The paper deals with the patterns of Cyanobacteria long-term succession at different stages of existence of the Kyiv Reservoir. The main ecological factors responsible for Cyanobacteria succession include current velocity, hydromorphological characteristics of the reservoir, the content of inorganic nitrogen (primarily of ammonium) and phosphorus, ΣN : P ratio, global climate changes, and the competition with higher aquatic plants in the shallow water zone. The trophic state of the hydroecosystem changed from mesotrophic (at the stage of the non-regulated river) to eutrophic (at the present time) depending on the intensity of Cyanobacteria development. With possible further deterioration of water quality, possible climate changes with an increase in water temperature within the range of even 1-2°C, the intensity of Cyanobacteria development will increase, especially in the pelagic zone of the reservoir, where they have practically no natural biological competitors.