The species composition of free-living planktonic ciliates and their distribution in the riverbed of the Middle and Lower Volga reservoirs during the end of the hydrological spring of 2016 and low water period of 2017 have been studied. The tendency of decreasing species diversity of ciliates downstream the cascade of the Volga reservoirs is shown and knowledge of the distribution of some rare species and the distribution of the main groups of structure-forming species from the orders Tintinnida and Choreotrichida along the Volga cascade has been broadened. The abundance of ciliates in the area from the dam of the Rybinsk hydroelectric power station to the city of Astrakhan varies from 20 to 4785 thousand cells/m³ and the biomass varies from 0.1 to 254 mg/m³. The highest species diversity and quantitative and productive characteristics of the ciliate communities in the Gorky and Cheboksary reservoirs, as well as in the unregulated part of the Lower Volga, are due to their traditionally higher trophic level. Factors determining seasonal and interannual differences in the distribution of ciliates in reservoirs of the Volga River are discussed.

The paper attempts to link changes in quantitative characteristics of the community of free-living planktonic ciliates and the nature of their vertical distribution with the operation mode of the hydroelectric power station in the reservoir. Low values of the abundance (8–1,560 thousand cells / m³) and biomass (0.1–66.9 mg / m³) of ciliates in the water column, and a fairly wide range of their variation in the reservoir are probably due to the oscillatory nature of the hydrological regime in the regulated river conditions. The vertical distribution of free-living planktonic ciliates of the near-dam section of the Kuibyshev reservoir was studied at all times of the day on all days of the week. The species were distributed throughout the water column to the bottom with some preference for the surface (0–2 m) and medium (4–10 m) layers. There was no clear attachment of individual species to certain layers in the conditions of homothermy. The preference for certain horizons is due to some biological characteristics of the species (for example, peritriches on the coenobia of algae in the surface layers), departure from “blooming spots”, etc. No data in favor of vertical migrations or “turbulent transport” has yet been revealed. The influence of the weekly regime of regulation of the Zhigulevskaya HPS, in which at the end of the working week, the fluctuation amplitude of many hydrological parameters probably decreases, is manifested in an increase in the abundance and biomass of planktonic ciliates at this time.The nature of the daily change in the average ciliates abundance in the water column was the same for a variant of the community in working days and was in antiphase with the change in the community abundance on weekends, namely: the maximum number on working days and on weekends was recorded in the evening (20.00) and in the daytime (14.00), respectively.

The features of the plankton community structure and transformation have been investigated in the lower course of the Kama River in the late autumn 2020. The transformation of organic matter and seston composition of the Kama River was affected by the inflow of the largest tributary (the Vyatka River) and by the operating mode of the Nizhnekamsk hydroelectric power station. Three structural groups of plankton have been defined: the Kama River community (upstream the confluence with the Vyatka River), the Vyatka River community, and the transformed community downstream of the Vyatka River confluence. The waters of the Vyatka River were characterized by the highest plankton biomass, which affected the quantitative indicators of the Kama River community downstream the tributary inflow. The transformation of the planktonic community under the influence of the Vyatka River inflow was manifested in the change of one dominant group (heterotrophic bacteria) to another (diatoms). In general, despite the lower volume of Vyatka runoff compared to the Kama River, 81% of the total organic carbon of plankton on weekends, and 61% on working days, was formed due to runoff from Vyatka. On the contrary, detritus mainly came directly from the Kama River waters (more than 97%). The influence of the Vyatka River on the Kama River ecosystem in its lower course and further on the Volga River was large, increasing even more on weekends as the amount of runoff through the hydroelectric power station reduced.

A comparative analysis of communities of plankton ciliates has been performed for two large clear-water, slightly alkaline lakes belonging to the hydrological natural monuments of the Southern Urals (Russia). The fauna of ciliates was typical for most oligo-mesotrophic reservoirs. However, in addition to tintinnids and strombidiids, small species of the genera Balanion, Urotricha, and Askenasia had higher abundance in the Lake Kandrykul, and large colonial Epicarchesium pectinatum, in the Lake Aslykul. In general, the ciliates abundance in the lakes did not exceed 789 Cells L¹ and 8,978 Cells L¹, the biomass, 45 mg m⁻³ and 70 mg m⁻³, respectively. The Lake Aslykul had higher water mineralization, a greater presence of mineral suspension, and a smaller area overgrown by higher aquatic vegetation; it was characterized by poor species composition, low taxonomic and structural diversity, and lower abundance and biomass of ciliates. The heterogeneity of the lake structural characteristics was less expressed in the Lake Kandrykul, due to the underwater associations of submerged aquatic plants, which smoothed the differences between ciliates communities in different ecotopes. The mixotrophic ciliates in both lakes contributed 20-25% of total abundance and 13-44% of total biomass. The degree of organic pollution corresponded to the (3-mesosaprobic in both lakes as followed by saprobity indicator species.

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.