Title screen

The >10 um fraction of surface water in ice-covered and ice-free MIZ regimes of the East Siberian Arctic Shelf (ESAS) was investigated to improve understanding of its source and composition as well as to provide end-member constraints on the planktonic contribution to the underlying sedimentary organic matter. Samples were collected during the SWERUS-C3 2014 expedition in outer shelf open waters (Laptev Sea) and ice-covered conditions (East Siberian Sea). Our analyses indicate a contrasting composition in the two regimes. The stable carbon isotopic composition (?13C) is more enriched in the ice-covered eastern ESAS where the radiocarbon age is also slightly depleted compared to the modern atmospheric value. By contrast, the western ESAS shows opposite trends with more depleted ?13C values but modern 14C ages (Fmod >1). Because the influence of terrigenous organic carbon in these samples is negligible (as documented at molecular level by lignin biomarkers), we interpret these compositional differences as a result of the HCO3- uptake by sea ice algae during carbon fixation due to the restricted access to CO2. This is consistent with the lipid biomarker IP25, unique for specific sea ice diatoms, which was found in relatively high concentration in the eastern shelf. Preliminary identifications of the phytoplankton taxa show also that open waters are characterized by a mixture of dinoflagellates, silicoflagellates and diatoms whereas the ice-covered region is mainly dominated by diatoms. In addition, while the distribution of nitrates in surface waters is fairly homogenous over the shelf, the nitrogen stable isotopic composition (?15N) is more depleted in the western than in the eastern shelf, likely mirroring the difference in phytoplankton taxa. Our results indicate that, as the sea ice reduces due to Arctic warming, phytoplankton assemblages will progressively adjust to the different climate conditions which will affect both trophic chain and the biogeochemistry of pelagic-benthic couplings.