Is salinity a driving factor for the phytoplankton community structure of a brackish shallow Mediterranean lake?

Phytoplankton is a well-studied group of organisms that can change rapidly with environmental conditions, providing a reliable response to these changes. We analysed phytoplankton biomass, diversity, and its response to environmental conditions in the shallow brackish Mediterranean Lake Vransko. Although protected as Nature Park, the Lake's succession is enhanced by intensive agricultural activity and an artificial connection to the sea. Analysis of phytoplankton reveals a specific community composition strongly influenced not only by nutrients but also by salinity gradient, with species composition shifting from freshwater to brackish. Conditions of higher salinity support the dominance of brackish species, often with low biomass, while periods of low salinity are characterised by dominance of cyanobacteria or other freshwater species capable of rapidly taking up nutrients and forming algal blooms. Changes in water transparency caused by phytoplankton dynamics strongly influence the overall lake system through the availability of macrophyte growth and sediment fixation. These findings are critical for the future lake management, particularly its hydrological regime and maintenance of natural oligohaline and mesotrophic conditions. Understanding the response of phytoplankton to environmental conditions, exacerbated by anthropogenic influence and recent climate change, contributes to the protection of Mediterranean shallow lakes at local and global scales.