Structure of the Autotrophic Component of the Biological Carbon Pump in the Western Periphery of the Central Arctic Basin in Summer 2021

In summer 2021, the unique data on a structure of the biological carbon pump were obtained in the near-surface ice zone and in the open water of the deep Arctic Ocean (Fram Strait and southern Nansen Basin up to 82° N) during the eighty-fourth cruise of the RV Akademik Mstislav Keldysh. The interaction of cold Arctic and warm Atlantic waters near the drifting ice edge led to a vertical thermohaline structure affecting the phytoplankton species composition, abundance and biomass. A characteristic feature of the phytoplankton structure was the predominance of large diatom Rhizosolenia hebetata f. hebetata and Rhizosolenia styliformis rather than small species of spring blooming. The study data were obtained for first time in relation to the fact that the mixoplankton trophic strategy prevailed in the phytoplankton biomass formation at most stations of the studied area in the late summer. The main role in the organic matter transfer from small (bacteria and phytoplankton) to large forms was played by dinoflagellates Gyrodinium lachryma and ciliates Mesodinium rubrum confined to the upper water layers (0–10 m). Large autotrophic diatoms, whose biomass correlated with nitrogen concentration (r = 0.44), accumulated near the lower boundary of the euphotic zone. As a consequence, the biological carbon pump was vertically differentiated in the western periphery of the Central Arctic Basin: in the upper layers, it was a system with a high organic matter transformation degree, whereas in the lower layers, it was a system with a high organic matter export.