Climate effects on sulphate flux from forested catchments in south-central Ontario

Net export of sulphate from watersheds may delay the response of surface waters to changes in acid deposition. Long-term (18-yr) sulphate budgets were calculated for 8 headwater streams located in the acid-sensitive region of Muskoka-Haliburton, south central Ontario. Sulphate deposition in this region has decreased by almost 40% over the last 2 decades, and sulphate export from catchments has also generally declined over time, but most catchments are still a net source of sulphate to drainage streams. Net export of sulphate occurred in the majority of catchments in most years of record, but was particularly large following dry, warmer than average summers, when stream flow ceased for up to several weeks at a time. In years with warm dry summers, such as occurred in 1983/84 and between 1987/88 and 1990/91, inclusive, stream export from most catchments was between 1.5 and 2 times greater than was input via bulk deposition. Annual average sulphate concentrations in streams were strongly correlated with stream dryness, and were greater in years in which streams were dry for longer periods of time. Temporal patterns of annual sulphate concentrations and export were highly coherent among the 8 streams, and net sulphate export occurred in both wetland-draining and predominantly upland streams. Climate variables, specifically temperature and precipitation act on a regional scale and are likely responsible for similar temporal patterns of sulphate retention among these 8 physiographically different catchments. Net sulphate export from catchments may delay the recovery of acid impacted surface waters, despite reductions in industrial SO2 emissions.