Assessment of changes in lake pH in southeastern Canada arising from present levels and expected reductions in acidic deposition

An integrated acid rain assessment model was used to estimate pH for six cl usters of lakes in southeastern Canada and scenarios of sulphate deposition that reflect the situation (a) before implementation of the SO2 emission c ontrols required by the Canada/U.S. Air Quality Agreement, (b) after implem entation of Canadian controls, acid (c) after implementation of Canadian an d U.S. controls. Modelled lake pHs were always less than their estimated or iginal values. To assess the ecological significance of the pH reduction, s cenario "damage" was quantified as the percentage of cluster lakes having p H < 6, a threshold criterion sufficient to protect most aquatic biota. Care was taken to account for naturally acidified lakes. The integrated acid ra in assessment model predicted that Canadian SO2 controls will reduce damage in Ontario and Quebec but have little effect in Atlantic Canada. implement ation of U.S. O-2 controls will further reduce damage throughout all region s, although it is conservatively estimated that from 5 to 24% of the lakes will still have pH < 6 depending on cluster. Extrapolating to the inventory of acid-sensitive lakes in southeastern Canada suggests that similar to 76 000 lakes and similar to 970 000 ha of lake area will remain chemically da maged unless additional reductions in SO2 emissions are implemented beyond those required by the Air Quality Agreement.