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Marine Chemistry 83 (2003) 103 – 120
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Review article
The biogeochemistry of the river and shelf ecosystem
of the Arctic Ocean: a review
Thorsten Dittmar *, Gerhard Kattner
Alfred-Wegener-Institut für 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
* 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).
0304-4203/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0304-4203(03)00105-1
104
T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
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
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 availability of a few elements. A major fraction of terrigenous nutrients is bound in organic molecules and
available to most primary producers only after bacterial 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 biogeochemistry 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
Ocean. In Section 3 we focus on natural organic
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 parallel 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 concentrations. 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 ammonium. 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,
T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
105
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).
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
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T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
Fig. 2. Seasonal variations of water discharge (Q), total organic
carbon concentration (TOC = DOC + POC) and silicate concentration 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 temperature, 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 significantly 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 generally 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; Köhler 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 comparable concentrations of DOC and POC (Telang et
al., 1991; Macdonald et al., 1998). Dissolved and
particulate organic nitrogen (DON, PON) concentrations, 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 concentrations generally exceed that of ammonium (Cauwet 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; Nöthig and Kattner, 1999;
Lobbes et al., 2000). Average concentrations in 12
Russian rivers entering the Arctic Ocean are 2.5 AM
for 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 evaluated by Holmes et al. (2000, 2001). Unfortunately,
many of these records are not reliable, and ammonium 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-
T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
107
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
Yenisey
Lena
Ob
Mackenzie
Pechora
Northern Dvina
Kolyma
Indigirka
Taz
Olenëk
Yana
Pur
Mezen
Onega
Nadym
Anabar
Watershed
area (km3)
Discharge
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)
2440
2430
2950
1680
312
348
526
305
100
198
224
95
56
56
48
79
562 – 577
524 – 533
404 – 419
249 – 333
135
106
71 – 98
50
33
32
31 – 32
28
20
16
15
13
4.1 – 4.9
3.4 – 4.7
3.1 – 3.2
1.3
2.1*
1.7*
0.46 – 0.7*
0.24 – 0.4*
–
0.32
0.09
–
0.25
–
–
–
0.17
0.47
0.31 – 0.6*
1.8 – 2.1
–
–
0.31
0.17
–
0.03
0.05
–
0.04
–
–
–
82
80 – 245
66*
27*
44*
35*
16
8.4
–
7.9
2.9
–
4.5
–
–
–
17
54
28* – 54*
160* – 190*
–
–
34
24
–
2.5
4.8
–
3.2
–
–
–
2.8 – 70
3.4 – 46
20 – 40
23.6*
7.1*
6.7*
2.5*
0.18 – 2.3*
0.75*
0.20 – 0.78*
1.2* – 1.7
0.74
0.71* – 1.3
0.99*
0.55*
0.09*
200 – 1223
890 – 1640
311
470*
–
–
–
0.7
–
21
61
–
10
–
–
–
6.0 – 6.9
3.5 – 6.5
7.9 – 23.5
1.5
4.2
2.0
0.76
0.11 – 0.35
2.8
0.03 – 0.23
0.08 – 0.36
3.0
0.27 – 0.44
0.15
2.0
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), Köhler 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), Nöthig 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 concentrations 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
108
T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
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). Verticalmixing 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 comparable to those in the central Arctic Ocean (e.g.
Anderson et al., 1994; Wheeler et al., 1997). Silicate
behaves non-conservatively and exhibits low concentrations in the surface water of the Kara and Laptev
Seas despite considerable riverine inputs. This indicates diatom growth and the availability of regenerated 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 inorganic 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 organic 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 determines the impact of the enormous flux of organically
bound nutrients on the marine ecosystem. Labile
compounds are rapidly available for primary producers 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 Canadian 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, analogous 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, Olenëk 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 gmol 1, and molar
UV-absorption coefficients (254 nm) of 72 – 78
m2mol 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
T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
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 concentrations (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 average) is evidence for an advanced diagenetic state of
lignin (Lobbes et al., 2000). A fraction of TDAA,
however, is probably not strongly associated to terrigenous 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
109
hydrolyzable sugars exhibit any correlation with lignin, 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 decomposition 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 significantly enriched in riverine TDAA relative to freshly
produced organic matter indicating an advanced diagenesis 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 organic matter is highly degraded. Relative to fresh organic
tissue, glucose in the dissolved organic matter of
Russian rivers is highly reduced exhibiting proportions 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-
Fig. 3. Concentrations of dissolved and particulate organic carbon (DOC, POC), and nitrogen (DON, PON), and the proportions of
carbohydrates (TCHO or THNS) and amino acids (TDAA, PAA). Average values and confidence intervals ( p < 0.05) for Russian rivers entering
the Arctic Ocean, the Siberian shelf, and surface waters of the Laptev Sea. As reference for marine-derived organic matter, values for the Arctic
deep-sea (Amundsen Basin) are given for comparison. Data from Dittmar et al. (2001a) and Engbrodt (2001).
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T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
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), Olenëk (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 formation 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 inorganic nutrient, DOC, DON and carbohydrate concentrations 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 Amazon 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 general a very minor fraction of total organic carbon in
the rivers, is constituted primarily of refractory compounds derived from vascular plant detritus, whereas
phytoplankton and living bacterial biomass is negligible in all Arctic rivers (Cauwet and Sidorov, 1996;
Sorokin and Sorokin, 1996). Chlorophyll a/POC ratios are low (Cauwet and Sidorov, 1996), and stable
carbon isotope analyses revealed a predominantly
terrestrial source for POC in the Russian rivers
T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
111
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 confirmed by lignin data. The carbon-normalized yield of
lignin phenols (Xlignin; Lobbes et al., 2000) is 2.3x
on 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 homogenous composition of riverine particles and soil erosion as one of the main factors controlling particle
composition. Yunker et al. (1991) made similar observations for the Mackenzie River. Particulate hydro-
112
T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
Fig. 7. Average mole percentages of individual dissolved neutral
sugars (THNS) in seven Russian Rivers and corresponding nearshore 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).
