in Arctic Ocean

surface waters is of labile nature which is low com-

pared to other world oceans.

Besides some patches where the release of dis-

solved organic matter from phytoplankton was evi-

dent, terrigenous humic substances dominated the

chemical signature of dissolved organic matter in

the Russian estuaries and shelves. The terrestrial

compounds pass the estuarine mixing zones largely

unmodified. In the Laptev Sea and eastern Arctic

Ocean the molecular signature, i.e. lignin-derived

phenols (Kattner et al., 1999), neutral sugar (Eng-

brodt, 2001) and amino acids (Dittmar et al., 2001a),

do not vary consistently from the conservative mix-

ing of recalcitrant marine (deep-sea) and riverine

dissolved organic matter. The invariantly high aro-

maticity of humic acids in the central Arctic Ocean

(Dittmar and Kattner, 2003) also shows the un-

changed molecular composition of this major terrig-

enous DOC fraction. Photodegradation, which would

reduce aromaticity (e.g. Opsahl and Benner, 1998),

does not significantly affect DOC composition and

concentration in the Arctic. Despite this composition-

al stability of terrigenous DOC during mixing, which

lasts years to decades in the Arctic Ocean (Schlosser

et al., 1995), the molecular size of humic acids (as

determined from size-exclusion chromatography;

Dittmar and Kattner, 2003) changes considerably.

The nominal molecular mass apparently decreases

from Mnc 1000 g�mol� 1 in the rivers to Mnc 600

g�mol� 1 in the coastal zone. This rapid size reduction

is probably due to intramolecular contraction or

coiling induced by the increases of ionic strength

and concentration of divalent ions in the brackish

zone (Engebretson and von Wandruszka, 1994),

which in turn leads to higher chromatographic reten-

tion times and apparently lower Mn values. This

physicochemical modification in the estuaries, how-

ever, does not lead to any phase transition or removal

of terrigenous dissolved organic matter from the

water column.

Due to the fact that removal mechanisms for DOC

in the Arctic estuaries are practically absent, the

enormous continental flux contributes substantially

to the DOC budget of the Arctic Ocean and dominates

the budgets on the shelves. For the whole Laptev Sea,

Kattner et al. (1999) estimated a terrigenous contri-

bution of about 60% to total DOC on the basis of

lignin analyses. In the Lena delta, concentrations of

this unequivocal tracer for vascular plants are on the

same order of magnitude as observed in other major

rivers (Opsahl and Benner, 1997) or coastal waters

(Moran et al., 1991; Dittmar et al., 2001b). In the

surface layer of the open Laptev Sea and the adjacent

Eurasian Basin, however, lignin yields are at least one

order of magnitude higher than those in the Atlantic

and Pacific Oceans (Opsahl and Benner, 1997). This

demonstrates the extraordinarily high proportion of

terrigenous compounds in the Arctic Ocean. Wheeler

et al. (1997) calculated that river runoff contributes

25% to the total DOC input to the Arctic Ocean, and

Opsahl et al. (1999) estimated a terrigenous fraction of

5–33% throughout the surface Arctic Ocean based on

lignin and stable carbon isotope analyses in ultra-

filtered dissolved organic matter. This represents 12–

41% of the annual river discharge of terrigenous

DOC. The latter authors deduced from low ratios of

syringyl versus vanillyl phenols (S/V) a dominant

gymnosperm (taiga) source of terrigenous dissolved

organic matter in the Arctic Ocean.

ine Chemistry 83 (2003) 103–120 115

3.4. Particulate organic matter in the estuaries and

shelf regions

Unlike dissolved organic matter, riverine suspended

solids are rapidly removed from the water column in

the estuaries and shelves. In the estuaries, flocculation

and coagulation of particles enhance sedimentation

(Moreira-Turcq and Martin, 1998; Droppo et al.,

1998). In the Russian Lena-Laptev Sea system, the

number of particles decreases exponentially with sa-

linity from 18.5� 105 cm� 3 in the river to 3.2� 105

cm� 3 in the open Laptev Sea (Moreira-Turcq and

Martin, 1998). POC decreases from 75 to < 10 AMin the Laptev Sea (Cauwet and Sidorov, 1996) and for

