CHANGES IN THE DOMINANT SPECIES AND SIZE STRUCTURE OF DIATOMS FROM THE SURFACE BOTTOM SEDIMENTS OF THE SEA OF AZOV OVER THE PAST 20 YEARS

The paper presents the results of diatom analysis of surface bottom sediments selected with a range of 20 years. Samples of surface sediments of the Sea of Azov collected in 1998 (before the salinization period) and in 2019 were studied. The results of the diatom analysis of the bottom sediments showed that the sediments were dominated by spores of diatoms from the genus Chaetoceros and stomatocysts of golden algae. In addition to spores and stomatocysts, diatoms from the genus Thalassiosira sp., Coscinodiscus sp., Ditylum brightwellii, Pseudosolenia calcar-avis and Thalassionema nitzschioides were often found in the surface soils of 1998. In the sediments selected in 2019, medium – and large-sized taxa (Actinoptychus senarius, Actinocyclus octonarius, Thalassiosira sp., Coscinodiscus sp.) began to predominate in frequency of occurrence and abundance. The appearance of these species can be associated with an increase in the average salinity of the waters of the Sea of Azov, since most of the representatives of the above taxa belong to marine and brackish-water-marine species. A decrease in freshwater runoff and an increase in salinity in the Sea of Azov can lead not only to a radical transformation of the native microalgae community, but also increases the risk of alien marine species of plankton and benthos. Large-sized diatoms are less attractive as a food base for zooplankton, which can also lead to changes in the native biota of the Sea of Azov. In general, these dangerous transformations can lead to a sharp decrease in the stability of the ecosystem of the Sea of Azov and irreversible consequences.