Dynamics of the composition and structure of phytoplankton communities in the Velikaya River in the period from 2016 to 2021

Freshwater phytoplankton is sensitive to environmental changes; therefore its monitoring is a crucial aspect of ecological, recreational and economic importance. Examination of the multi-year alterations in phytoplankton populations is of paramount importance, as it enables to elucidate the underlying drivers of the change and to formulate projections for future scenarios. The present work studies the dynamics of the dominant species of summer phytoplankton in the Velikaya River delta from 2016 to 2021, including the relationship between the species composition and the quantitative characteristics of phytoplankton with changes in pH and water temperature. In general, there was a tendency of decreasing the species diversity and the quantity of phytoplankton in the delta of the Velikaya River during the study period. In 2020 and 2021, there was a sharp decline in the number/biomass of the phytoplankton species that usually dominated in the river. These changes first affected green microalgae and cyanobacteria (2020), and then diatoms (2021). During the same period (2020–2021), the largest decrease in cell numbers was observed in the cyanobacterium Aphanocapsa delicatissima , which allows us to use this species as an indicator of the status of the phytoplankton in the Velikaya River delta. Water temperature and pH significantly influenced the numbers of cells of the dominant phyla (Bacillariophyta, Chlorophyta, and Cyanobacteria) and correlated weakly with the species composition. In particular, water temperature and pH influenced the numbers of cyanobacterial cells, indicating the importance of these factors in the initiation of blooms in water bodies and suggesting that the effect is enhanced by their simultaneous action.