Dynamics of local phytoplankton communities in the upper-cascade Dnieper reservoirs under present conditions

Important factors, affecting the spatial and temporal dynamics and structure of local phytoplankton communities in the upper-cascade Dnieper reservoirs include global climate change manifesting in abnormally high water temperatures and the unsteady mode of HPP operation, which is registered in abnormally high or low water level in reservoirs. A strong positive correlation has been found between water temperature and Cyanobacteria and Chlorophyta biomass and strong inverse correlation for Bacillariophyta. Two groups of algae have been distinguished, prevailing under different temperatures: 1) Bacillariophyta—Chlorophyta, when the water temperature is close to climatic norm; 2) Cyanobacteria—Chlorophyta, when the water temperature is abnormally high. Besides, the reduction in phytoplankton cell size is observed at cenotic level. The findings of comparative analysis of retrospective and present data show an increase in Cyanobacteria biomass during the recent decades. High cell count and biomass of Cyanobacteria are recorded in the lower part of the Kyiv reservoir distinguished by large shallow-water areas, which quickly get warm, and in the upper part of the Kaniv reservoir, where Cyanobacteria get during the HPP discharge. This phenomenon can be considered as mass effect – presence of species in untypical localities due to high speed of dispersal from other localities. The phytoplankton responded to abnormal fluctuations of water level by increase in the Sorensen species similarity indices between the local communities under the abnormally high and low levels, as compared with the normal level. When the water level is abnormally high, species migration among the local communities becomes more intense, and the mass effect is observed. During the abnormally low water level some algal species from benthos and periphyton migrate to phytoplankton. Besides, during the abnormally high and low levels, as compared with the normal level, the distance decay of similarity slows down, which can also be regarded a manifestation of mass effect.