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Cyanobacterial blooms are regarded as a global environmental problem. The role of cyanobacteria in the food web of water bodies is still unclear. We have carried out an experimental study of invertebrate trophic relationships during the bloom of the cyanobacterium Aphanizomenon flos-aquae. The key links of the food web—plankton, microperiphyton, detritus, and benthic invertebrates—were collected for stable isotope analysis of carbon (δ13C) and nitrogen (δ15N) after 20-day exposure in mesocosms simulating the conditions of the near-shore biotope of the Rybinsk Reservoir without cyanobacteria (variant I) and with high abundance of cyanobacteria (variant II). Statistically significant differences in the δ15N values for crustaceans (Cladocera, Copepoda, Asellus aquaticus and Gmelinoides fasciatus), and mayfly larvae have been revealed between the variants: these values were 1–4‰ lower in the variant with cyanobacteria than in the variant without them. The cyanobacteria-induced shifts in the δ15N value for consumers indicate an assimilation of diazotrophic nitrogen and incorporation of cyanobacterial nutrients into their food chains.

New data on the chlorophyll a (Chl a) content in the water of the Volga River reservoirs are presented. In years with different temperature regimes (2017 and 2018), the content of Chl a varied from minimal 2.2–13.8 to maximal 7.6–50.5 and 25.8–104 µg/L, respectively. As before, the Chl a concentration in the Volga cascade decreases from the upstream reservoirs to downstream ones; the maximum is recorded in highly eutrophic Shosha reach of the Ivankovo Reservoir and in the estuarine section of the Oka River in the Cheboksary Reservoir; and the local increase in Chl a is also recorded in tributaries and their mouth areas, protected shallow areas, and near-dam zones. The mean Chl a concentrations characterize the Ivankovo, Uglich, and Cheboksary reservoirs as eutrophic and the Saratov and Volgograd reservoirs as mesotrophic; the trophic status of the Rybinsk, Gorky, and Kuibyshev reservoirs varies from mesotrophic to moderately eutrophic and eutrophic.

The aim of the present study is investigation of the impact of mass development of cyanobacteria and Elodea on population characteristics (abundance, biomass, fecundity) and "health" parameters (the state of embryos, heart rate, and thermal resistance) of aquatic invertebrates in experiments with controlled conditions. In July-August 2018, in four variants of microcosms, the conditions were modeled characteristic of the coastal zone of the Rybinsk Reservoir in the period of mass development of cyanobacteria. Mass species were placed in microcosms of planktonic (with domination (80%) Daphnia longispina) and bottom animals (Unio pictorum, Gmelinoides fasciatus, Asellus aquaticus), as well as Elodea canadensis. In the variants with cyanobacteria, the toxic microcystins LR were revealed (24-53% of the sum of toxins, or from 6.6-66.5 μg/L). The presence of cyanobacteria (Aphanizomenon flos-aquae, Microcystis aeruginosa, and Gloeoitrichia spp.) influenced structural parameters of plankton: the quantity to green algae decreased, small-sized crustaceans increased. Influence of cyanobacteria caused reliable changes in parameters of aquatic animal from those of the control: thermal resitance of mollusks and amphipods decreased, time of restoration of heart rate after loads (salinity test) in mollusks increased; in amphipods, individual fecundity decreased, frequency of abnormalities and of mortality of embryos increased. The presence of Elodea intensified these effects. Thus, mass development of cyanobacteria and Elodea influences ecological-physiological parameters of aquatic animals, decreasing their adaptivity to natural stresses and deteriorating the quality of the progeny.

Data on the chlorophyll a (Chl a) content in water of the Volga River reservoirs sampled at 64–73 sampling stations in the summer period of 2015 and 2016, analyzed using the standard spectrophotometric method, are presented. Chl a content varied from a minimum of 0.9–3.0 to a maximum of 7.6–32.1 μg/L in June 2016, from 2.1–15.5 to 9.4–86 μg/L in August 2015, and from 1.1–6.7 to 15.1–62.7 μg/L in August 2016. A local increase in Chl a occurred in the near-dam areas of the reservoirs, as well as downstream of the confluence with tributaries. The highest Chl a concentrations were constantly recorded in the highly eutrophic Shoshinskii Reach in the Ivankovo Reservoir and at the Oka River mouth in the Cheboksary Reservoir. The average values of Chl a concentrations make it possible to classify the Ivankovo, Uglich, Rybinsk, and Cheboksary reservoirs as eutrophic; the Gorky and Kuibyshev reservoirs as moderately eutrophic; and the Saratov and Volgograd reservoirs as mesotrophic.