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The structure of phytoplankton during the under-ice period in northern oligotrophic lakes (Lake Krivoe and Lake Krugloe) has been considered and its level of development during ice melt is assessed. In winter 2019–2021, cyanobacteria, diatoms, and cryptophyte algae were noted in phytoplankton. The phytoplankton spring increase in Lake Krivoe was due to the development of the dinophyte Peridinium aciculiferum and, in Lake Krugloe, to the diatom Aulacoseira subarctica. Biomass varied from 0.003 to 0.083 mg/L in winter and up to 0.65 mg/L in spring. The spring peak (April–May) of phytoplankton is associated with the development of dinophyte algae and exceeded summer values. The average Chl a concentration in plankton reached 0.3–0.4 μg/L. The cryoflora contained algae frozen into the ice in an inactive state (0.003–0.04 mg/L). The development of phytoplankton under the ice makes an important contribution to the productivity of northern lakes. [ABSTRACT FROM AUTHOR]