The study is aimed at determining the features of seasonal variability of the content of dissolved organic matter under the influence of the process of mass development of cyanobacteria in the Kuibyshev Reservoir. For this purpose, monitoring data from the Pridaminny reach of the reservoir in the period 2017-2022 were used. Water samples were taken monthly from the surface horizon (0.5 m) for the following indicators: chlorophyll a (Chl); biochemical oxygen demand (BOD₅); permanganate oxidizability (PO) and chemical oxygen demand (COD). In the summer of 2021, the following were additionally determined: structure, abundance and biomass of phytoplankton. It was found that the average annual content of dissolved organic matter is: 1.6 mgO/dm³ for BOD5; 7.5 mgO/dm³ for PO and 25 mgO/dm³ for COD. Seasonal variability includes four periods, which differ in the directions of changes in the dissolved organic matter content. In the future, the problem of organic pollution of the Kuibyshev Reservoir will only worsen in the context of the intensification of the MRC process due to global warming.

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