Life Stage and Population Variation in Resistance and Tolerance ofHyalella azteca(Amphipoda) to Low pH

Lethality experiments revealed that resistance of Hyalella azteca (Amphipoda) to low pH was directly related to size and developmental stage. Exposure of adults to water below pH 5.0 during pulses of acid snowmelt, or of juveniles to below pH 5.5 through gradual lake acidification, could result in population decline. Hyalella azteca from moderately acidic Ontario lakes (pH 5.6–5.7) survived longer at lethal pH than did amphipods from circumneutral lakes (pH 6.4–7.2) where spring pH depressions do not occur. Resistance and tolerance to low pH was neither readily lost by tolerant amphipods exposed to neutral water for 10 d nor readily gained by nontolerant amphipods exposed to sublethal low pH for a similar duration. This absence of physiological plasticity of individual H. azteca suggests that population differences in acid tolerance may result from processes of selective mortality with or without a genetic basis.