Effects of acidic pH on predation rates and survivorship of damselfly nymphs

Environmental variables such as pH critically affect the physiology and ecology of aquatic organisms. Our lack of information regarding indirect effects of acid precipitation requires investigation of acid stress and response to acid stress by species of freshwater communities. Damselfly nymphs Enallagma civile (Odonata: Coenagrionidae) are abundant aquatic predators and food items for other secondary consumers.We investigated the effects of low pH on mortality, survivorship, predation rate, respiration rate, and caloric content of nymphs of E. civile. We hypothesized that acid stress would increase mortality and decrease predation rate among surviving individuals. We hypothesized that respiration rate and caloric content as measures of physiological condition would decrease in acid-stress conditions.We cultured individual early-instar nymphs in the laboratory with a controlled diet of mosquito larvae. The nymphs were reared to final instar in pond water and artificially reconstituted soft water (RSW) adjusted to pH 3.5, 4.5, 5.5, and ambient conditions of the collection site.Predation rate was significantly lower in RSW than in pond water in treatments of equivalent H⁺ concentrations. This indicated stress from differences in dissolved solids in RSW compared with pond water. Acid conditions significantly affected predation rates in pond water, but only at pH 3.5 and only for the largest instar. Mortality increased and survivorship decreased at low pH. Early-instar mortality and survivorship were particularly susceptible to acid stress. Treatments of pH 4.5, 5.5 and ambient did not significantly alter respiration rate or caloric content. However, respiration rate and caloric content increased at pH 3.5.Damselfly nymphs tolerated pHs as low as 4.5 without significant changes (including predation rate). However, the effects of pH 3.5 were significant and greatly lowered the predation and fitness of the nymphs.author for correspondence