Data of our long-term observations of the bacterioplankton development in the mouth section of the Usinsky bay (the Kuibyshev reservoir) during the summer–autumn periods are presented. The total abundance of heterotrophic bacterioplankton varied within (1.61–7.44)×10⁶ cells/mL, the biomass was 39.01–249.53 µgC/L, which corresponds to III–V water quality classes. Between 2010 and 2017 the total abundance of heterotrophic bacterioplankton was nonmonotonically decreasing, and since 2021 its significant increase is recorded. Among heterotrophic bacteria, single freefloating cells predominated, mainly cocci and coccobacilli, and the proportion of rod-shaped cells increased during cyanobacterial bloom. Picocyanobacteria and picodetritus particles were almost always present in plankton, but their average contribution to the total organic carbon of picoseston was insignificant.

The structure and spatial distribution of picoseston in the Saratov reservoir in July 2011 and August 2014 are described. On average, the total abundance of cells and particles forming picoseston was 1.87±0.73×10⁶ cells (part.)/mL; and the total biomass of picoseston was 43.8±19.4 µg C/L. During the study period, solitary heterotrophic bacteria and phototrophic picocyanobacteria prevailed in the picoseston of the Saratov reservoir, amounting to 77–100% of its total abundance and biomass. The proportion of picodetrital particles was extremely small (on average, 4.1% of the total biomass of picoseston). The high heterogeneity of picoceston distribution (Cv is 130% and 110% for abundance and biomass, respectively) was found in the Saratov reservoir during the study period. On average, the abundance and biomass of heterotrophic bacterioplaknton and picodetritus at stations of the tributary estuaries were slightly higher than in the riverbed. At the same time, the abundance of picocyanobacteria was higher, and the biomass was lower at the estuarine stations compared to the riverbed ones. The total number of bacterioplankton and autotrophic picoplankton in the Saratov reservoir in 2011 and 2014 corresponded to the mesotrophic level of productivity. Picodetrital particles, in contrast to the Upper Volga and Kama reservoirs, make no significant contribution to the planktonic food webs of the Saratov reservoir, at least during the study period.

—The structure and spatial distribution of picoseston in the Saratov Reservoir in July 2011 and August 2014 are described. On average, the total abundance of cells and particles forming picoseston was 1.87 ± 0.73 × 10⁶ cells (part.)/mL; and the total biomass of picoseston was 43.8 ± 19.4 μg C/L. During the study period, single heterotrophic bacteria and phototrophic picocyanobacteria prevailed in the picoseston of the Saratov Reservoir, amounting to 77–100% of its total abundance and biomass. The proportion of picodetrital particles was extremely small (on average, 4.1% of the total biomass of picoseston). The high heterogeneity of picoceston distribution (Cv was 130 and 110% for abundance and biomass, respectively) was found in the Saratov Reservoir during the study period. On average, the abundance and biomass of heterotrophic bacterioplaknton and picodetritus at stations of the tributary estuaries were slightly higher than in the riverbed. At the same time, the abundance of picocyanobacteria was higher and the biomass was lower at the estuarine stations compared to the riverbed ones. The total number of bacterioplankton and autotrophic picoplankton in the Saratov Reservoir in 2011 and 2014 corresponded to the mesotrophic level of productivity. Picodetrital particles, in contrast to the Upper Volga and Kama reservoirs, make no significant contribution to the planktonic food webs of the Saratov Reservoir, at least during the study period.

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.

Data of our long-term observations of the development of bacterioplankton in the mouth section of Usinskii Bay (Kuibyshev Reservoir) during the summer–autumn periods are presented. The total abundance of heterotrophic bacterioplankton varied within (1.61–7.44) × 10⁶ cells/mL, and the biomass was 39.01–249.53 μg C/L, which corresponds to water quality classes III–V. Between 2010 and 2017, the total abundance of heterotrophic bacterioplankton was nonmonotonically decreasing, and since 2021 a significant increase has been recorded. Among heterotrophic bacteria, single free-floating cells predominated, mainly cocci and coccobacilli, and the proportion of rod-shaped cells increased during a cyanobacterial bloom. Picocyanobacteria and picodetritus particles were almost always present in plankton, but their average contribution to the total organic carbon of picoseston was insignificant.

The paper describes the size-morphological structure and spatial distribution of heterotrophic bacterioplankton in the Nizhnekamskoe, Votkinskoe and Kamskoe Reservoirs in August – September 2022. The total abundance of heterotrophic bacterioplankton in the studied reservoirs varied within the range of (1.91–8.74) · 106 cells/mL (on average (3.8±1.2) · 106 cells/mL), biomass – 44.1–190.8 µg C/L (91.0 ± 29.3 µg C/L), which corresponds to the mesotrophiceutrophic level of productivity. The composition of bacterioplankton was dominated by small single free-floating heterotrophic bacteria, mainly cocci and cocobacilli. Unicellular filamentous cells constantly present in the samples during the observation period, and were an integral component of heterotrophic bacterioplankton, despite their small abundance at most stations. In the mouth areas of the tributaries, the number of bacteria was 1.5–2.0 times higher than in the adjacent areas of the reservoirs. This may be due to the fact that mouth areas are a kind of barrier between the tributary and the reservoir, where transformation and accumulation of river flow occur, i.e. they represent ecotones characterized, among other things, by an increase in the abundance of organisms. Aggregated bacterioplankton was mainly associated with detritus particles and made a significant contribution to the total abundance of bacteria, mainly in the mouth areas of tributaries. Picodetritic particles were constantly present in all samples and exceeded the mass of bacterioplankton by 2.8 times, and their greatest number was found in the mouths of tributaries. The water in all reservoirs of the cascade corresponded to the IV (polluted) quality class in terms of the number of bacterioplankton.