Abstract
The study presents the results of a comprehensive hydrobiological and hydrochemical investigation of the Caspian Sea water environment within the Mangystau and Atyrau regions conducted in 2024–2025. The aim of this study is to analyze the hydrochemical characteristics of the aquatic environment within the Mangystau and Atyrau regions and to assess their influence on the structure, abundance, and biomass of phytoplankton during 2024–2025. The main water quality parameters were assessed, including pH, dissolved oxygen content, concentrations of biogenic elements (ammonium nitrogen, nitrites, nitrates, phosphates), total iron, and salinity levels. The aquatic environment was found to be characterized by an alkaline reaction and generally favorable oxygen conditions; however, pronounced spatial heterogeneity of hydrochemical parameters was observed. Local exceedances of nitrites, phosphates, and total iron were recorded, indicating areas of increased anthropogenic pressure and potential local eutrophication processes. An analysis of the taxonomic composition, abundance, and biomass of phytoplankton was carried out. A predominance of diatoms, green algae, and cyanobacteria was established, with cyanobacteria dominating in some areas and contributing significantly to total community abundance. In 2025, an increasing proportion of small-celled phytoplankton forms was observed alongside a decrease in biomass, indicating a structural reorganization of plankton communities. The obtained results confirm the necessity of continuous monitoring of the Caspian Sea ecosystem to assess its trophic state dynamics and ecological stability.