Abstract <p>The data were obtained in the eastern Kara Sea in September 2015 and 2022 and in the seasonal ice formation period in the last ten days of October 2023 during the cruise 63, 89, and 92 cruises of the R/V <i>Akademik Mstislav Keldysh</i>. In the ice formation period, phytoplankton was characterized by poor species composition dominated by dinoflagellates and diatoms. The maximum abundance did not exсeed 3.5 × 10⁴ cell/L, and biomass was 5.0–25.2 µg/L, the lowest values in the whole vegetation period in the Kara Sea. Higher phytoplankton species diversity, a two to three times higher abundance, and five to ten times higher biomass were noted in September. Latitudinal areas with different features of the pelagic environment, algae species assemblages, and quantitative characteristics of phytoplankton were outlined. In all observations, the continental slope and the outer shelf boundary were characterized by maximum algae abundance and biomass. The shallow shelf was characterized by high variability of environmental conditions and phytoplankton communities. The phytoplankton assemblage was stable, and seasonal suссession processes were similar in the vast water area of the middle shelf of the Kara Sea, freshened by river discharge, with a surface salinity from 25 to 30 PSU. In September–October, the “depth” maximum in the phytoplankton vertical distribution characteristic for earlier seasonal succession periods was absent.</p>