Surveys showed phytoplankton in exposed and sheltered parts of the Lake Onego littoral zone to comprise 116 algal taxa of below-genus level. Its species diversity was mainly constituted by diatoms (44% of the total number of species) and green algae (28%). Throughout the open water period, the plankton was dominated by diatoms, which on average contributed 74–97.5 % to total phytoplankton abundance and 33.2–96.4 % to its total biomass. Average phytoplankton amounts in the lake’s littoral zone did not exceed 276,500 cells/l in abundance and 0.52887 g/m3 in biomass in spring, 137,900 cells/l and 0.23581 g/m3 in summer, and 95,500 cells/l and 0.39867 g/m3 in autumn. The biomass of the fine fraction (nanoplankton) was the highest in summer (July).