Peculiarities of Phytoplankton Development in the Zaporizhzhya Reservoir under Conditions of Anthropogenic Load

The taxonomic structure of phytoplankton and regularities of inter-annual and seasonal dynamics of its cell numbers and biomass were studied in different sections of the Zaporizhzhya Reservoir during vegetation seasons in 20192021. The highest phytoplankton numbers at all stations were registered during vegetation season in 2020, whereas the highest biomass - in 2019. During almost the whole period of investigations, Cyanobacteria dominated forming water bloom. It has been found that the content of heavy metals (lead, zinc, copper, iron, manganese, and nickel) significantly influences the hydroecosystem of the Zaporizhzhya Reservoir. It has been shown that the content of artificial radionuclides <sup>137</sup>Cs and <sup>90</sup>Sr in phytoplankton of the Zaporizhzhya Reservoir was rather low. The specific activity of natural radionuclides (<sup>226</sup>Ra, <sup>232</sup>Th and <sup>40</sup>K) was 80-320 times higher than that of artificial ones, which is accounted for by their higher concentration in the ecosystem of the Zaporizhzhya Reservoir. The correlation analysis of the relations between hydrochemical, toxicological, radioecological parameters and species richness and quantitative indices of phytoplankton development was performed. The main factors influencing phytoplankton development in the Zaporizhzhya Reservoir at the present time were revealed.