Recent lake acidification and recovery trends in southern Quebec, Canada

A total of 51 lakes in southern Quebec, Canada, were sampled between 1985 and 1993 to study changes in water chemistry following reductions in SO2 emissions (main precursor of acid precipitation). Time series analysis of precipitation chemistry revealed significant reductions in concentrations and deposition of SO42- from 1981 to 1992 in southern Quebec as well as reductions in concentrations and deposition of base cations (Ca2+, Mg2+), NO3- and H+ in the western section of the study area. Reductions in atmospheric inputs of SO42- have resulted in decreased lakewater SO42- concentrations in the majority of the lakes in our study, although only a small fraction (9 of 37 lakes used in the temporal analysis) have improved significantly in terms of acidity status (pH, acid neutralizing capacity - ANC). The main response of the lakes to decreased SO42- is a decrease in base cations (Ca2++Mg2+), which was observed in 17 of 37 lakes. Seventeen lakes also showed significant increases in dissolved organic carbon (DOC) over the period of study. The resulting increases in organic acidity as well as the decrease in base cations could both play a role in delaying the recovery of our lakes.