Zooplankton of Volga River Reservoirs: Structure, Abundance and Dynamics

The taxonomic structure and spatial distribution of the late-summer zooplankton in the Volga River from the upper Ivankovo Reservoir to the Volga Delta near Astrakhan in 2020–2021 have been studied. Zooplankton biomass in most of the Volga reservoirs is dominated by cladoceran Daphnia galeata and copepod Mesocyclops leuckarti (up to 45–84%). However, different dominant species are observed in the Volgograd Reservoir. Cladocerans Chydorus sphaericus and Bosmina cf. longispina formed there up to 43% of the total zooplankton biomass. Rotifers and Ponto-Caspian copepod Heteroscope caspian are dominant in the Volga River below the dam of the Volzhskaya Hydroelectric Station (56% and 18% of the total zooplankton biomass, respectively). Among invasive species, most abundant are the East Asian copepod Thermocyclops taihokuensis (up to 230 000 ind./m³ and locally >90% of the total biomass) and the North American copepod Acanthocyclops americanus (up to 86 000 ind./m³ and >35% of the total biomass). The maximum abundance of the zooplankton (0.8–1.6 g/m³) is observed in the Upper Volga reservoirs, while the minimum abundance (0.1–0.2 g/m³) is found in the Lower Volga. The maximum abundance of zooplankton (0.8–3.3 g/m³) is recorded in the mouth areas of the reservoir tributaries, while the minimum abundance (0.1–1.4 g/m³) is in the pelagic zone of the reservoirs. A decrease in the zooplankton community biomass from the Upper to Lower Volga is observed in all biotopes. A negative correlation between the community abundance and daily water inflow into the reservoirs and a positive correlation between the community abundance and water temperature are revealed. Long-term variations in zooplankton biomass and their relationship with the trophic state and thermal and oxygen regimes of the water body are discussed.