Leeches in acidified lakes of central Ontario, Canada: Status and trends

Lakes in the acid-sensitive Legions of Sudbury, Algoma, and Muskoka. Ontario (Canada), were examined to assess relationships between leech populations and certain chemical and physical characteristics of the lakes (pH, conductivity, total nitrogen, dissolved organic carbon, and water depth). Thirteen leech species were trapped, and leeches occurred in 81% of study lakes. Leech species richness was higher in lakes with high pH (i.e., less acid) and low conductivity. Occurrence and abundance of some species were significantly increased in lakes with higher pH and lower conductivity however, abundance model, explained low portions of data variability (10-13%). Temporal trends of leech occurrence, species richness, and abundance in the Sudbury study area were examined in four separate years over a nine-year interval. Most lakes had no significant change in leech richness or abundance over this period. However, a substantial subset of the lakes showed declines in richness or abundance despite dramatic reductions in acidic deposition across eastern North America and some subsequent improvements in lake chemistries. Our results suggest that leech declines were not directly related to changes in take chemistry. Hence, we suggest that leeches are not suitable as direct indicators of chemical recovery from acidification.