Characteristic of the Population State in the Southern Invasive Species Dreissena polymorpha (Pallas) in Ivan’kovskoe Water Reservoir

The state of the invasive species Dreissena polymorpha (Pallas) was studied in Ivan’kovskoe (Ivan’kovo) Water Reservoir in 2018–2022 to obtain the quantitative parameters of the mollusk development at the larval and definitive stages in all pleses (moderately wide stretches of a water body that is deeper compared to the surrounding waters) of the water reservoir during different seasons, as well as to show the interannual dynamics of their population size and spatial distribution. In the Ivan’kovo Water Reservoir, the southern invasive species Dreissena polymorpha (Pallas) has adapted rather successfully and has occurred continuously in the subsaline water conditions with a persistent lack of dissolved oxygen in summer in the near-bottom waters, which caused a shift in the mollusk niche down to a depth of 4 m; the mussel did not occur in silty biotopes. Range expansion of the southern immigrant D. polymorpha to the northern regional water bodies is influenced by climate warming, which prolongs the reproductive process of this mollusk. Currently, the end dates of mussel spawning in the Upper Volga water reservoirs approach those in southern water bodies. Eutrophication enhancement due to climate changes may pose a serious threat to Ivan’kovo Water Reservoir. Overall, the mussel population (adult mollusks + veligers) play a significant role in the process of self-purification of the water reservoir.