Abstract
Floodplain lakes play a significant role in maintaining biological diversity and providing a food base for aquatic organisms. In 2023–2024, for the first time, we studied phytoplankton of five floodplain lakes of the transboundary Irtysh River in Kazakhstan. A total of 149 species and forms of planktonic algae were recorded, with a low level of similarity between the lakes. The ratio of indicator species (predominance of eutraphents and meso-eutraphents), abundance (3301.6–168,961.1 thou. cells L−1), biomass (2.41–83.67 mg L−1) of phytoplankton communities, and composition of dominant phyla and species (Cyanobacteria: Microcystis pulverea, M. aeruginosa, Aphanizomenon flos-aquae; Chlorophyta: Volvox globator; Dinoflagellata: Ceratium hirundinella and others) testified to a high level of organic pollution of floodplain lakes. Chemical variables (nitrogen compound content, PI) supported this conclusion. Analysis of the RDA revealed that the biomass of Cyanobacteria was controlled by nitrate nitrogen, while phosphates controlled that of Chlorophyta. The applied integrated approach showed an improvement in the trophic status of lakes in a high-water year and can be useful in assessing the ecological state of aquatic ecosystems in other regions.