Ecological State of the Eastern Sivash Waters in Early Summer of 2020

Abstract

The paper studies the ecosystem of the Eastern Sivash in the context of natural and anthropogenic influence. The data collected during the expedition of the Azov-Black Sea Branch of the FSBSI “VNIRO” conducted in early summer 2020 were used as original empirical evidence. Specific features of hydrological and hydrochemical conditions, as well as species composition, distribution, and abundance of ichthyoplankton were indicated. It was shown that the values of pH, nitrates, nitrites, and phosphates did not exceed MPCs established for fishery water bodies within the entire water area under study; the range of dissolved oxygen content was wide: 3.63–9.13 mg/dm3 . By comparison of the environmental status of the biocoenoses during early summer 2020 with the data collected during a similar survey in 2019, it was found that the water temperature at site 4 is lower by 1.9–2.7 °C. Further salinization of the Sivash was recorded: the lowest salinity in 2019 ranged within 32–38 ‰ and the highest one ranged within 83–86 ‰; in 2020, these values were 36–43 ‰ and 91–97 ‰, respectively. In terms of spatial distribution, the range of the increase in salinity was 4.36–16.69 ‰, with the highest value being recorded at site 2 (11.51–16.69 ‰) and the eastern part of site 3 (13.22 ‰). To a lesser extent (by 4.36–6.26 ‰), an increase in salinity was recorded at site 4 and the western part (5.27 ‰) of site 3. The results of plankton net surveys showed that ichthyoplankton was represented almost exclusively by larvae of big-scale send smelt, which had the densest distribution at site 1 at a salinity of 89 ‰, and on the border between sites 2 and 3 at a salinity of 74 ‰. The upper limit of salinity at which this fish reproduction is possible lies within the range 75–90 ‰. The absence of so-iuy mullet eggs and juveniles at a salinity higher than 32–36 ‰ in May – June of 2019–2020 during the peak of so-iuy mullet spawning does not provide a reason to include the Sivash Bay among the Azov Sea areas important for reproduction of this fish species