RESPONSES OF THE FRESH-WATER AMPHIPOD HYALELLA-AZTECA TO ENVIRONMENTAL ACIDIFICATION

Effects of acidification on the freshwater amphipod Hyalella azteca were investigated by examining the role of pH on natural distribution of the species in 30 northwestern Ontario lakes and by determining its responses to acidification in a series of field and laboratory experiments. The amphipod was absent from all five lakes that had pH 5.03-5.40 or Ca concentration < 1.24 mg/L and was present in 23 of the 25 remaining lakes that had pH > 6.0 and Ca > 1.24 mg/L. In the field and laboratory experiments, amphipods transplanted to acidified lakes (pH 5.45, 5.66) or held in 500-L bags (pH less-than-or-equal-to 5.8) or 4-L aquaria (pH less-than-or-equal-to 5.6) showed lower survival than those held in corresponding neutral-pH, control treatments (P < 0.05). Neonate H. azteca were less tolerant of low pH than those in later life-history stages (P < 0.05). Populations of H. azteca will decline in habitats where the pH remains < 5.8, a condition extant in a substantial proportion of lakes in eastern North America.