Eutrophication of the Volgograd Reservoir: impact of climate transformation or successional processes?

The changes in the main nutrients – phosphorus and nitrogen, as well as the quantitative and structural characteristics of the phytoplankton of the Volgograd Reservoir in 2001–2021 are shown. Under the conditions of global climatic transformation, we recorded changes in the thermal regime, intra-annual redistribution of water runoff, decrease in precipitation, negative trends in dissolved ammonium and nitrates. In the genesis of phosphates, the role of internal water processes decreased and the influence of hydrological factors increased. Negative trends in the total biomass of phytoplankton, as well as diatoms and green algae, have been found. At the same time biomass of Cyanobacteria increased. The ratio of taxonomic groups of phytoplankton has changed: the proportion of diatoms has decreased with an increase in cyanobacteria, cryptophytes and dinophytes. These negative trends in pelagial can be considered as the processes of regressive succession inside the biggest cyclic processes, and could be caused by changing climatic conditions. At the same time, the increase in the biological activity of benthal communities (higher aquatic vegetation) indicates the ongoing eutrophication of the reservoir according to the macrophyte type.