Input of organic carbon as determinant of nutrient fluxes, light climate and productivity in the Ob and Yenisey estuaries

Spectral light attenuation profiles and concentrations of total and dissolved carbon (C), nutrients and chlorophyll a (Chla) were studied along transects running from the river mouth to the Kara Sea during late summer 2003 for the Yenisey and fall 2005 for the Ob estuaries. Earth Observation data were used to generate composite images of water color and Chla distribution over the estuaries and the Kara Sea to reveal the spatial impact of the river efflux in terms of optical properties. evels of total nitrogen (N), total phosphorus (P), silicate (Si) and iron (Fe), but low levels of inorganic N and P and Chla were found in the estuaries. More than 90 % of total organic C was in dissolved form (DOC). The high concentrations of DOC, mostly terrigenous, humic compounds, gave extremely high attenuation coefficients for both visible and ultraviolet light. For UV-B, Z10% (the depth at which 10% of surface light remains) was <10 cm, while Z10% for visible light (PAR) generally ranged between 1 and 3 m for both transects. The light attenuation rapidly decreases when the freshwater is mixed with the coastal water outside off the coast. This leads to a strong light limitation and low productivity in the inner estuaries, while the high load of N and P associated with DOC eventually could promote primary production in the Kara Sea and further upstream the coastal current in the Arctic Ocean as the organic matter becomes diluted and photooxidized. On the other hand, the high inputs of colored dissolved organic matter (CDOM) provide an efficient screening of potential harmful UV-radiation over vast areas of the Arctic Ocean. A rising trend of riverine efflux to the Arctic seas is observed, and further increases in freshwater runoff as well as eventual permafrost thawing, will accentuate the freshwater impact in the estuaries and the Kara Sea.