Based on field observations carried out during the 2015–2020 summer low water period, the interannual variability of abiotic characteristics and chlorophyll content in the Lower Volga is considered. In years with different thermal conditions and water content, the temperature, transparency, color and electrical conductivity of water are characterized by small variability and demonstrate changes from north to south, according to the zonal features of the Volga cascade. The average nutrient content (0.81–0.99 mg/L Ntot and 101–134 μg/L Ptot) changes insignificantly in the Saratov and Volgograd reservoirs, but decreases in the unregulated lower part of the Volga. The content of N-N\({\text{O}}_{3}^{ - }\) and P-P\({\text{O}}_{4}^{{3 - }}\) in the total nitrogen and phosphorus pool respectively, is 4–9 and 69–74%, the ratio Ntot/Ptot 10 indicates a possible nitrogen limitation of phytoplankton. The Chl a content corresponds to the mesotrophic category in the Saratov and Volgograd reservoirs (5.3 ± 0.6 and 7.2 ± 0.9 μg/L), and eutrophic in the lower section (13.9 ± 1.5 μg/L). The trophic status of the Lower Volga has not changed in comparison with the last decade of the 20th century. It was found that abiotic factors have a weak effect on the Chl a content in the Saratov reservoir, moderate in the Volgograd reservoir and almost completely control the development of phytoplankton in the unregulated lower part of the Volga (R2 = 0.21, 0.59, and 0.91). The data obtained supplement the observations of previous years and form the basis for long-term monitoring of ecosystems of large artificial reservoirs.

This chapter contains a comprehensive description of the Volga River, the largest river in Europe, and its main tributaries, the Kama, Oka, Sheksna. It starts with a review of human history and biogeographical setting including paleogeography of the basin, as well as description of physiography and climate, and features of hydrological and biogeochemical regime in the past and present. Problems of the management, conservation, and economic importance of the basin are also under consideration. Much attention is paid to aquatic and riparian biodiversity. These sections include description of the species composition of macrophytes, algae, aquatic invertebrates, and fish with an assessment of the abundance, seasonal, and long-term dynamics of biological communities. Data on invasive species from Ponto-Caspian and Boreal-Arctic complex are given. Molecular studies performed during the last 10 years significantly clarified fauna composition as well as phylogeographic patterns.

The interannual and long-term variability of abiotic characteristics and chlorophyll content is studied in reservoirs of the Middle Volga River by field observations in the summer period of 2015–2020. Water transparency increases from 1.2 ± 0.1 m in the Gorky Reservoir to 1.5 ± 0.1 m in the Kuibyshev Reservoir and electrical conductivity from 206 ± 2 to 315 ± 7 µS/cm, respectively. Water color decreases from 53 ± 1 to 38 ± 1; all these features reflect the pronounced zoning of the Volga River cascade. The water temperature depends on local weather conditions. The concentration of P tot increases from 68 ± 3 μg/L in the Gorky Reservoir up to 145 ± 7 μg/L in the Kuibyshev Reservoir; the concentration of N tot (1.08 ± 0.05–1.14 ± 0.06 mg/L) changes insignificantly. The content of Chl a averages 10.9 ± 0.7, 26.7 ± 3.9, and 9.2 ± 1.0 μg/L in the Gorky, Cheboksary, and Kuibyshev reservoirs, respectively; this parameter ranges widely and is characterized by interannual variability. The trophic status of the Gorky and Kuibyshev reservoirs in different years varies from mesotrophic to moderately eutrophic and eutrophic; the Cheboksary Reservoir is characterized as eutrophic throughout the entire 6-year period. The low correlation coefficients between the content of Chl a and abiotic characteristics indicates the complex and multicomponent influence of external conditions on the phytoplankton development.

The content of Chl a and nutrients in the Upper Volga reservoirs in the summer of 2015–2018 has been studied. With Chl a content typical of the summer phytoplankton maximum (from 19.9 ± 3.9 to 48.0 ± 12.0 μg/L), the reservoirs are classified as eutrophic; the Rybinsk Reservoir was classified as mesotrophic only in cool 2017 (10.3 ± 4.7 μg/L). The content of nutrients (from 0.71 ± 0.06 to 1.54 ± 0.07 mg/L Ntot and from 51 ± 12 to 106 ± 3 μg/L Ptot) also corresponds to the eutrophic status of the Upper Volga reservoirs. The average values of the Ntot/Ptot ratio (11–16) indicate the absence of the biogenic limitation of phytoplankton, and a deficiency of nutrients was recorded only in the Rybinsk Reservoir in August 2015 and 2016 and the Ivankovo Reservoir in 2015. In most cases, low values of correlation coefficients (r < 0.7) were noted between the Chl a and nutrients, indicating the complex and multicomponent effect of the latter on phytoplankton. Multiple correlation analysis confirms the high importance of hydrological conditions for the development of autotrophic plankton of the Upper Volga reservoirs.

We studied the distribution of cyanotoxins and potential producers, as well as the variability of microcystin to biomass parameters (chlorophyll-a; MC/Chl-a; and biovolume; MC/BV) in 12 drinking water reservoirs of the world's largest reservoir system, the Volga-Kama-Don cascade (European part of Russia) during the summers of 2016 and 2018. MC concentrations varied from below 0.1 mu g L-1 in June up to 16.4 mu g L-1 in August and exceeded 1 mu g L-1 in 25 % of the samples. This MC variability was associated to changes in the abundance of widespread bloom formers such as Microcystis spp., Dolichospermum spp. and Planktothrix agardhii. Ratios of MC/Chl-a and MC/BVcyano ranged up to 0.88 mu g mu g(-1) and 4.5 mu g mm(-3), respectively. Together with microcystin profiles MC/BV eya ,, o ratios characterized cyanobacterial populations along the reservoir cascade and they indicated a potential toxin hazard better than MC/Chl-a. The neurotoxin anatoxin-a was observed only in the most southern and hypereutrophic Tsimlyansk Reservoir (maximum 0.01 mu g L-1). Toxin gene analysis revealed that MC mostly originated from Microcystis and Dolichospermum. During their co-existence up to 14 MC congeners cooccurred. Cuspidothrix issatschenkoi cf. Rcuphidiopsis mediterranea was identified as possible neurotoxin producers.

The paper deals with the protective function of latent coverings in Moina macrocopa egg at the postpause stage. All egg coverings are found to serve as a barrier for toxic substances of the bottom silt, and CuSO4, CdSO4 and K2Cr2O7 as well. This barrier is rather strong one. The selective permeability is saggested to be characteristic of egg coverings at the dispersal stages in other paleolinmetic invertebrates.