11 Russian rivers from 115 to 13 AM on average

(Dittmar et al., 2001a). For the Canadian Beaufort

Shelf, Macdonald et al. (1998) estimated that about

50% of the sediment supply is trapped in the delta,

about 40% on the shelf and the remainder escapes the

shelf edge. Also the nature of the particles changes

dramatically during the transition in the estuaries. The

proportion of organic to total particles (organic +min-

eral) increases from 26% on average in the upper Lena

River and delta, to 42% in the coastal region, and 66%

in the Laptev Sea, the remainder being mineral par-

ticles (Moreira-Turcq and Martin, 1998). On a weight

basis, the proportion of minerals appears even higher

in the rivers: The average carbon content of suspended

solids is only 2% on average for the Russian rivers

(Lobbes et al., 2000), and 4% and 7% for Ob and

Yenisei, respectively (Fernandes and Sicre, 2000). In

the Kara Sea, the POC proportion increases to 12% on

average (Fernandes and Sicre, 2000). In the Laptev

Sea, POC increases from about 4% in the Lena up to

20% on the shelf (Cauwet and Sidorov, 1996).

The source indicators Xlignin and d13C point to a

changing composition of suspended organic matter

towards lignin-poor and less 13C-depleted phyto-

plankton-derived compounds in the eastern Arctic

shelf (Dittmar et al., 2001a) showing that riverine

particulate organic matter is rapidly replaced on the

shelf by algal detritus. Yunker et al. (1991) and

Fernandes and Sicre (2000) found relatively constant

n-alkane particle loads in the Beaufort and Kara Sea

shelves, indicating a main terrestrial source of POC.

In the Laptev Sea, however, compositional features of

suspended n-alkanes indicate significant planktonic

inputs (Broyelle, 1997) that are not observed in the

T. Dittmar, G. Kattner / Mar

Kara Sea. Peulve et al. (1996) exposed suspended

organic matter and sediments of the Laptev Sea to

Curie-point pyrolysis (CuPy) and identified a suite of

products with gas chromatography-mass spectrometry

(GC-MS). The pyrolysates of surface-suspended par-

ticles suggested mixed inputs of algae and terrestrial

sources. Some fatty acids, phytadienes and n-alkylni-

triles were assigned to planktonic organisms while

polysaccharides, phenolic substances, and lipid con-

stituents were attributed to higher terrestrial plants.

With increasing water depths, the proportion of plank-

ton-derived phytadienes and fatty acids decrease due

to biotransformation. Preferential settling out of lithic

material relative to less dense higher plant debris also

may modify the organic signature of suspended or-

ganic matter. This was evident from modifications of

hydrocarbon markers in the Mackenzie estuary

(Yunker et al., 1995).

Due to the refractory character of riverine POC, a

major fraction (50–60%) is preserved in delta and

shelf sediments of the Kara Sea (Fernandes and Sicre,

2000) and Beaufort Shelf (Macdonald et al., 1998).

Primary production, which adds a similar amount of

POC to the carbon budget as the rivers, is almost

entirely (97%) recycled and is not preserved in the

sediments of the Beaufort Shelf (Macdonald et al.,

1998). In the Laptev Sea, sedimentation rates are

comparable with primary productivity values of

open-ocean environments (f 50 g C m� 2 year� 1).

Accumulation rates, however, are several orders of

magnitude higher in the Laptev Sea than in the

general open ocean (450 versus 0.005 g C m� 2

ky� 1; Stein, 1991; Stein and Schubert, 1996), which

reflects differences in the refractory versus labile

character of terrigenous and marine compounds. This

is consistent with the finding of Dittmar et al. (2001a)

that particulate D-amino acids are present even off-

shore in the euphotic zone, indicating microbial bio-

mass and a fast turnover of decaying phytoplankton.

Significant amounts of terrigenous particulate or-

ganic matter are transported from the shelves further

offshore by different processes such as freezing into

sea-ice, ocean currents and turbidity currents (Harms et

al., 2000). Schubert and Calvert (2001) and Belicka et

al. (2002) found evidence for a considerable contribu-

tion of terrigenous compounds to central Arctic Ocean

sediments (up to 30% of organic carbon). However,

most of the terrigenous organic matter accumulates in


coastal zones. Fernandes and Sicre (2000) estimated,

using the constant ratio of high molecular weight odd

n-alkanes to organic carbon (f 11 Ag g� 1) in Ob and

Yenisei rivers, that more than 70% of the organic

carbon preserved in Kara Sea shelf sediments is

terrestrially derived. A different approach using d13Cvalues and assuming binary dilution of riverine and

marine organic carbon led to comparable estimates

(Fernandes and Sicre, 2000; Krishnamurthy et al.,

2001). The amino acid signature also indicates a

primary terrestrial source for organic nitrogen in sur-

face sediments of the Ob and Yenisey estuaries (Neu-

mann, 1999). In the Laptev Sea, sedimentary organic

matter is on average 78% terrigenous, as evident from

maceral analysis (Boucsein and Stein, 2000). Fahl and

Stein (1997) obtained a detailed picture of the organic

matter source in recent sediments of the Laptev Sea

using bulk parameters and molecular biomarkers.

