Evidence of Acidification Effects on Stream Insect Communities in Central Ontario Between 1937 and 1985

Aquatic insect surveys were conducted in two low-alkalinity streams in Algonquin Provincial Park, Ontario, Canada, that were first studied 48 yrs ago (1937–42). At a site where current spring pH fluctuations are small (pH 6.4–6.1), most of the same taxa that were present in 1984–85 were observed during the original surveys. However, at sites where large spring pH depressions are currently observed (pH 6.4–4.9), many mayfly and stonefly taxa that were present 48 yr ago and that are known to be intolerant of low pH (pH <5) were not recorded in 1984–85. An invasion of acid-tolerant mayfly and stonefly taxa occurred at sites with large pH depressions. Species replacements from acid-sensitive to acid-tolerant insects suggest that poorly buffered surface waters in this region have acidified sometime within the last five decades.