Acid Deposition

Photo: Boreal Shield lakes © Roy Neureuther, EC/WS&T
Boreal Shield lakes

Trends in sulphate levels and acidity in Boreal Shield lakes

Sulphates (micro-equivalents per litre), 1972 to 2008
Map and graphs: trends in sulphate levels and acidity in boreal shield lakes
The response of Clearwater Lake is related to its proximity to the sulphur dioxide emission sources at Sudbury.
Source: adapted from Jeffries et al., 200319


From 1980 to 2006, sulphur dioxide emissions in Canada and the U.S. declined by about 45% and emissions of nitrogen oxides declined by about 19%.20 Although significant declines in lake sulphates followed closely behind the emission reductions,19-21 the response of lake acidity, measured by pH, has been slow and less widespread, due in part to declines in calcium which are also related to acid deposition.20 Declines in calcium also threaten keystone zooplankton species.22 Encouraging biological improvements have been seen in some locations.1, 21, 23-26 Even with chemical recovery, however, biological communities are likely to remain altered from their pre-acidification state because many factors beyond acidity influence biological recovery.23, 27