Generally, high C/N ratios of >7 and low hydrogen

indices (HI; after Espitalie et al., 1977) of < 100 mg

HC/gC indicate the dominance of terrigenous organic

matter. High concentrations of long-chain n-alkanes

and long-chain wax esters support this finding. The

terrigenous influence generally decreases with increas-

ing distance from the source to the continental slope.

Despite the dominance of the terrigenous input in the

whole area (up to 99% of total organic material is

terrigenous), relatively high concentrations of diatom-

specific fatty acids indicate high algae production at

the ice edge and in ice-free polynyas. These values are

correlated with high concentrations of chlorophyll a,

phaeopigments and biogenic opal (Fahl and Stein,

1997). The amino acid signature of PAA also indicates

a predominant diatomaceous source for this region

(Dittmar et al., 2001a). Relatively high amounts of

marine organic matter (20–40%) are restricted to

regions of phytoplankton blooms having a fluvial

nutrient supply and open-water conditions at the ice

edge. The composition of macromolecules in sedi-

ments of the Laptev Sea, investigated via CuPy-GC-

MS (Peulve et al., 1996), indicated the preservation of

refractory terrigenous constituents. The presence of

some algal constituents suggested that also a fraction

of marine-derived organic matter is resistant to degra-

dation in the sediments.

In the Beaufort Shelf, Yunker et al. (1995) were

able to use individual sterols and n-alcohols as unam-

biguous markers of terrestrial and marine organic

matter after principal component analysis (PCA).

The Mackenzie River is the dominant source for n-

alkanes, n-alcohols, sterols and triterpenoids from

higher plants. Seasonal marine production of a suite

of alkenes, sterols and alcohols from phytoplankton

and zooplankton is evident in water column and

sediment trap samples, but these labile compounds

tend not to be preserved in surficial sediments (Yunker

et al., 1995). Goni et al. (2000) quantified and sub-

jected to PCA over 60 different compounds derived

from alkaline CuO oxidation of suspended matter and

sediments from the Mackenzie River and shelf. Lignin

and cutin products as well as stable carbon isotopes

indicate an abundance of terrigenous compounds (50–

80%) derived from non-woody vascular plants in shelf

sediments. Proteins, polysaccharides and lipids, which

are primarily derived from plankton, likely from dia-

toms, are mainly present in the outer mid-shelf. In

general, the CuO biomarker signature is highly vari-

able on the Beaufort shelf, which highlights the

heterogeneous nature of the particle load exported by

the Mackenzie River. Some of this variability may be

related to seasonal changes. Yunker et al. (1993) found

that the hydrocarbon composition in sediments of the

Mackenzie River is linked to the relative amount of

peat material, which is mobilized at freshet.

Although the terrigenous material seems to be a

major fraction in particulate matter in the Arctic

estuaries there are some controversial interpretations.

Zegouagh et al. (1996) suggested from the signature

of carboxylic acids that microalgae, which underwent

substantial bacterial reworking, contributes the major

fraction to sediments of the Lena delta and Laptev

Sea. Zegouagh et al. (1998) interpreted the abundance

of long-chain, odd n-alkanes as not being evidence for

a major contribution of higher plant waxes. They

deduced a major dilution of terrestrial input due to

high primary production in the summer, promoted, in

particular, by the nutrients provided by the Lena

River, and a high level of degradation of organic

matter transported by the Lena River.

4. Conclusions and research perspectives

Organic matter concentrations in the Arctic rivers

are among the highest reported in world’s rivers.

