Natalia G. Tarasova

Kuibyshev Reservoir, the largest in the Volga basin, is poorly covered by modern molecular studies. The results of a metabarcoding study of pro- and eukaryotic microbial plankton in its lower section during the summer period are presented. Bacterioplankton composition was typical for most temperate freshwater bodies and characterized by the dominance of cyanobacteria, Pseudomonadota, Bacteroidota, Actinomycetota, and PVC superphylum (Verrucomicrobiota and Planctomycetota), with a somewhat increased proportion of the latter. The protist community was dominated by Cryptista, principally phototrophic, and various ciliates. Several picoeukaryotic groups were newly detected in the reservoir. A relationship between the composition of both bacterioplankton and protist communities and the stage of phytoplankton succession, including the cyanobacterial bloom, was observed. Some inconsistency between the cyanobacterial bloom phase and the structure of other parts of the microbial plankton is obviously due to some temporal delay, spatial station position, and inflow from tributaries. Heterotrophic bacterioplankton indicator species of the main bloom stage include OTUs representing both the phycosphere of colonial cyanobacteria and free-living species. Among the protists, sessile ciliates benefit most from plenty of substrates for colonization, while cyanobacterial grazers and parasites were minor. Overall, the cyanobacterial bloom creates new niches for the plankton community and significantly modifies its structure.

The comparative analysis of indicators of development of various components of plankton community in reservoirs of the Kama cascade in various hydrological seasons, and on various sites (in coastal and waterway) was carried out. The difference in the quantitative development of unicellular pro- and eukaryotes in different areas (coastal and riverbed) was more pronounced than in different hydrological seasons (spring flood and summer low water). Cyanobacteria make the greatest contribution to the formation of the total biomass of phytoplankton in the waterway. The contribution of heterotrophic component (bacteria and ciliata) is more significant in the coastal area. It was shown the significant correlation of the biomass of unicellular plankton and picodetritus particles, which was present in considerable amounts in the waters of the Kama reservoirs cascade.