Short term effects of salinization on the plankton community of an oligotrophic mountain lake

Freshwater salinization is of increasing concern for integrity and functioning of freshwater habitats worldwide. Experiments so far often have studied drastic salt additions, while gradient designs have been performed less commonly. We tested the effect of freshwater salinization in a mesocosm exposing the plankton community of the oligotrophic Lake Lunz, Austria, to a four-fold salinization gradient (control, 0.2, 1, a 5 ppt salt). Salinity was manipulated in a factorial design with enrichment, with 10 g L-1 and 30 g L-1 phosphorus, resulting in 8 treatments with 3 replicates each. We followed the effects of salinization on diversity, community composition and resource use over 36 days. Community composition was assessed by amplicon sequencing, Diversity loss and community turnover followed upon salt addition. All levels of salinization caused pronounced changes in community composition, with 5 ppt causing the most drastic changes. Salinization caused trophic downgrading by kicking out especially protistan consumers and rotifers, while some green algae and chrysophytes were especially tolerant, resulting in reduced phylogenetic and functional diversity with increasing salinization. In line with reduced top down control, salinization affected temporal variability in chlorophyll-a (chl-a) and resource use (RUE), with higher salinity causing more extreme fluctuations in chl-a and RUE. Enrichment overall aggravated salinization, enhancing temporal turnover and temporal fluctuations in resource use.