Spring Picophytoplankton of the Kara Sea

The abundance, biomass, chlorophyll “a” concentration of picophytoplankton, contribution of picoalgae to total chlorophyll “a” and species composition of phototrophic picoeukaryotes (cells size less than 3 µm) were studied during 83 cruise of R/V Akademik Mstislav Keldysh in the Kara Sea in June 2021. The picophytoplankton abundance varied from 0.25 to 4.91 × 10 9 cell/m 3 , biomass–from 0.34 to 2.49 mg С/m 3 . The minimum abundance and biomass were found in the areas adjacent to the ice edge. The contribution of picophytoplankton to total chlorophyll a concentration in photic layer varied from 3 to 28% with maximum found in the stations free of ice more than 5 days. Picophytoplankton was represented by picoeukaryotes and cyanobacteria. The cyanobacteria contribution was low and varied from 0.2 to 1.8%. The Illumina sequencing of V4 region of 18S rRNA gene revealed that in the surface layer Chaetoceros socialis dominated in picoeukaryotes near ice edge during the diatom bloom. In the ice-free areas for more than 5 days small flagellates, such as Micromonas polaris, Phaeocystis pouchetii and Pyramimonas diskoicola , begin to develop in the surface layer after large phytoplankton descends to the lower horizon of upper mixed layer.