Structure and Functioning of Plankton Communities in the Rybinsk Reservoir under the Conditions of Climate Change

Based on the data of complex environmental studies, which have been regularly carried out at six standard stations of the Rybinsk Reservoir since the middle of the 20th century, the orientation of changes in the elements of the reservoir ecosystem associated with global climatic events has been analyzed. During the period of climate warming, which began in 1977 and continues into the 21st century, the air temperature in the warm season has increased by 0.9°C, the water temperature by 1.4°C, the average annual inflow by 7.5%, and the duration of the ice-free period by 2 weeks. An increase in electrical conductivity and color of water and a decrease in transparency are noted. With significant interannual variations in biological characteristics, in the 21st century, the number of bacterioplankton has increased 1.7 times and bacterial production has doubled. Chlorophyll content has increased 1.4 times and mean values >15 μg/L reflecting the eutrophic state of the reservoir are observed more often. In the biomass of phytoplankton, the proportion of small cell forms has increased. The total abundance of phytoplankton has increased due to the development of cyanobacteria, which form a long summer maximum in the seasonal dynamics of the community. The increase in water mineralization promoted the progressive spread of alien brackish-water algae. The biomass of zooplankton has increased 2.5 times. An increase in the abundance of crustaceans (Cladocerans by 1.6 times and Copepods by 1.9 times) has caused a change in the structure of zooplankton and the formation of a strong late summer peak of biomass. The intensification of hydrobiological processes was clearly manifested after the abnormally hot 2010, the conditions of which not only stimulated the development of plankton communities, but also caused the formation of oxygen deficiency in the bottom layers. Warming has significantly transformed the ecosystem of the Rybinsk Reservoir, intensified eutrophication processes, and worsened water quality. Changes in hydrometeorological characteristics have gone beyond the mild scenario of climate warming.