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

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Abstract
An integrated acid rain assessment model was used to estimate pH for six clusters of lakes in southeastern Canada and scenarios of sulphate deposition that reflect the situation (a) before implementation of the SO2 emission controls required by the Canada/U.S. Air Quality Agreement, (b) after implementation of Canadian controls, and (c) after implementation of Canadian and U.S. controls. Modelled lake pHs were always less than their estimated original values. To assess the ecological significance of the pH reduction, scenario "damage" was quantified as the percentage of cluster lakes having pH < 6, a threshold criterion sufficient to protect most aquatic biota. Care was taken to account for naturally acidified lakes. The integrated acid rain assessment model predicted that Canadian SO2 controls will reduce damage in Ontario and Quebec but have little effect in Atlantic Canada. Implementation of U.S. SO2 controls will further reduce damage throughout all regions, 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 ~76 000 lakes and ~970 000 ha of lake area will remain chemically damaged unless additional reductions in SO2 emissions are implemented beyond those required by the Air Quality Agreement.

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