Degradation of Individual Biotic and Abiotic Components of the Ecosystem in Southeastern Sivash Bay (Azov Sea) under Long-Term Salinity Changes

Sivash Bay is the world’s largest hypersaline lagoon, which has undergone several changes in recent history due to anthropogenic impact. The Southern Sivash is rapidly transforming (~40 psu salinity rise for 3 years) from a brackish lagoon into an ultra-hypersaline body of water. The purpose of this research was to analyze changes in the state of individual biotic and abiotic components of the Southern Sivash ecosystem under conditions of shifting salinity. The components studied included the bacteriobenthos community (heterotrophic, thionic, and sulfate-reducing bacteria), meio- and macrozoobenthos, and phytoplankton. Additionally, the salinity of the lagoon, the hydrogen index (pH), and the redox potential (Eh) of bottom sediments were analyzed. The study showed that the number of taxa in the meiobenthos community remained unchanged, but the number of organisms decreased almost 12-fold. No correlation was found between the spatial variability of salinity and pH under the studied conditions, while the relationship between Eh and salinity showed a negative trend. From a spatial perspective, it was possible to identify relationships between the state of various biotic components—such as specific bacterial groups (sulfate-reducing and thionic bacteria) and microalgae groups—and the distribution of physicochemical environmental indicators. It can be concluded that no significant changes have been observed in the microbiota of bottom sediments at the level of bacterial groups, despite the temporal transformation of environmental factors. However, the salinity levels have reached critical thresholds for many aquatic organisms, as indicated by the collapse of the macrozoobenthos community, the marked decline in meiobenthos, and the reduced abundance and diversity of microalgae.