Formation of summer deficiency of dissolved oxygen in the water of the Kuibyshev reservoir

The content of dissolved oxygen is the most important indicator of the ecological state and quality of water in the Kuibyshev Reservoir, used for drinking water supply, fisheries and recreation. There is an opinion that the formation of the oxygen regime is significantly influenced by the process of mass development of cyanobacteria. Therefore, a quantitative assessment of the seasonal variability of dissolved oxygen and its deficiency under conditions of massive development of cyanobacteria is very relevant. Systematic observations were carried out at the outlet section of the Kuibyshev reservoir. The research results show that seasonal variability in the concentration of dissolved oxygen includes four periods, distinguished by different directions of the processes of formation of the oxygen regime. Within a year, two minimum levels of dissolved oxygen are observed: winter and summer. During the summer minimum, a deficiency of dissolved oxygen occurs when its concentration decreases to critical values and amounts to 4-5 mg/dm3. The formation of the summer oxygen minimum is significantly influenced by: an increase in water temperature and the process of mass development of cyanobacteria. As a result of photosynthetic aeration, a thin surface layer of water supersaturated with oxygen is formed, which disrupts the process of oxygen absorption from the atmosphere. In conditions of increasing nutrient load and global warming, the intensity and scale of the process of mass development of cyanobacteria in the Kuibyshev Reservoir will only increase, which will lead to a further increase in oxygen deficiency, deterioration of water quality and the emergence of risks for water use and fisheries.