Scientists of the Institute of Hydrobiology of the National Academy of Sciences of Ukraine monitored the development of algal blooms in the Kyiv and Kaniv Reservoirs throughout the summer of 2026. Their observations showed that the intensity of the phenomenon varied considerably over the course of the season: while algal blooms were almost absent in different areas of the reservoirs in mid-June, they became much more intense and heterogeneous from mid- to late August, particularly in the Kyiv Reservoir.

The study is being conducted within the framework of the National Research Foundation of Ukraine project entitled “Development of New Scientifically Grounded Approaches and Methods for Controlling Harmful Cyanobacterial Blooms in Large Reservoirs as a Tool for Ensuring the Environmental Security of the State” (State Registration No. 0126U002967).

Throughout the summer, specialists from the Institute of Hydrobiology of the NAS of Ukraine carried out more than five field trips to the Kyiv and Kaniv Reservoirs. The research team included phycologists, ichthyologists, hydrologists, and biochemists.

During the expeditions, the scientists collected water samples for subsequent laboratory analysis, including the determination of chlorophyll a concentrations, a parameter widely used to assess the development of phytoplankton. Some physical and hydrochemical parameters were measured directly in the field. The species composition of the fish fauna was also investigated.

Throughout the summer, the researchers recorded different levels of phytoplankton development, ranging from minor to intensive algal blooms. The composition of the dominant organisms in the water also changed. In addition to cyanobacteria, the samples contained green algae, probably representatives of the genus Desmodesmus. The exact species composition of the phytoplankton will be determined after the collected samples have been examined by specialists in phycology.

Seasonal changes became particularly pronounced in August. While algal blooms were almost absent in various areas of both reservoirs in mid-June, they intensified considerably from mid- to late August. This was most clearly observed in the Kyiv Reservoir, where the blooms were both intense and unevenly distributed.

Changes in phytoplankton development were also reflected in the hydrochemical characteristics of the water. In particular, the scientists recorded low levels of dissolved oxygen. The combination of these factors also affected fish behaviour in the coastal zone, as observed by the researchers during the most recent summer expedition.

During intensive algal blooms, water may acquire a characteristic blue-green or turquoise colour due to pigments produced by cyanobacteria, including phycocyanin. This water-soluble pigment is involved in light absorption during photosynthesis and can serve as one of the indicators of cyanobacterial development.

At the same time, the scientists emphasise that the colour of the water alone cannot be used to determine either the intensity of an algal bloom or its potential harmfulness. Water colour depends not only on phycocyanin concentration but also on the number of cells, water depth and transparency, lighting conditions, suspended particles, and the presence of other pigments.

Assessing the nature and scale of an algal bloom requires laboratory studies, including the determination of phytoplankton composition and abundance, concentrations of chlorophyll a and phycocyanin, and, where necessary, testing for the presence of cyanotoxins. Laboratory analysis of the samples collected throughout the summer will therefore be the next stage of the researchers’ work.

The researchers also point out that not every algal bloom is harmful. Combining regular field observations with laboratory analyses will make it possible to better understand the patterns governing the development of this phenomenon in the large Dnipro reservoirs and to assess its impact on aquatic ecosystems.

The project team expresses its gratitude to the Kyiv Fish Protection Patrol for its cooperation and assistance in conducting the field studies.

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