We compared the tolerance of three common Ontario crayfish species to low pH under natural and laboratory conditions in water varying in aluminum concentration and hardness. Both transplant and laboratory experiments indicated that exposure to a pH range of 5.4–6.1 in soft water was toxic to attached juvenile stages of Orconectes rusticus and O. propinquus but not to females carrying the broods. In contrast, stage III juveniles of Cambarus robustus molted and survived in soft water at pH 4. Cambarus robustus is clearly far less sensitive to low pH stress than the two Orconectes species. No increased mortality due to the presence of elevated aluminum occurred among stage III juveniles of the three species exposed to pH 4.5–5.0 in soft water. Physiological differences, augmented by differences in life cycle, may account for the continued presence of C. robustus in acid-stressed lakes and streams.

Field surveys of the distribution of mayfly nymphs suggest that Stenonema femoratum are more acid-sensitive than Leptophlebia cupida. To assess whether this apparent difference in sensitivity of nymphs is reflected in differences in the degree of whole-body loss of [Na], [Cl], [Ca] or [K] under laboratory conditions, we exposed nymphs of both species to low pH for 96-192 h in soft water ([Ca] = 0.1 mM). Although mortality and loss of whole-body [Na] and [Cl] occurred in both species at pH 3.5, unexpectedly they were considerably greater in L. cupida than in S. femoratum. Ion loss was not size related within the range of nymphal weights used (2-14 mg dry wt) for S. femoratum. Exposure to the environmentally more common pH 4.5 had no effect on whole-body [Na] and [Cl] or on mortality in either species. However, in L. cupida, molting by nymphs increased at both pH 3.5 and 4.5. A decrease in whole-body [Ca] occurred, and the loss of whole-body [Na] and [Ca] at pH 3.5 appeared to cease following the period of molting. In S. femoratum no molting or Ca loss occurred and whole-body [Na] and [Cl] decreased between 96 and 192 h exposures.

The lower depth limit of a Mysis population in Crystal Lake, southern Ontario, gradually moved upward through the summer stagnation period, generally following a position just above the 1.0 mg/L dissolved oxygen isopleth. Mysis held in bioassay chambers demonstrated a 16-h LC50 value of 1.0 mg dissolved oxygen/L. Avoidance chamber experiments revealed that Mysis could detect and move away from areas of low dissolved oxygen concentrations. It is suggested that Mysis, which has been used as an indicator of oligotrophic conditions, could survive in a mesotrophic or eutrophic lake that undergoes oxygen depletion in the hypolimnion by moving upward in the water column.

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Cambarus robustus is more tolerant of low environmental pH than Orconectes rusticus and this tolerance reflects a difference in ion regulation physiology. Chronic exposure (96 h) of the acid-tolerant C. robustus to pH 3.8 soft water did not significantly change haemolymph [Na+] or [Ca2+] of the adults or total body [Na+] of the juveniles relative to the control (pH 6.5). In contrast, the intolerant O. rusticus showed a significant decrease in [Na+] and increase in [Ca2+] in adult haemolymph (Wood and Rogano. 1986. Can. J. Fish. Aquat. Sci. 43: 1017–1026) and an increase in total body [Na+] of stage III juveniles following acute exposure to pH 3.8 compared with the pH 6.5 control.