Blinded by the stink: Elevated pH impairs the anti-predator responses of freshwater gastropods

Eutrophication of ponds and lakes often results in elevated pH. Because most aquatic animals use waterborne chemical cues to assess predation risk, shifts in pH may have important effects on animal sensory capability, but these sublethal effects of pH shifts have not been studied. Using laboratory and field experiments, we investigated whether the ability of  freshwater gastropods to detect predators depends on pH. In field experiments, we manipulated the pH of outdoor mesocosms with nutrient additions resulting in midafternoon pH values ranging from 8.5 to 10.4. The snails Physa acuta and Helisoma trivolvis were stocked into microcosms containing water drawn from each mesocosm and exposed to the chemical cues of their predator, the pumpkinseed sunfish. Both species moved to avoid fish at a pH < 9.0 but showed no avoidance at higher pH.  In another study, conducted indoors, we manipulated pH directly with buffers, establishing six pH levels ranging from 7.5 to 10.0. At low pH, snails responded to fish cues by moving into safe habitats, but avoidance became impaired at pH > 9.0. Thus, in eutrophic conditions, the ability of snails to detect predators is impaired. This discovery represents an important but previously unknown consequence of nutrient pollution.