Alteration of biogeochemical cycles in lakes and streams is usually accompanied by a shift in hydrogen ion concentration. For example, eutrophication of ponds and lakes often results in elevated pH, as nutrients additions stimulate plant productivity, which in turn depletes bicarbonate. Extreme pH can exclude most species from aquatic communities and cause a shift in community composition to tolerant taxa. Less extreme shifts in pH may have important effects on animal sensory capability, but these sublethal effects of pH shifts have not been studied. Using both laboratory and field experiments, we investigated how the ability of freshwater gastropods to detect predators depends on pH. Snails, like most aquatic animals, use waterborne chemical signals to detect and avoid predators. We hypothesized that predator recognition would be impaired at high pH because the structure of the amino acids used to detect predators is pH dependent.
Results/Conclusions
In one set of 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, Physa acuta responded to fish cues by moving into safe habitats, but avoidance became impaired at a pH of 9.5. Helisoma trivolvis responded to fish in a similar manner, but their avoidance behavior became impaired at a pH of 9.0. Thus, in eutrophic conditions, the ability of snails to detect predators is impaired. Sensory impairment is likely a common occurrence in nature, as our field surveys show that about 1/3 of the ponds in northwestern Pennsylvania have daytime pH values above 9.0. Given the diversity of freshwater animals that depend upon perception of chemical cues, it is likely that eutrophication affects the sensory abilities of many other freshwater taxa. This discovery is of considerable applied interest, as it represents an important but unknown consequence of nutrient pollution. It is also of large interest to ecologists in general, many of whom have expended much effort in recent years studying chemically-mediated behavior without any appreciation of the contingency of such behavior on environmental conditions.
