Proceedings of the International scientific and practical conference ―Science at the Frontier of Civilizations: Challenges and Perspectives‖ (December 27-29, 2025) / Publisher website: www.naukainfo.com. – Helsinki, Finland, 2026. – 252 p.

22 monitoring [1, 2]. Under conditions of intensive technogenic pressure, which is typical for the territories of Eastern Ukraine, bioindication approaches become especially important, as they allow an integrated evaluation of the level of anthropogenic impact on water bodies [3, 4]. Among such approaches, the analysis of algal flora is particularly informative, since algae respond rapidly to changes in the chemical composition of water, the concentration of organic substances, and the trophic status [5, 6]. The Seversky Donets River basin is the main source of water supply for the Kharkiv region; therefore, studying its ecological status is important for both scientific and practical purposes [7, 8]. The study of indicator algal species makes it possible to determine the actual level of water pollution, assess the intensity of eutrophication processes, and characterize the dynamics of water saprobity [9]. Analysis of recent studies and publications. The species composition of algae is formed under the influence of a complex of hydrological and hydrochemical factors. Flowing water bodies, particularly lowland rivers of Eastern Europe, are typically dominated by diatoms (Bacillariophyta), which reflect high water mineralization and constant variability of flow conditions. Diatoms are highly sensitive to the concentration of organic matter, making them important indicators of saprobity. In stagnant water bodies (lakes, ponds, oxbow lakes), the accumulation of organic compounds and reduced water exchange are characteristic, which contributes to eutrophication. Under such conditions, representatives of Euglenophyta, as well as green algae (Chlorophyta), actively develop, as they are capable of inhabiting environments rich in organic matter. The accumulation of biomass and dead remains enhances siltation processes, accompanied by an increase in water saprobity. The system of water quality assessment using saprobity zones (oligosaprobic, β- mesosaprobic, α-mesosaprobic, polysaprobic) is widely used in hydroecology and has proven its effectiveness for water bodies of Ukraine.