Response of Hypolimnetic Water and Bottom Sediment Microbial Communities to Freshwater Salinization—A Microcosm Experiment

The introduction of NaCl in freshwater caused by winter runoffs is a problem whose consequences are still little understood. We sought to analyze the effect of NaCl addition on microbial communities of the hypolimnion and bottom sediments of a Canadian lake. Using microcosms comprising a salinity gradient varying between 0.01 and 3.22 ppt (10–3220 mg/L−1) NaCl, we investigated the effect of salinity on prokaryotic absolute abundance and diversity, following a three- and six-week exposure, and detected the presence of a salinity threshold for microbial communities’ differentiation. We observed a significant decline of bacterial diversity after six weeks in hypolimnetic samples. In the sediments, no clear effect of NaCl was observed on abundance or diversity, despite the presence of variations throughout the salinity gradient. The implication of nutrient fluctuations as well as the co-occurrence of species and inter-domain interactions is likely and would strongly contribute to the development of salt-exposed prokaryotic communities. In hypolimnetic water and sediments, the archaeal and eukaryotic communities differed significantly from 0.93 ppt (930 mg/L−1), while only conclusive at 1.9 ppt (1900 mg/L−1) NaCl in bacteria, meaning that the regulations in place are possibly suitable for the protection of the microbial communities in the hypolimnion and sediment lake layers.