<p>The dynamics of the layered hydrological structure was monitored in the meromictic Lake Elovoe, which was separated from the subarctic White Sea due to postglacial uplift. Within its water column, the following layers were identified: freshwater mixolimnion, brackish aerobic layer, chemocline, and anaerobic monimolimnion. The taxonomic composition of phytoplankton, its carbon biomass in different layers, and seasonal dynamics of biomass were investigated from March to October 2021. The chemocline exhibited the highest phytoplankton biomass throughout the year. In the surface water layer biomass peak occurred in summer, when diatoms proliferated en masse. The seasonal dynamics of phytoplankton abundance showed two increases: a spring increase in late May and a summer-autumn increase in late August-early September. The seasonal trend of integrated biomass dynamics and changes in predominant feeding modes were mainly shaped by the predatory dinoflagellate <i>Oxyrrhis marina</i>, which developed in the chemocline during winter, spring, and autumn. Autotrophic algae predominated in summer in both the surface layer and in chemocline. This is consistent with the data of underwater illuminance: conditions were dark under the ice in winter whereas during the polar day (June–July), light conditions favored autotrophs. In the chemocline, where always less than 1% of solar energy reached, heterotrophic algae prevailed. Statistical analysis confirmed the separation of communities of the surface freshwater, middle brackish, and chemocline layers. The phytoplankton structure and dynamics were compared between lakes at adjacent stages of isolation.</p>