Abstract
A comprehensive analysis of the dynamics of hydrobiological communities in the Kazakh sector of the Caspian Sea for the period 2010-2024 was carried out. The study covers three key components of the marine ecosystem: phytoplankton, zooplankton and macrozoobenthos. It was found that the species diversity of phytoplankton decreased by 3.7 times (from 74 to 20 taxa), with the dominance of diatoms (Cyclotella choctawhatcheeana) and blue-green algae (Phormidium angustissimum). Zooplankton is characterized by a sharp increase in biomass – 47.7 times by 2024. This growth is due to an increase in the number of facultative plankters that are resistant to pollution and hypoxia. Macrozoobenthos communities, on the contrary, demonstrate a pronounced decrease in biomass — 5.2 times compared to the initial values. The most significant reduction was recorded among polychaetes (Hediste diversicolor), which may be associated with the deterioration of conditions in the bottom horizons, including pollution, oxygen deficiency and siltation. The obtained data indicate a profound transformation of the trophic structure of the ecosystem associated with a complex of natural and anthropogenic factors, including climate change, oil pollution and invasion of alien species. The results emphasize the need to adapt existing environmental protection measures and develop new approaches to monitoring the state of the Caspian Sea.