Salt stratification and predator cues: Impacts on freshwater species depend on habitat choice

Freshwater bodies worldwide are experiencing salinization, often with severe ecological consequences. An underexplored topic in freshwater salinization research is the potential for waterbodies to stratify their incoming salty water (due to its higher density) and the effects that this stratification may have on freshwater organisms. We conducted a mesocosm experiment to test whether salt stratification can occur in small freshwater bodies and its ecological consequences. We created salt-stratified mesocosms by piping salty water to the bottom of a 1,200-L mesocosm and examined the duration of the created stratification. We discovered that salt stratification in mesocosms can last for several weeks. We then compared the habitat selection and survival of two species of freshwater snails (the native pouch snail, Physa acuta, and invasive banded mystery snail, Viviparus georgianus) when exposed to the presence or absence of salt stratification, crossed with the presence or absence of predation cues (i.e., crushed conspecifics). Mystery snails preferred the bottom habitats, regardless of the presence of salt or predator cues, and consequently suffered near total mortality in the salt-stratified mesocosms (97% survival without salt vs. 3% survival with). In contrast, the pouch snails reduced their use of the bottom habitat in response to stratified salt and predator cues. While they still suffered mortality in the salt-stratified mesocosms, it was much less than the mystery snails (82% survival without salt vs. 47% survival with). These results suggest that the stratification of salt pollution in freshwater bodies can persist for prolonged periods and differentially affect freshwater species as a result of their habitat preferences.