The use of phytoplankton as an indicator of internal hydrodynamics of a large seaside reservoir – case of the Sasyk Reservoir, Ukraine

The Sasyk Reservoir, one of the largest seaside reservoirs on the northwest shore of the Black Sea, the former river delta currently detached from the sea with a narrow bar of sediments, was investigated for spatial distribution of water quality characteristics with the use of bioindication method. A three day preliminary sampling campaign of phytoplankton held in the summer 2013 revealed that the reservoir's dominating species belonged to moderate temperature, freshwater, low acidic and low alkaline indicative groups. They suggest standing to slow-moving hydrodynamics of water masses, that contain a medium amount of oxygen. The indices of ecosystem status, such as index of organic pollution according to Watanabe, modification of Sládeček, water quality classes based on saprobity, trophic state indicator groups, nitrogen metabolism indicator groups, as well as Shannon index coincided, showing medium polluted Class III (saprobity) eutrophic water with prevailing moderate concentrations of organically bounded nitrogen. Environmentally homogeneous water masses were dominated with local anomalies on the locations of inflows of waters of different physicochemical characteristics from rivers and the Danube-Sasyk Canal, saline waters from the sea, and nitrogen enriched groundwater from intensive agricultural area. Two types of ecosystems were identified, the one connected with the main water body and the other with the shallows. The sampling was performed in the condition of no outflow, but the wind direction and the wind driven internal dynamics of the reservoir were the major influencing factors for phytoplankton distribution. A gradient of water pollution in the direction of the north–south section of the receiving water from the Danube-Sasyk Canal was revealed. Although the reservoir ecological processes seems to be effective enough in metabolizing the incoming pollution, the symptoms of progressing deterioration of water quality (small-cell forms of organisms) indicate the need for further monitoring and proper management. The bioindicative method appeared to be in some cases more sensitive in ecosystem and abiotic factors analysis then a standard physicochemical analysis, which creates a ground for further developing of its predictive potential.