Acid Rain Still Plaguing Lakes and Loons

A steady decline in the breeding success of loons over the past decade and the results of ongoing monitoring indicate that, despite progress in reducing sulphur dioxide emissions (SO₂), Canada's lakes continue to suffer from the effects of acid rain.

Sulphur dioxide and nitrogen oxide (NO_x) emitted by metal smelters and the burning of fossil fuels in electrical utilities, factories and motor vehicles are the primary cause of acid rain. When these pollutants come into contact with moisture in the atmosphere, they are converted into dilute solutions of sulphuric acid and nitric acid that fall to the earth as rain, snow, fog and other forms of precipitation.

To meet commitments made under the Canada-U.S. Air Quality Agreement, Canada has reduced its SO₂ emissions by 50 per cent over 1980 levels. This goal was achieved in 1994. The United States is more than halfway to its reduction target, and is expected to meet the 2010 deadline. In contrast, NO_x emissions have remained relatively constant over the past 20 years.

Environment Canada scientists have also been studying acid rain for more than two decades, and recent findings indicate that acid levels in our inland waters have responded little to the SO₂ reductions. A study of 152 lakes in southeastern Canada indicates that only 41 per cent are less acidic today than they were 20 years ago, 50 per cent have not changed, and 9 per cent are actually more acidic. Scientists estimate that to protect all of these ecosystems would require reducing SO₂ emissions by 75 per cent over current targets.

Southeastern Canada is particularly vulnerable because it receives more acid rain than any other part of the country and is highly sensitive because of the weak neutralizing ability of the Canadian Shield's granite bedrock.With many lakes and wetlands in the region receiving twice as much sulphate as they can tolerate, models predict that up to one quarter of the lakes in Eastern Canada will remain chemically damaged after 2010.

 

Maps of eastern Canada showing the predicted improvement in the suitability of nesting habitat for fish-eating birds. The first map shows the improvement between 1982 and 2010 as a result of meeting current sulphur emission reduction targets, and the second map the improvement if sulphur emissions were reduced by an additional 75 per cent. Dark blue represents no change, medium blue a slight improvement, and yellow a significant improvement.

Loons are an excellent indicator of how lakes are recovering or suffering from acid rain, because acidification causes significant declines in the populations of the fish and invertebrates for which they prey. Lakes with a pH (a measure of acidity) of less than 6 (7 is neutral) support either a different mix or fewer species than those above the threshold. When pH drops to 5 or below, the number of fish species surviving plunges dramatically.

Since two adult loons require up to 180 kilograms of fish during the summer to fledge one chick, breeding success is lower on acid lakes, where young may starve from lack of food. Acid rain also leaches toxic metals, such as mercury, from soil and sediments. These metals can bioaccumulate in the food chain and affect loon reproduction.

For the past 20 years, surveyors with the Canadian Lakes Loon Survey (CLLS)—a volunteer-based program supported by Environment Canada and other partners and administered by Bird Studies Canada—have monitored the breeding success of loons on up to 800 lakes annually across Canada. These surveys have shown that, between 1981 and 1997, the proportion of successfully breeding loons has declined, and that the rate of decline was more extreme on lakes with high acid levels than on well-buffered lakes—especially in recent years.

One reason that acid levels remain so high is that there has been a drop in the level of acid-neutralizing bases in the atmosphere—a phenomenon scientists are still trying to understand. The same is happening in forest soils, where decades of acid rain have leached away calcium and magnesium, leaving them less able to neutralize surface water before it reaches lakes and streams. Another is that hot, dry weather, which is becoming more common due to climate change, converts sulphur that has accumulated in wetlands and soils over past decades into sulphuric acid. When wet weather returns, some of this acid washes into nearby lakes.

Scientists cannot say whether species that have disappeared from an acidified lake will ever return—even if pH levels return to normal. However, near Sudbury, Ontario, where sulphur dioxide emissions from local smelting plants have declined dramatically, invertebrates are reappearing and fish populations are being successfully established naturally or through restocking. Similar effects are being witnessed in Europe, where SO₂ reductions began a decade earlier than in North America.

Restoring lakes damaged by acid rain is a long-term process, but scientists expect that more strict emission-reduction targets will help put these ecosystems on the road to recovery.

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