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

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 muM C. The total amount of DOC discharged by rivers into the Arctic Ocean is 18 - 26 x 10(12) g C year(-1) and similar to that of the Amazon. The discharge of particulate organic carbon is much lower with 4 - 6 X 10(12) 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 muM; phosphate: 0 - 0.8 muM), with the exception of silicate in some rivers (0.5 - 110 muM). 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. (C) 2003 Elsevier B.V. All rights reserved.