Heterotrophic bacterial activity and fluxes of dissolved free amino acids and glucose in the Arctic rivers Ob, Yenisei and the adjacent Kara Sea

Various parameters of bacterial activity were measured in the Kara Sea system (Arctic) including the freshwater endmembers of the rivers Ob and Yenisei during August/September 2001. Average bacterial production in surface water was highest in the rivers (13.5 mug C l(-1) d(-1)) and decreased towards the estuaries (5.8 mug C l(-1) d(-1)) and the open Kara Sea (2.4 mug C l(-1) d(-1)). Bacterial production in the salinity gradient was significantly correlated to chlorophyll a concentrations (r = 0.78, p < 0.001), indicating a tight coupling between primary production and bacterial growth. Similar to bacterial production mean turnover-rate constants of dissolved free amino acids (DFAA) and glucose decreased from the rivers (1.5 to 1.9 d(-1)) towards the Kara Sea (0.4 d(-1)), In combination with low in situ concentrations of DFAA (< 50 nM) and glucose (< 10 nM) and the immediate stimulation of bacterial growth upon addition of glucose in incubation experiments this suggests that bacteria were limited by the availability of organic carbon and that the supply of substrates governed the distribution pattern of bacterial growth. Mean uptake of DFAA contributed more to bacterial production (24 to > 100%) than glucose uptake (< 10%). Addition of filtered water from the Yenisei to untreated water from the Kara Sea caused no increase in bacterial production relative to the controls, indicating that riverine dissolved organic material is not the primary source of labile compounds for the bacterial community of the Kara Sea. Based on a bacterial growth efficiency of 27% that was derived from bacterial production and respiration measurements, our estimates for the bacterial carbon demand confine the export of surplus primary production from the Kara Sea to the central Arctic.