The paper deals with comparative analysis of the main abiotic factors and phytoplankton taxonomic characteristics in various ecosystems of the Dnieper River, including the head cascade Kyiv Reservoir and the natural Dnieper estuary. Investigations were carried out in summer seasons of 2014-2020. The studied water bodies differ significantly in their physical, geographic, climatic, hydrological, and hydrochemical characteristics. However, they have some common features: both water bodies are large highly productive lentic ecosystems with similar morphometric parameters. As regards phytoplankton taxonomic diversity, the similarities include the same rank distribution of leading divisions and high species richness of summer phytoplankton. A number of differences has also been observed: a higher share of Bacillariophyta in the reservoir and higher shares of Cyanobacteria and Miozoa in the estuary; higher species richness of phytoplankton in the Dnieper estuary compared to the Kyiv Reservoir; a large number of species, occurring only in one of the studied ecosystems - 75 species in the Kyiv Reservoir and 88 species in the Dnieper estuary. The obtained data can serve as a background for assessing an adverse effect of the Russia's military aggression and arising hazards for the Dnieper ecosystems of Ukraine.