How do freshwater prey respond to combinations of predation risk and salinity?

Freshwater salinization is a growing problem worldwide, and negatively affects many freshwater organisms. Many of the prior experiments studying effects of salinity on freshwater organisms have examined the effects of salinity in isolation from natural stressors such as predation risk. However, stressors such as predation risk can synergistically interact with many pollutants to affect prey survival and growth, so it is important to determine if it interacts with salt pollution. In this experiment we looked for interactive effects of salinity and predation risk on freshwater snails using a full-factorial mesocosm experiment that crossed four salt concentrations (no added salt, 250 mg Cl⁻ l^–1, 500 mg Cl⁻ l^–1 and 1000 mg Cl⁻ l^–1) with three predator regimes (no predator, crayfish cues and water bug cues). Snails were raised in the mesocosms from juveniles to reproductively mature adults, and we measured their survival, mass, reproduction, and shell morphology. We discovered a synergistic effect of salinity and predation risk on the survival of snails at the highest salt concentration. At the highest salinity, adding crayfish cues caused a 66% increase in snail mortality whereas adding water bug cues caused a 143% increase in snail mortality. In contrast, the effects of predation risk and salinity on snail mass, reproduction, and shell morphology were additive. Our results suggest that experiments testing salinity in isolation from other stressors may underestimate the full impact of salt pollution in freshwater environments.