Nikolenko Y. The features of phytoplankton development of the Zaporizhzhia Reservoir under conditions of anthropogenic load

Currently, the Zaporizhzhia Reservoir undergoes increased anthropogenic pressure. Its priority pollutants are organic substances, biogenic substances, heavy metals and radionuclides. Phytoplankton is a sensitive element of aquatic ecosystems, and therefore the first to respond to pollution. Anthropogenic load can cause changes in the chemical composition of water, including an increase in the concentration of potentially toxic substances; this leads to changes in the floristic structure of phytoplankton, indicators of its abundance and biomass. The study of the features of phytoplankton development under conditions of anthropogenic load allows us to understand the mechanisms of the impact of pollution on these important microorganisms, thus contributing to the development of strategies for the protection and restoration of aquatic ecosystems, as well as the prediction of future changes. The aim of the work was to establish and analyze the features of phytoplankton development in different areas of the Zaporizhzhia Reservoir under conditions of modern anthropogenic load. To achieve this aim, a retrospective analysis of phytoplankton studies of the Zaporizhzhia Reservoir was conducted; literary data on the influence of heavy metals and radionuclides on the development of phytoplankton were analyzed; the taxonomic and ecological structure of phytoplankton in different parts of the Zaporizhzhia Reservoir was established; the peculiarities of the interannual and seasonal dynamics of phytoplankton abundance and biomass indicators in different parts of the Zaporizhzhia Reservoir were clarified; the peculiarities of the distribution and accumulation of heavy metals and radionuclides by phytoplankton in different parts of the Zaporizhzhia reservoir were investigated; by conducting a correlation analysis between hydrochemical, toxicological, radiobiological indicators and qualitative and quantitative indicators of phytoplankton development, the factors affecting the development of phytoplankton of the Zaporizhzhia Reservoir at the current stage of existence were identified. The work was performed following generally accepted methods of hydrobiological, toxicological, radiobiological, and hydrochemical research. Generally accepted statistical methods were used for calculations and data comparison. During the studied period, the phytoplankton of the Zaporizhzhia reservoir was represented by 116 species and 119 intraspecific taxa (IST) belonging to 72 genera, 51 families, 31 orders, 11 classes and 6 divisions (Chlorophyta (41 IST), Bacillariophyta (37 IST), Сyanobacteria (23 IST), Charophyta (10 IST), Euglenophyta (4 IST) and Ochrophyta (4 IST)). It is worth noting that during the studied period, 60 genera that formed 58% of the phytoplankton floristic structure were represented by only 1–2 representatives. This may indicate the influence of several environmental factors on the phytoplankton grouping; the latter is consistent with literature data and typical for anthropogenically polluted water bodies. During three years of phytoplankton research in the water area of the Zaporizhzhia Reservoir, the largest number of species was recorded in the samples taken in the area of the lower part of the reservoir and on the Monastyrsky island; the smallest one was in the Samara Bay. For all sampling points, 39 phytoplankton species were common. In 2019, the phytoplankton groups of the Zaporizhzhia Reservoir in all studied areas were dominated by representatives of the Chlorophyta division, accounting for 46% of the total phytoplankton diversity in the Samara Bay and 60% in the lower part of the reservoir. In 2020–2021, the share of Chlorophyta decreased to 30–38%, while the share of Bacillariophyta increased, from 30% in Samara Bay (2020) to 41% in the Festivalny Wharf area (2020–2021). A seasonal increase in the number of phytoplankton species occurred between April and August-September, varying by sampling point, with a subsequent decrease in October. This trend is mostly associated with changes in water temperature. According to the ecological characteristics, the basis of the phytoplankton of the Zaporizhzhia Reservoir was planktonic (53% of the total), moderate species (61%), β-mesosaprobic species (64%), slow-flowing species (68%), indifferent to salinity species (68%) species with medium pH tolerance (50 %). The highest rates of population indicators, at all research points, were recorded during the growing season of 2020, which exceeded those in 2019 and 2021 by 1.3–1.7 and 1.2–1.5 times respectively. The seasonal distribution of phytoplankton abundance was observed, so the maximum number during the three years of research, at all sampling points, was recorded between August and the beginning of September. The lowest numbers were recorded in April and October. The latter is typical for phytoplankton of the temperate zone and is consistent with similar studies.