ACID AND ALUMINUM EFFECTS ON SODIUM HOMEOSTASIS AND SURVIVAL OF ACID-SENSITIVE AND ACID-TOLERANT CLADOCERA

Acid-sensitive (Daphnia galeata mendotae) and acid-tolerant (Bosmina longirostris) cladocerans were exposed to 0, 100, or 200 mug Al/L at pH 4.5, 6.0, or 7.5. Whole-body Na content and survivals were determined after 12- and 24-h exposures to the nine treatments. Daphnia galeata mendotae experienced significant decreases in body Na content and survival at pH 4.5. Aluminum effects were pH dependent. At pH 7.5 and 6.0, Na content and survival declined with increasing Al; at pH 4.5, the highest Al concentration enhanced Na content and prolonged survival. Bosmina longirostris Na content and survival were only slightly reduced at pH 4.5, and there were no significant Al or pH x Al interaction effects. The results Support the view that (1) the extinction of D. galeata mendotae and the relative increase of B. longirostris during lake acidification are largely due to differential impacts of acid stress on osmoregulation and (2) Al toxicity might also be a factor responsible for D. galeata mendotae population declines, which are most pronounced near pH 6.0.