The effect of El Nino-related drought on the recovery of acidified lakes

Although SO2 emissions and deposition rates have declined substantially since the implementation of sulphur emission control programmes in North America [1], recovery (measured as decreases in SO42- concentrations) of affected lakes in central Ontario has been much less substantial than anticipated based on the decrease in deposition. The slow recovery is attributed to the reoxidation and release of stored sulphur in catchments. Reduced sulphur retained in previous years when sulphur deposition was higher is exposed to air and oxidized during severe droughts, then exported during subsequent wet periods Elevated stream SO42- concentrations and export rates occur in the autumns of years with prolonged severe droughts, particularly in catchments with extensive wetlands. Drought in our study catchments occurred in years following strong El Nino events. When the Southern Oscillation Index (SOI) was strongly negative (1976-77, 1982-83, 1986-87, 1991-92, 1993-94) the frequency of occurrence of drought the following summer in small catchments with shallow overburden was extremely high. A lake's rate of recovery from acidification depends upon the amount of excess reduced S that has been stored in anoxic zones in the catchment (largely a function of the extent of wetlands) during years of elevated S deposition rates, and the frequency and severity of droughts. If long-term changes in global or regional climate alter the frequency or magnitude of El Nino-related droughts, the recovery of acidified lakes will be affected.