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Three species of stream invertebrates, the caddis fly Lepidostoma liba (Ross), the isopod Asellus intermedius (Forbes), and the snail Physella heterostropha (Say), were found to be highly vulnerable to depression to pH 4.0 with 70% sulfuric acid (H2SO3) and 30% nitric acid (HNO3) from a control pH of 6.7 to 7.2. This vulnerability was directly correlated to size with smaller individuals being much less tolerant to pH 4 depression than were larger individuals. Smaller size classes, including egg masses of P. heterostropha, were totally eliminated by exposure to pH 4. Survival increased as initial body mass increased past a threshold size (0.5 mg dry weight per individual for A. intermedius, 0.6 mg for L. liba, and at a shell length of 4 mm for P. heterostropha). Depression of pH from 7 to 4 also depressed growth rates for A. intermedius, and L. liba. This experiment was conducted in recirculating laboratory streams operated near ambient temperatures at 8-week intervals throughout the year.
pH effects, stream invertebrates, mortality, growth, size dependency
Graduate research assistant, Michigan State University, East Lansing, MI
Professor, Michigan State University, East Lansing, MI