Rapid improvement of lake acid–base status in Atlantic Canada following steep decline in precipitation acidity

Abstract

Policy measures resulted in reductions in acidic emission of sulfur and nitrogen in North America. Here, we conducted trend analyses on a 37-year database for atmospheric depositions and lake (n = 78) chemistry in the provinces of Nova Scotia, New Brunswick, and Newfoundland and Labrador, in Canada. Sulfate, and particularly nitrate deposition, began to decrease strongly around the year 2000, resulting in strong pH increases in precipitation. Recovery was observed in most lakes with significant increases in pH and Gran alkalinity (AlcG). A piecewise regression approach detected coherent breaks and over-lapping confidence intervals in the break years for precipitation pH and lake AlcG (to a lesser extent for lake pH), suggesting a rapid lake response to the sudden changes in acidic inputs. However, an increase in organic acid after 2000 decreases the rate of acid neutralizing capacity recovery nearly by half in about a third of the studied lakes. Despite the positive pH and AlcG trends, a significant proportion of lakes still have pH and calcium values below the recommended thresholds for aquatic life.