Crayfish populations as an indicator of littoral zone recovery in historically acidified lakes with and without predatory fish in the Killarney Region, Ontario

Killarney Provincial Park, Ontario suffered extensive aquatic biodiversity loss in the early 1960s from acidification, particularly in the highly sensitive ridgetop lakes. In recent decades, effective North American legislation and emission reductions have resulted in substantial chemical recovery, however biological recovery has lagged considerably behind. A set of 19 lakes were reassessed after a 25-year period to quantify multi-faceted ecological changes. Crayfish populations were our focal indicator, which we complemented with habitat, water quality and littoral zone fish community surveys. Significant increases in pH and DOC and decreases in sulphate, calcium and metals, as well as increases in centrarchid fish predators, appeared to be interacting to affect crayfish populations. Surprisingly, neither the acid-sensitive faxonius or acid-tolerant cambarus exhibited population growth with pH recovery, except in a chain of 5 small fishless lakes. Our results demonstrate the importance of the interactions between historical and contemporary stressors and provide evidence that Killarney’s historically acid-damaged lakes are no longer limited by acidic conditions but are now impacted more by contemporary stressors, especially the expansion of warmwater fishes.