The first data on the composition and diversity of the summer community of planktonic protists in Usinsky Bay and the adjacent water area of the Kuybyshev Reservoir, obtained using high-throughput sequencing of the 18S ribosomal RNA gene, are presented. In the composition of the protist community, 1150 operational taxonomic units (OTU) were found; among them, representatives of the Stramenopiles–Alveolata–Rhizaria (SAR) supergroup predominate. In the complete library, Stramenopiles predominate by the number of OTU, represented (in descending order) by Chrysophyceae (8.5%), Opalozoa (8.3%), Diatomea (4.7%), and Oomycetes (Peronosporomycetes) (3.7%); by the number of nucleotide sequences, Alveolata (62.0%) and, first of all, Ciliata (56.9%). There are four interconnected communities of protists: of the river area, the bay itself, the mouth zone, and the reservoir outside the influence of the bay (above the mouth). The communities of the river area and the reservoir area above the mouth have the greatest contrast in composition. In general, the estuarine system studied has features of both ecocline and ecotone. The competitive relationships with cyanobacteria have the main effect on the development of the phototrophic component of eukaryotic plankton. Cyanobacterial “bloom” also affects the structure of the heterotrophic part of the community, although this effect is less pronounced. The development of Archaeplastida, Rhizaria, and minor groups of protists positively correlates with the portion of Metazoa sequences in the samples, which reflects the intensity of the metazooplankton press. Sequences belonging to Bolidophyceae and Rhodelphida, the taxa rare for freshwater, were found. These findings expand the biogeography of these groups that were found so far in very few freshwater reservoirs.

During summer thermal stratification, a broad transition zone with hypoxic conditions is formed in meromictic ferruginous Lake Kuznechikha between the thermocline and the main gradient of water mineralization. In this zone, the chemical composition of water undergoes an ecologically significant transformation due to overlapping gradients of nutrient concentrations and redox conditions. We present an analysis of a strongly vertically structured community of prokaryotic and eukaryotic phototrophs developing in the lake as a whole and especially in the transition zone. In early summer 2009, a sequence of phototrophic organisms with depth in order Chlorophyceae → Chromatiales → Chloroflexales → Euglenales → Chlorobiales was observed in the transition zone, while cyanobacteria were almost completely absent. Biomass maximum of anoxygenic phototrophic bacteria was located between the peaks of phototrophic picoplankton and euglenoids. Such a coexistence of oxygenic and anoxygenic phototrophs in a wide range of depths is highly unusual and sharply distinguishes Lake Kuznechikha from waterbodies with sulphide-containing monimolimnion.

The phylogenetic structure of bacterioplankton of the Usa Bay and the adjacent area of the Kuibyshev Reservoir, as well as three hydrologically connected urban lakes of the Kaban system (Kazan), was analyzed using the results of high-throughput sequencing of the V3–V4 hypervariable region of the 16S rRNA gene. In the studied water objects, mass cyanobacterial development was observed, dominated by members of the phylogenetic lines Aphanizomenon/Dolichospermum and Cyanobium and the genus Planktothrix. Alpha- and betaproteobacteria dominated in the heterotrophic bacterioplankton of all stations. A significant proportion of its composition was made up of mixotrophic bacteria with the rhodopsin type of photosynthesis (for example, Ca. Fonsibacter, Ca. Nanopelagicus, and Ca. Planctophila). A characteristic feature of the studied samples was a high proportion of bacteria of PVC superphylum, especially Planctomycetota. The relationship between the composition and structure of bacterioplankton and the composition of the dominant cyanobacterial complexes was assessed, and heterotrophic OTUs assemblages associated with different types of cyanobacteria were identified. The most numerous assemblage is formed around Aphanizomenon–Dolichospermum–Microcystis and mainly consists of bacteria included in the phycosphere of colonial cyanobacteria, as well as representatives of the PVC superphylum. Two small assemblages form around Limnothrix redekei and Cyanobium rubescens and consist of typical planktonic bacteria belonging mainly to the order Flavobacteriales and the family Nanopelagicaceae.

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.