Effects of freshwater salinization on a salt-naïve planktonic eukaryote community

Salinization of freshwater ecosystems is a widespread issue, but evidence of ecological effects on aquatic eukaryote communities remains scarce. We experimentally exposed naive planktonic communities of a north-temperate, freshwater lake to a gradient of chloride (Cl-) concentration (0.27-1400 mg Cl-.L-1) with in-situ mesocosms. Following six weeks of exposure, we measured changes in the diversity, composition, and abundance of eukaryotic 18S rRNA gene. Total phytoplankton biomass remained unchanged, but we observed a shift in dominant phytoplankton groups with elevated salt concentration, from Cryptophyta and Chlorophyta that dominated in lower chloride concentrations (<185 mg Cl-.L-1) to Ochrophyta that dominated at higher conductivity (>185 mg Cl-.L-1). Most zooplankton and rotifer taxa were sensitive to the salinity and disappeared at low chloride concentrations (<40 mg Cl-.L-1). While ciliates thrived at low chloride concentrations (<185 mg Cl-.L-1), fungal groups dominated at intermediate chloride concentrations (185 mg Cl-.L-1 to 640 mg Cl-.L-1), and only phytoplankton remained at the highest chloride concentrations (> 640 mg Cl-.L-1).