Acid Deposition

Status and Trends
for eastern Canada, improving; for some parts of western Canada, getting worse
Concern, some improvements, some worsening
trends are clear
High confidence in finding

KEY FINDING 13. Thresholds related to ecological impact of acid deposition, including acid rain, are exceeded in some areas, acidifying emissions are increasing in some areas, and biological recovery has not kept pace with emission reductions in other areas.

This key finding is divided into four sections:

Acid deposition, sometimes referred to as acid rain, is produced when sulphur and nitrogen-based pollutants react with water in the atmosphere and are deposited on the Earth’s surface.1 More than just acid rain, it includes acidifying gases and dry particles. The pollutants originate from industrial processes and can travel thousands of kilometres. It is the combination of acid deposition and the sensitivity of the land, water, flora, and fauna to acid that determines the severity of the impact on biodiversity. Acid deposition is important because algae, invertebrates, fish, amphibians, and birds are affected by increased acidity through reduced survival, growth and reproductive success, and loss or alteration of prey species.1-6 The acidification of aquatic systems can lead to increases in methylmercury, which bioaccumulates, affecting embryos and young animals.7-10 Acidification may also negatively affect the growth rate and health of trees, for example, sugar maple and red spruce in northeastern North America.11, 12


Global Trends

Once recognized as a problem only in Europe and parts of North America, acid deposition is now also an environmental issue in Asia and Pacific regions.31 Significant reductions in sulphur emissions have been achieved in parts of Europe.32






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Terrain sensitivity and thresholds

Ecosystems have different sensitivities to acid depending upon their geology and soils. Thus the maximum level of acid deposition that terrain can withstand without harming ecological integrity, called the “critical load,” differs across ecosystems.13 Acid-sensitive terrain is generally underlain by slightly soluble bedrock and overlain by thin, glacially derived soils14 and has less buffering capacity.

Areas where the critical load has been exceeded in the Boreal Shield
Number of units above critical load, 2009
Map: areas where the critical load has been exceeded in the Boreal Shield. Click for graphic description (new window).
Source: adapted from Jeffries et al., 201018

Critical loads can be exceeded either when extremely sensitive terrain receives low levels of acid deposition or when less sensitive terrain receives high levels of acid deposition. The inset map shows where critical loads have been exceeded in the Boreal Shield Ecozone+. The potential for critical loads to be exceeded in northwest Saskatchewan is also a concern due to the high degree of acid sensitivity of many of the lakes in this area (68% of 259 lakes assessed in 2007/2008) and its location downwind of acidifying emissions from oil and gas developments.15 Similarly, transportation-related sulphur emissions in southwest B.C. are an emerging issue, with terrestrial critical loads exceeded in 32% of the Georgia Basin in 2005/2006.16

Sensitivity of terrain to acidity
Critical Load Index, 2008
Yellow through red categories are considered acid sensitive terrain
Map: sensitivity of terrain to acidity. Click for graphic description (new window).
Source: adapted from Jeffries et al., 201017

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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

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Impact of acidification on Atlantic salmon

Status of salmon rivers in Nova Scotia, 1996
Map: impact of acidification on Atlantic salmon. Click for graphic description (new window).
Source: adapted from Watt et al., 200028

Despite having the lowest rates of acid deposition in eastern North America, the Atlantic Maritime Ecozone+ has some of the most acidic waters due to the poor buffering ability of the terrain.29, 30 There has been no measurable change in pH despite declines in sulphur dioxide emissions. This has resulted in the most heavily impacted fish habitat in North America.29 Atlantic salmon are highly sensitive to acidity, and by 1996, 14 runs in coastal Nova Scotia were extinct because of water acidity, 20 were severely impacted, and a further 15 were lightly impacted.28 Recovery of water chemistry and ecology is expected to take several more decades in Nova Scotia than in other parts of Canada.28-30

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