Elsevier

Marine Chemistry

Volume 83, Issues 3–4, November 2003, Pages 103-120
Marine Chemistry

Review article
The biogeochemistry of the river and shelf ecosystem of the Arctic Ocean: a review

https://doi.org/10.1016/S0304-4203(03)00105-1Get rights and content

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

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

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?

Section snippets

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

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

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 regenerated nutrients. Therefore, nutrient concentrations are very low in the photic zone, but increase with depth. In the spring,

Acknowledgments

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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