Hydrological, Hydrochemical and Hydrobiological Studies of Lake Donuzlav (Western Crimea, Black Sea) Based on the Results of Expeditions in 2018

Abstract

The ecosystem of unique Lake Donuzlav has been under significant anthropogenic pressure over a long period. Therefore, to assess the long-term changes occurring in the lake, regular monitoring studies are required. In June and October 2018, complex hydrological, hydrochemical and hydrobiological studies were carried out in the lake water area. Temperature, salinity, pH, dissolved oxygen content, biochemical oxygen demand on the fifth day (BOD5), permanganate oxidizability, concentration of silicon, mineral and organic forms of nitrogen and phosphorus, abundance and biomass of phytoplankton were determined by standard methods. It is shown that the thermohaline structure of waters directly affects the distribution of hydrochemical and hydrobiological indicators. In shallow water, a low oxygen content was recorded due to significant heating and poor water exchange, but no oxygen deficiency was found. Mussel and oyster farms and sewage have a significant impact on the ecological state of the lake. Especially high values of BOD5, oxidizability, maximum concentrations of organic forms of nitrogen and phosphorus were noted in these places. It was found that the biomass of phytoplankton increases and the species diversity decreases with distance from the sea to the apex part of the lake. Potentially toxic alga Dinophisis sacculus is found in small quantities, which does not affect the nutritional value of the molluscs. To assess the long-term changes in the phytoplankton community and its food reserves, regular monitoring studies of Lake Donuzlav are required in different seasons.