Nutrients, on the other hand, are present in very low


concentrations, in particular nitrogenous compounds

and phosphate. The stable stratification on the Arctic

shelves, caused by the huge freshwater discharge of the

rivers, furthermore hampers upward transport of regen-

erated nutrients. Therefore, nutrient concentrations are

very low in the photic zone, but increase with depth. In

the spring, patchy phytoplankton blooms are present

where the ocean becomes ice-free. There, nutrients are

rapidly consumed and lost via sinking particles. How-

ever, regeneration in the photic zone is still sufficient to

enable diatoms to consume the considerable silicate

supply from the rivers. Phytoplankton-derived organic

matter is labile and rapidly turned over in the Arctic

Ocean. Riverine organic matter, in contrast, is refrac-

tory and mainly composed of soil-derived humic sub-

stances. Nitrogen and phosphorus are principally

discharged in organic compounds. However, this dis-

charge has probably no direct influence on marine

productivity, since terrigenous organic matter seems

to be highly resistant to degradation in the Arctic

Ocean. Suspended organic matter discharged by the

rivers is removed from the water column mainly in the

estuaries and shelves. However, due to the low pro-

ductivity of the Arctic Ocean and the differences in

labile versus refractory character for marine and terrig-

enous organic matter, the terrigenous signature can be

Fig. 10. Abundance of terrigenous organic carbon in sediments and dissol

Average values for the Laptev Sea (Boucsein and Stein, 2000; Kattner et al

et al., 2000), maximum and minimum ranges for the central Arctic Ocean (O

distance from coast in the Kara Sea (Krishnamurthy et al., 2001). Percenta

data for the Laptev Sea (grain%).

found even in sediments of the central Arctic Ocean

(Fig. 10). Most of organic carbon is discharged by the

rivers as dissolved organic matter. There are practically

no removal mechanisms for dissolved organic matter in

the estuaries and shelves, and the proportion of terrig-

enous dissolved compounds in the Arctic Ocean is an

order of magnitude higher than in the Pacific or

Atlantic. The organic signature of dissolved organic

matter in the Arctic Ocean can largely be explained as a

conservative mixture of refractory compounds of

allochthonous marine and terrestrial origin.

The total amount of organic matter discharged by

rivers into the Arctic Ocean is similar to that of the

Amazon. Because of the practical lack of removal

mechanisms, in particular photodegradation, the Arc-

tic river discharge plays an important role as a

contemporary sink in the global carbon cycle. How-

ever, terrigenous dissolved organic matter has a short

residence in the Arctic Ocean prior to export the

Atlantic Ocean (probably < 10 years; Opsahl et al.,

1999), and the reactivity of the organic matter may

increase entering surface waters of lower latitudes.

The vegetation of the drainage areas has probably no

considerable influence on the chemical composition

of dissolved organic matter, with the exception of

lignin-derived phenols, which can be used as chemo-

ved organic matter for the Arctic shelves and central Arctic Ocean.

., 1999), Kara Sea (Schubert and Calvert, 2001), Beaufort Sea (Goni

psahl et al., 1999; Fernandes and Sicre, 2000) and the decrease with

ges are on an organic carbon basis, with the exception of sediment


taxonomic tracers. It can therefore be speculated that

changes in vegetation due to climate warming may

not necessarily come along with compositional

changes of dissolved organic matter discharged into

the Arctic Ocean. Calculations of Lobbes et al. (2000)

furthermore indicate that the vegetation in the catch-

ment areas does not considerably influence carbon

export rates, despite the enormous difference in car-

bon fixation rates between taiga and tundra.

However, biogeochemical information and dis-

charge data for nutrients and organic matter are so

far rough and base mostly on single-point measure-

ments. Reliable discharge data are essential to assess

future changes and the possible influence of vegeta-

tion. Long-term collection of basic data, in particular

concentrations of nutrients, organic matter and its

principal chemical constituents over the seasonal

cycle, is urgently needed. Such data would enable

us to compare discharge dynamics of drainage basins

with different types of vegetation and interannual

weather variations. On the basis of these comparisons,

future changes may reliably assessed.

Our knowledge on the preservation and cycling of

terrigenous organic matter in the ocean is still sketchy.

We do not yet know how a fraction of natural

dissolved organic matter can persist thousands of

years in the ocean (Williams and Druffel, 1987; Bauer

et al., 1992). It is not so far possible to relate chemical

structures to recalcitrance. The Arctic Ocean provides

a model system for studying marine and terrigenous

refractory organic compounds in the ocean, because of

the huge terrestrial input, the lack of removal mech-

anisms and the comparatively low autochthonous

production. Future research should be focused on

linking the chemical structure of organic matter to

the different sources and diagenetic processes under

varying environmental conditions. Thus, the impact of

the enormous continental flow of organic matter on

the marine and global carbon cycle could be better

assessed, and changes in the catchment areas may be

related to quantity and quality (bioavailability) of

riverine organic matter discharge.

Acknowledgements

We thank Leif Anderson and Ronald Benner for

reviewing the paper and for their very constructive

comments and suggestions. We are also grateful to

Kenia Whitehead and Ralph Engbrodt for valuable

discussions. This work was supported by Deutsche

Forschungsgemeinschaft (DFG grant no. DI 842/2),

the German Academic Exchange Service (DAAD

grant no. D/0103746) and the National Science

Foundation (NSF grant no. INT-0128796).

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