Seasonal Dynamics of Phytoplankton in Lake Kislo-Sladkoe Partially Isolated from the White Sea during Two-Year Meromixis.

Natural or artificial separation of a water body from its parent water area leads to stratification of its water column and the development of a vertical sequence of planktonic communities. Data on succession of algal communities are sparse. Seasonal variability in the composition and abundance of microalgae in different water layers of the semienclosed lagoon called Lake Kislo-Sladkoe in 2021 was studied during stable stratification and bottom anoxia. Species composition and carbon biomass at different depths, as well as the integrated phytoplankton biomass in the water column (B_int), were determined. An analysis of the similarity of the phytoplankton structure in different water layers was carried out. A total of 53 species and/or genera with a predominance of marine forms and 11 higher rank taxa were identified. During the ice-covered period, most of the water column was anaerobic, and the freshwater cyanobacterium Microcystis ichthyoblabe dominated under the ice. After ice thaw, the biomass maximum shifted towards the boundary of the sulfide zone (into the chemocline). In May–early June, the highest biomass was determined by green alga Chlamydomonas sp., while that in June-July was by the diatom Cyclotella choctawhatcheeana. The type of nutrition changed from predominantly autotrophic at the beginning of the season (March–July) to predominantly heterotrophic (August–October). The highest biomass values were recorded in July–September (149.8 mg С/m²) due to the dominance of the predatory dinoflagellate Oxyrrhis marina. A comparison was made with water bodies of a similar type. [ABSTRACT FROM AUTHOR]