The phytoplankton community and its distribution were investigated in the eastern branch of the St. Anna Trough and over the adjacent Kara Sea shelf in September 2011 and in August 2014. The spatial and seasonal differences in the phytoplankton structure were analyzed in connection with ice melting and changes in the temperature, salinity, and biogenic regime. Four 4 zones were defined: the Kara Sea shelf, the upper part of the St. Anna Trough slope, the lower part of the St. Anna Trough slope, and its deep part. It was shown that over the Kara Sea shelf where riverine runoff influence was observed, the phytoplankton community differs from the community of the St. Anna Trough in both investigated periods: in September and August. Rearrangement of the phytoplankton community was observed in the frontal zone associated with the upper part of the slope in September of 2011. The main features of this rearrangement were a change in dominant species composition and a 1.5-2-fold increase in the number and biomass of algae. Over the lower part of the slope and deep area of the St. Anna Trough in August 2014 and September 2011, the phytoplankton community was at different successive stages: in August, at the late spring and summer stage, and in September, at the summer-autumn stage.
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