Spring phytoplankton of the Ural River and Iriklinskoe Reservoir in the low-water period

The qualitative and quantitative composition of phytoplankton of the unregulated section of the Ural River and different types of channelized shoulders of the Iriklinsky Reservoir in spring 2016–2022 was analyzed. It is shown that the decrease in the amount of precipitation, the volume of inflow to the reservoir and its level, the volume of discharge and water exchange in 2019–2022 has a significant impact on phytoplankton communities: species richness, number of species in the sample, abundance and biomass, average individual cell mass, and Shannon index decrease. On average for the reservoir during this period, the share of golden algae in the total phytoplankton abundance and biomass increases significantly. The most striking changes in communities occur in the upper section of the reservoir, characterized by shallower depths and proximity to the main river. No increase in the number and proportion of indicator species of α-mesosaprobic and polysaprobic conditions was observed in the composition of the dominant complex of organisms. In case of significant increase in air and water temperature during the low-water period, changes in phytoplankton indicators characteristic of later stages of eutrophication are observed in the upper section, while in the lower section – for earlier stages. During the low-water period, a statistically significant decrease in the saprobic index values was detected. The trophic status of the river waters and different types of the Iriklinskoye Reservoir, determined by phytoplankton biomass, decreases in the lowwater period. The revealed features of oligotrophization cannot be attributed to positive consequences of water availability reduction. Firstly, the reduction of quantitative and qualitative characteristics of phytoplankton indicates the impoverishment of the food base of planktonic consuls and, indirectly, the food base of fish population, and the reduction of biological self-purification potential. Secondly, the scenario of phytoplankton changes in summer and fall seasons of the low-water period can be quite different and demonstrate negative consequences of water availability reduction.