Indication of Long-Term Changes of Algae Communities in a Hydrologically Transformed Estuary Sasyk, Black Sea, Ukraine

Forty years ago, the transformation of the estuary of the Black Sea to a freshwater reservoir was started by its connection with the Danube River through the Danube-Sasyk Canal. Today, the inflow of the Danube water into the Sasyk is the main component of the water balance, affecting the internal and external water exchange, and it is, in general, responsible for the quality of the aquatic environment. In addition, the channel is a migration route for hydrobionts and the main source of invasion. We describe the changes in the composition of algal communities in Sasyk, which was converted from an estuary to a reservoir during three stages: estuary-lake in 1967-1977 (stage I), the forming of the reservoir from 1980-1990 (stage II), and the reservoir from 2013-2019 (stage III). Average salinity decreased from 7790 to 491 mg L-1 chlorides, and the species richness of algae increased from 259 to 289 taxa during the Sasyk transformation. Analysis of the algal communities revealed that salinity could have an effect on their composition. The species list for the Sasyk, including 586 taxa of algae and cyanobacteria (613 infraspecific taxa), is presented here for the first time. Rare marine and freshwater taxa present in the algal community at the modern stage of the reservoir and their transformation are discussed. An ecological analysis of the different stages of the Sasyk transformation from an estuary into a reservoir was conducted. The increase in species richness for stages II and III along with an increase in desalination was noted. To keep the current ecological status of the reservoir and its hydrological stability, the role of the Danube River should be considered. Bioindicators, statistical analysis of species, and environmental variables' relationships confirm salinity as a major regulating factor that can be observed by way of long-term monitoring.

Automated summary

Forty years ago, the estuary of the Black Sea was transformed into a freshwater reservoir through the Danube-Sasyk Canal. The inflow of the Danube water into the Sasyk plays a crucial role in the water balance and affects the aquatic environment. The changes in algal communities during the transformation stages were studied, and the influence of salinity on their composition was observed. The species list of algae and cyanobacteria in the Sasyk was presented for the first time, and the importance of the Danube River in maintaining the ecological status of the reservoir was emphasized.