Tracing dissolved organic substances and nutrients from the Lena River through Laptev Sea (Arctic)

Dissolved organic substances and inorganic nutrients were determined in the central and northern Laptev Sea north of the Lena delta region in August 1995. These results are discussed with data from the Lena River from July 1994. Enhanced concentrations of dissolved organic matter (DOM) and silicate in the Lena River generate a concentration gradient extending towards the oceanic regions of the Laptev Sea. Several parameters show higher concentrations in a plume of low salinity water (<31.2) across the Laptev Sea shelf and slope and the boundary of the Eurasian Basin. Dissolved organic carbon (DOC) and nitrogen (DON) in the Lena delta region averaged 550 μM C and 11 μM N, about 5 and 2 times higher than in the Laptev Sea, respectively. The C/N ratio of DOM in the Laptev Sea ranged from 18 to 25, half of that in the Lena. The inorganic nitrogenous nutrients and phosphate concentrations were generally low in the Lena delta region. Silicate concentrations were high (60 μM) and about 10 times higher than in the Laptev Sea. Nitrate exhaustion occurred at some stations in the ice free regions, coinciding to some extend with enhanced chlorophyll a concentrations. While inorganic nutrients increased with depth, all components of DOM decreased. Of these, carbohydrates decreased most rapidly. The distribution of DOC is largely controlled by conservative mixing. A correlation between salinity and silicate indicates its riverine input. Most of the silicate values are below the theoretical dilution line between river and ocean reflecting biological uptake. The distributions of carbohydrates and amino acids were quite variable, with the higher concentrations generally associated to low salinity and high chlorophyll values. Lignin phenols, which are unequivocal tracers for terrestrially derived organic matter, were about 23 nM in the Lena delta region, 7 nM in the lower salinity water and about 2 nM in areas less influenced by river runoff. From these values we estimate that the contribution of riverine derived freshwater to the water budget in the Laptev Sea is about 30 to 10%, decreasing towards the Eurasian Basin. Calculation based on this proportion and on the concentrations of DOC in the Lena River and the Laptev Sea indicate that about 60% of the DOC in the surface layer of the Laptev Sea is of terrigenous origin.