Effects of regional reductions in sulphur deposition on the chemical and biological recovery of lakes within Killarney Park, Ontario, Canada.

The lakes in Killarney Provincial Park, located 40-60 km southwest of Sudbury, Ontario, were some of the first lakes in North America to be acidified by atmospheric pollutants. Acidification affected thousands of fish and invertebrate populations in dozens of lakes. Since the 1970's, water quality has improved in response to atmospheric pollution reductions and some lakes have already recovered to approximately their pre-industrial pH levels, as inferred from diatom microfossils in lake sediments. Since the 1970's, fish species richness has not changed substantially, but zooplankton species richness has increased in acidified lakes. The critical sulphur load, the amount of SO2-derived acid deposition that can occur while still maintaining suitable water quality, was estimated to be exceeded in 38% of the park area in 1997. Depending on which of four possible North American emission control scenarios (CLR = currently legislated reduction; CLR + 25%; CLR + 50%; CLR + 75%) is achieved by 2010, the projected critical loads will be exceeded in about 0-30% of the park area in the future. There are many factors that can affect biological recovery rates of damaged lakes, but it is expected that biological recovery will lag considerably behind observed chemical recovery rates.