Impact of the process of mass development of cyanobacteria on the formation of dissolved organic substances in the water of the Kuibyshev reservoir

The study is aimed at determining the transformation of seasonal variability in the content of dissolved organic matter (DOM) under the influence of the process of mass development of cyanobacteria (MDC) in the Kuibyshev Reservoir. For this purpose, monitoring data on the Pridamtinny reach of the reservoir in the period 2017-2022 was used. Water samples were taken monthly from the surface horizon (0.5 m) for the following indicators: chlorophyll “a” (Chl “a”); biochemical oxygen demand (BOD5); permanganate oxidation (PO) and chemical oxygen demand (COD). In the summer of 2021, the structure, abundance and biomass of phytoplankton were additionally determined. It has been established that the average annual content of DOM is: 1.6 mgO/dm3 according to BOD5; 7.5 mgO/dm3 according to PO and 25 mgO/dm3 according to COD. Seasonal variability includes four periods, which are distinguished by multidirectional trends. In the first period, the DOM content increases from December to May and covers the winter low-water period before the onset of the spring flood. The second period is characterized by a decrease in the content of DOM and is observed during the spring flood. The third period is characterized by an increase in DOM content due to the MRC process and lasts 2 months during the summer low-water period. During this period, the highest DOM content is observed due to the MRC process. The fourth period is characterized by a decrease in DOM content and lasts from September to December. The MRC process, controlled by the Chl a content, transforms the seasonal variability of the DOM content. In summer (July-August) there is an increase in the DOM content under the influence of the MRC process by 20-25% for BOD5, by 13-15% for PO and by 4-6% for COD. 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.