Blinded by the stink: Nutrient enrichment impairs the perception of predation risk by freshwater snails

The acquisition of sensory information is central to all species interactions. Most aquatic organisms use chemical cues to assess predation risk and other key ecological factors, but chemoreception may be disrupted in systems with elevated pH. Elevated pH in lakes and rivers is often associated with eutrophication. We used laboratory and mesocosm experiments to test whether elevated pH impairs perception of predation risk by the freshwater snails Physa acuta and Helisoma trivolvis. In one set of experiments, nutrients were added to outdoor mesocosms, resulting in mid-afternoon pH values of 8.5-9.7. Both snail species moved to avoid fish in water with pH < 9.0 but showed no avoidance at higher pH. In a laboratory study, we used buffers to establish six pH treatments ranging from 7.5 to 10.0. At lower pH Physa acuta responded to fish cues by moving into safer habitats, but avoidance became impaired at a pH of 9.4. Helisoma trivolvis also responded to fish at lower pH, and their avoidance behavior became impaired at a pH of 8.8. Given the diversity of aquatic organisms that depend on reception of chemical cues and the broad extent of eutrophication, chemosensory impairment is likely a common occurrence in nature.