The biogeochemistry of Lena River: organic carbon and nutrients distribution

The Lena River is one of the most important rivers flowing to the Arctic Ocean. Draining the Siberian forest and tundra, it is characterized by black waters enriched in organic matter. Compared to other Arctic or subarctic rivers, the Lena River is very similar in the content of ammonia, phosphates, organic nitrogen and phosphorus, but three times richer in silica and nitrate. The distribution of POC, DOC, DIC and suspended matter during two cruises in September 1989 and 1991 was comparable and was influenced by the water input from the river. DOC and DIC exhibit a very conservative behaviour to salinity. The TOC discharge, is on a yearly basis directly connected to water discharge with a maximum during the flood time in June–July. From about 330 μM during the low stage period (November to April), the TOC concentration increases up to 1200 μM during the flood. The organic carbon content of suspended matter depends upon the level sampled and decreases with the suspended load. Surface samples range between 4 and 20% while samples collected in bottom waters are less rich (6 to 3%). Waters from the Buor-Khaya Bay are richer (20 to 10%). The concentrations of the nutrients (SiO 2, PO 4, TDP, TDN, NH 4, NO 3) are different in surface and bottom waters, and vary from summer to winter. Plotted against chlorinity, these parameters exhibit a characteristic behaviour. Silica is always more concentrated in bottom water, decreasing with salinity. Phosphate and nitrate are more concentrated in bottom water, suggesting mineralization of organic matter and regeneration of nutrients. On the contrary, ammonium is more concentrated in surface water. Total dissolved nitrogen, mainly represented by organic nitrogen (DON), is decreasing rapidly in summer at low salinities (0–2%.), and slowly increases seawards. In winter the concentration is not lower but slowly decreases all along the salinity gradient. The behaviour of organic carbon and nutrients are linked to the inputs by the river and marine production and to the degradation step in the sediment.