Climate-induced episodic acidification of streams in central ontario.

In this study we have analyzed the hydrochemical effect of drought conditions during 311 hydrological episodes in nine headwater streams in central Ontario over the past 20 years. Acid Neutralization Capacity (ANC) was logarithmically correlated (p<0.05) to antecedent discharge in eight of the nine streams, with the largest decline in ANC occurring after low antecedent flow. In eight of the nine streams SO4(2-) was the most important driving mechanism of ANC decline, but dilution as well as organic acidity was important in several streams. No decrease in the SO4(2-) driven ANC decline was observed over the 20 year study period despite a approximately 40% reduction in SO4(2-) deposition. The strong correlation between ANC decline and low antecedent discharge demonstrates that episodic acidification during rain events is strongly associated with preceding drought conditions, especially in wetland-dominated catchments. The results have important implications for recoveryfrom acidification, especially in northern ecosystems where climate scenarios forecast that warmer and drier conditions will be more common.