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). Accordingly, 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
(Goñi 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 refractory soil-derived compounds. Taiga is the main veg-
etation of northern and central Eurasia and is
characterized by coniferous woods and bogs. Adjacent to the north, treeless permafrost tundra dominates. 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 chemotaxonomic 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 significantly 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. 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.
T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
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. However, a significant correlation between the chemical
signature of the particulate organic matter and watershed 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 Köhler et al. (2003, Kara Sea) reported, in
general agreement, a tight fit of DOC concentrations
with the conservative mixing lines for Russian estuaries and shelves. No losses or gains of DOC are
evident. These findings are consistent with laboratory
experiments with dissolved organic matter from Yenisei, which showed no loss after mixing with marine
113
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 (Köhler et al., 2003: crosses).
waters, and only very minor losses due to bacterial
consumption (Köhler et al., 2003). For the Canadian
Mackenzie River, Macdonald et al. (1998) also suspected that riverine DOC simply transits the shelf to
enter the surface pool of the interior ocean.
Single datapoints, however, may significantly differ from conservative mixing. The possibility that a
minor fraction of terrigenous organic matter is reactive 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 conservative 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 behavior of salts and organic solutes. Additionally, autochthonous 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
114
T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
(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) analyzed 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 phytoplanktonderived compounds. Systematic increases of D-enantiomers in phytoplankton-derived DON and the general finding of low TDAA yields in marine DON also
point towards a fast microbial turnover of phytoplankton-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 compared to other world oceans.
Besides some patches where the release of dissolved organic matter from phytoplankton was evident, 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 (Engbrodt, 2001) and amino acids (Dittmar et al., 2001a),
do not vary consistently from the conservative mixing of recalcitrant marine (deep-sea) and riverine
dissolved organic matter. The invariantly high aromaticity of humic acids in the central Arctic Ocean
(Dittmar and Kattner, 2003) also shows the unchanged molecular composition of this major terrigenous 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 compositional 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 Mn c 1000 gmol 1 in the rivers to Mn c 600
gmol 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 retention times and apparently lower Mn values. This
physicochemical modification in the estuaries, however, 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 contribution 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 ultrafiltered 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.
T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
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 salinity 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 AM
in 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 + mineral) 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 particles (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 phytoplankton-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
115
Kara Sea. Peulvé 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 particles suggested mixed inputs of algae and terrestrial
sources. Some fatty acids, phytadienes and n-alkylnitriles were assigned to planktonic organisms while
polysaccharides, phenolic substances, and lipid constituents were attributed to higher terrestrial plants.
With increasing water depths, the proportion of plankton-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 organic 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 offshore in the euphotic zone, indicating microbial biomass and a fast turnover of decaying phytoplankton.
Significant amounts of terrigenous particulate organic 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 contribution of terrigenous compounds to central Arctic Ocean
sediments (up to 30% of organic carbon). However,
most of the terrigenous organic matter accumulates in
116
T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
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 d13C
values 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 surface sediments of the Ob and Yenisey estuaries (Neumann, 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 Espitalié 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 increasing 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 diatomspecific 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 sediments of the Laptev Sea, investigated via CuPy-GCMS (Peulvé 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 degradation in the sediments.
In the Beaufort Shelf, Yunker et al. (1995) were
able to use individual sterols and n-alcohols as unambiguous markers of terrestrial and marine organic
matter after principal component analysis (PCA).
The Mackenzie River is the dominant source for nalkanes, 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). Goñi et al. (2000) quantified and subjected 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 diatoms, are mainly present in the outer mid-shelf. In
general, the CuO biomarker signature is highly variable 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
T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
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 regenerated 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. However, 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 refractory and mainly composed of soil-derived humic substances. Nitrogen and phosphorus are principally
discharged in organic compounds. However, this discharge 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 productivity of the Arctic Ocean and the differences in
labile versus refractory character for marine and terrigenous organic matter, the terrigenous signature can be
117
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 terrigenous 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 Arctic river discharge plays an important role as a
contemporary sink in the global carbon cycle. However, 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-
Fig. 10. Abundance of terrigenous organic carbon in sediments and dissolved organic matter for the Arctic shelves and central Arctic Ocean.
Average values for the Laptev Sea (Boucsein and Stein, 2000; Kattner et al., 1999), Kara Sea (Schubert and Calvert, 2001), Beaufort Sea (Goñi
et al., 2000), maximum and minimum ranges for the central Arctic Ocean (Opsahl et al., 1999; Fernandes and Sicre, 2000) and the decrease with
distance from coast in the Kara Sea (Krishnamurthy et al., 2001). Percentages are on an organic carbon basis, with the exception of sediment
data for the Laptev Sea (grain%).
118
T. Dittmar, G. Kattner / Marine Chemistry 83 (2003) 103–120
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 catchment areas does not considerably influence carbon
export rates, despite the enormous difference in carbon fixation rates between taiga and tundra.
However, biogeochemical information and discharge data for nutrients and organic matter are so
far rough and base mostly on single-point measurements. Reliable discharge data are essential to assess
future changes and the possible influence of vegetation. 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 mechanisms 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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