Salinity as a Key Factor on the Benthic Fauna Diversity in the Coastal Lakes

SIMPLE SUMMARY: Salinity is a stress factor for benthic invertebrates. Based on a 2-year study of 9 coastal lakes along the southern Baltic Sea, representing freshwater, transitional, and brackish ecosystems, we have shown that benthic fauna was structured by sea water intrusion (=fluctuation of salinity). The increase in salinity gradient resulted in a decreasing trend in the richness and abundance of benthic species, while the diversity showed a slightly positive trend, but below statistical significance (p < 0.05). The abundance of benthic organisms was the highest in brackish costal lakes, where the marine component of fauna was identified. Due to the greatest instability of environmental conditions in lakes periodically linked with the sea (transitional), we found the lowest species number, α-diversity, and abundance, regardless of the season. Salinity appears as a key factor in controlling the functioning of the ecotone (coastal lakes, lagoons) between the marine and terrestrial environments. Salinity is a prerequisite for the proper assessment of the ecological status of coastal lakes and the development of effective methods of protecting coastal lakes. ABSTRACT: Benthic communities were studied in nine Polish coastal lakes of the Baltic Sea; representing three levels of hydrological connection with the sea (isolated, periodically connected, and permanently connected), with resultant differences in salinity (freshwater, transitional, and brackish). The lakes classified in this way allowed us to investigate biodiversity in relation to the degree of environmental pressure. Stress intensity in coastal water bodies, resulting from contrasting marine and terrestrial influences, varied from mild to severe. Spatial variation in environmental predictors affected species richness more strongly than seasonal fluctuations. The broader the spatial salinity gradient, the smaller the species number recorded. Differences in the intensity of natural instability only slightly affected species number and α-diversity. In Baltic coastal lakes, characterized by low salinity (max. 7.5 PSU), benthic faunal communities were dominated by large populations of opportunistic species. This applied primarily to closed systems and those periodically influenced by seawater intrusion. The marine component of fauna played a more important role in increasing the diversity of benthos in permanently open water bodies (brackish). The highest density of benthic fauna was recorded in them, whereas low values were associated with the strongest instability, observed in lakes periodically linked with the sea (transitional).