This study states fundamental patterns of development for species and taxonomic composition, structure and seasonal population dynamics, biomass and informational diversity of phytoplankton in the Basivkut water store. Over the course of this research (June-October, 2022), we-identified 121 algae species represented with 125 intraspecific taxa that belong to 84 genera, 45 families, 29 orders, 12 classes and 8 divisions. Floristic aspect of plankton algae is presented with <i>Chlorophyta</i> (40.1% of the total species number), <i>Bacillariophyta</i> (26.4% ), <i>Euglenozoa</i> (13.2% ) and <i>Cyanobacteria</i> (12.4% ). The population of phytoplankton in the Basivkut water store ranged from 4654 thousand cells/dm<sup>3</sup> to 14212 thousand cells/dm<sup>3</sup> (August), and the population of biomass ranged from 0.8732 mg/dm<sup>3</sup> (July) to 9.4828 mg/dm<sup>3</sup> (October). The Shannon index fluctuated between 0.62-5.07 bit/mg according to biomass, or 2.13-4.50 bit/sp. according to the population. The average values of the saprobic index ranged from 1.74 to 2.14, corresponding to the third class of water quality (poorly polluted). The author highlights the increasing <i>Cyanobacteria </i>development, whose population and biomass reached 86.1% and 33.2% causing intensive algal blooms in all summer months and is the impact of algal groups, on the anthropogenic factors effect and climate changes. <i>Aphanizomenon flos-aquae </i>(10.5-32.2% and 10.6-12.4% ) and <i>Cuspidothrix issatschenkoi</i> (11.6-31.2% and 10.2-14.5% ) have dominated according to their population and biomass. <i>Bacillariophyta</i> impacted on population (39.5-74.0% ) and biomass (88.7-98.4% ) in autumn. Small cell type <i>Stephanodiscus hantzschii </i>has been characterized with significant population (16.9-65.4% ) and biomass (51.1-92.9% ) increase in the Basivkut water storage.