Microbial marker for seawater intrusion in a coastal Mediterranean shallow Lake, Lake Vrana, Croatia.

Climate change-induced rising sea levels and prolonged dry periods impose a global threat to the freshwater scarcity on the coastline: salinization. Lake Vrana is the largest surface freshwater resource in mid-Dalmatia, while the local springs are heavily used in agriculture. The karstified carbonate ridge that separates this shallow lake from the Adriatic Sea enables seawater intrusion if the lakes' precipitation-evaporation balance is disturbed. In this study, the impact of anthropogenic activities and drought exuberated salinization on microbial communities was tracked in Lake Vrana and its inlets, using 16S rRNA gene sequencing. The lack of precipitation and high water temperatures in summer months introduced an imbalance in the water regime of the lake, allowing for seawater intrusion, mainly via the karst conduit Jugovir. The determined microbial community spatial differences in the lake itself and the main drainage canals were driven by salinity, drought, and nutrient loading. Particle-associated and free-living microorganisms both strongly responded to the ecosystem perturbations, and their co-occurrence was driven by the salinization event. Notably, a bloom of halotolerant taxa, predominant the sulfur-oxidizing genus Sulfurovum, emerged with increased salinity and sulfate concentrations, having the potential to be used as an indicator for salinization of shallow coastal lakes. Following summer salinization, lake water column homogenization took from a couple of weeks up to a few months, while the entire system displayed increased salinity despite increased precipitation. This study represents a valuable contribution to understanding the impact of the Freshwater Salinization Syndrome on Mediterranean lakes' microbial communities and the ecosystem resilience.