Linked regional chemical and biotic models predicted that at least 20% of all lakes in 15 of 38 secondary watersheds in eastern Canada had lost at least 20% of their potential species richness given 1980 acidic sulphate deposition levels; 20% of lakes in 15 watersheds represents circa 55 000 lakes. Fish and molluscs were the most affected of seven aquatic taxonomic groups, and rotifers the least. Mean percent loss of potential richness in all of eastern Canada ranged from 5.0 to 9.5% for the seven groups of biota under 1980 acidic deposition. Sulphur dioxide emission reductions of 42% in Canada and the United States were predicted to reduce the number of affected watersheds to seven, leaving circa 25 000 lakes with 20% or more loss of potential richness. Greater acidic deposition reductions will be needed to minimize the biotic damage affecting large areas of eastern Canada.