GASTROPODS FROM SMALL NORTHEASTERN ONTARIO LAKES - THEIR VALUE AS INDICATORS OF ACIDIFICATION

Gastropoda were sampled in 15 small lakes covering pH 5.0-7.5 northeast of Sudbury, Ontario. Twelve species were found among nine lakes with pH>6.0. The most widespread was a freshwater limpet, Ferrissia sp., which was the only one occurring in those lakes with pH 5.2-6.0. In less intensively sampled lakes in the area, other species were recorded between pH 5.5 and 6.0. In lakes with pH>6.0, there was no evidence of a relationship between the total number of gastropods, dominated by Helisoma anceps and Physella gyrina, and pH, alkalinity, or calcium ion concentration. However, the log(e) total number of gastropods was significantly correlated with total phosphorous concentrations (r=0.72, n=9, p<0.05). Above minimum pH thresholds, gastropod densities in small oligotrophic lakes appear to be limited by food resources, and not by calcium concentrations or alkalinity.