The mouth areas of large rivers can serve as a good model of heterogeneity sites with a pronounced trophic gradient to assess the impact of the degree of eutrophication on different plankton communities. The aim of this research was to identify the possible response of the diversity indicators of phyto- and zooplankton communities to trophic gradients in the mouth area of two large tributaries of the reservoir, formed in the Middle Volga River (Russia). Both linear regression models and canonical correlation analysis (CCA) were used to assess the role of abiotic and biotic predictors in the structural organization of plankton communities and to assess the changes in the parameters of the species plankton community structure in the trophic gradient. It was found that the species diversity (Adjusted R² = 0.116) and evenness (Adjusted R² = 0.114) of phytoplankton significantly decreased with an increase in the degree of eutrophication, while the species diversity (Adjusted R² = 0.059) and evenness (Adjusted R² = 0.073) of zooplankton increased. According to the CCA models, electrical conductivity (EC) explained the largest proportion of the observed dispersion. The Trophic State Index (TSI) explained 3.0% of the total variance in the phytoplankton community species structure and 7.8% in the zooplankton one. The variation in phyto- and zooplankton dominant complexes generally corresponded to the well-known patterns of plankton species succession in the gradient of trophic conditions and can be considered as a classic manifestation of the cascade effect in the food chains of freshwater plankton communities. Our results highlight the necessity of studying the mouth river areas, as well as applying an integrated approach to investigating the response of plankton communities to eutrophication processes of continental water bodies.

Gypsum karst lakes are unique water ecosystems characterized by specific habitat conditions for living organisms, including phytoplankton species, as primary producers and mediating biogeochemical cycles in the water bodies. Studies of diversity and structure of phytoplankton communities can be used to identify the specific and typical lake features and plan basin-wide monitoring. The aim of this research was to analyze the structural variables of algocenoses in the small gypsum karstic Lake Klyuchik (Middle Volga basin), atypical for the subzone of mixed coniferous and deciduous forest zone high values of water mineralization (brackish water) and low temperatures. The lake has two water areas, connected by a shallow strait (ecotone zone) and differing from each other in the chemical compositions and physical properties of the water. A total of 133 species of phytoplankton with prevalence percentages of Bacillariophyta (46%), Chlorophyta (24%), and Ochrophyta (11%) were found; α-diversity varied from 4 to 30 specific and intraspecific taxa per sample. According to Spearman’s correlation coefficients, the diversity indices (Shannon, Pielou, Simpson) were mainly determined by the number of dominant species. The uniquely high (up to 130 g/m³) biomass of phytoplankton was noted in the ecotone, on the border between the water column and the bottom. The formation of mono- and oligo-dominant nannoplankton diatom communities with a predominance of the rare species Cyclotella distinguenda Hustedt was demonstrated there. The roles of flagellate algae and cyanobacteria were found to be less significant.

A new species, Cryptomonas uralensis Martynenko, Gusev, Kulizin & Guseva sp. nov., is described from western slopes of the Ural Mountains (Russia) based on morphological and molecular data. Phylogenetic relationships inferred from nuclear SSU and LSU rDNA sequences show that the new species forms a clade with C. tetrapyrenoidosa Skuja emend. Hoef-Emden & Melkonian. Comparison of secondary structures of nuclear rDNA ITS2, including analysis of Compensatory Base Changes (CBC), confirms the separation between C. uralensis sp. nov. and C. tetrapyrenoidosa. Cell morphology and sizes of C. uralensis sp. nov. are very similar to C. tetrapyrenoidosa and C. pyrenoidifera, and C. uralensis sp. nov. may thus represent a species that can only be reliably identified using molecular data.

Three algal species, Cryptomonas loricata, C. gyropyrenoidosa, and C. lundii, identified by light microscopy and molecular genetic methods, have been observed in Russia for the first time. The species are illustrated with original photographs. The current state of the taxonomy of Cryptomonas, representing the largest genus of cryptophytes, is reviewed. The problems of species identification within this genus are discussed, as is the necessity of using an integrative approach for correctly identifying species.

The paper provides information on the composition, structure and dynamics of phytoplankton of three morphologically different tributaries of the Cheboksary reservoir (the Vetluga, the Kerzhenets and the Vishnya rivers). The species composition of algae includes 826 species and intraspecific taxa, the flora was formed by diatom, green and euglena algae. It reflects the watercourses particularities of the southern taiga zone. Based on long-term studies of phytoplankton a change in the composition of algoflora by more than half was noticed, especially in the groups of Charophyta, Cryptophyta, Miozoa, Ochrophyta and Euglenophyta. An increase in quantitative indicators of algocoenoses from oligotrophic to oligotrophic-mesotrophic level in the middle reaches of rivers by mesotrophic-eutrophic level in estuarine areas is shown. In a fifty-year research series a tendency of phytoplankton reduction in size by more than 3,5 times has been established. It reflects the impact of eutrophication and gradual warming of the climate. The period of abnormally hot summer of 2010 led to increased vegetation in the middle reaches of rivers of diazotrophic blue-green algae and dinophytes. Since the early 2000s, gradual penetration and naturalization of both planktonic invasive algae species and representatives of benthic algocoenoses have been noticed. The mentioned changes reflect the nature of phytoplankton rearrangements as the essential component of the biota and emphasize the importance of long-term research with a possibility to predict negative consequences.

Based on electron-microscope studies of the phytoplankton of two left-bank tributaries of the Cheboksary reservoir, 30 species of silica-scaled chrysophytes from the Chrysophyceae class, including 3 species new for the Russian flora and 1 species potentially new for science, have been found. Data on the abundance and biomass of phytoplankton with the role of chrysophyte algae are presented. The dominance of silica-scaled chrysophytes in spring and early summer algocenoses of the Kerzhenets River is noted.