Lakes and waterways in North America are struggling to recover from the effects of acid rain, despite reduced emissions of the pollutants that cause it. Without further cuts, it could be millennia before the worst-affected sites recover, say environmentalists.
Although the 1990 US Clean Air Act has reduced acid rain in northeastern North America, many lakes in eastern Canada are still beyond their critical load - the amount of acidification that harms the organisms living there, researchers told a meeting of ecologists in Montreal.
Acid rain is caused largely by sulphur dioxide and oxides of nitrogen emitted by industrial activities such as coal burning. The gases dissolve in rainwater to form acids. Much of this industry is based in the United States, but the weather exports pollution north of the border. Between 50% and 70% of Canada's acid rain comes from the United States, while only 2-10% of America's pollution in this area comes from Canada.
The Clean Air Act reduced US sulphur dioxide emissions from 16 million tonnes a year in the 1980s to 11 million tonnes in 2000. Rain is less acidic, but Ontario's lakes are not recovering, Shaun Watmough of Trent University in Peterborough, Ontario, told the annual meeting of the Ecological Society of America.
Many of the province's 31,000 small lakes have a pH value of about 5, making them dangerously acidic for fish and plants, he says.
"We've had 20 years of reductions and things still haven't got better," says Watmough. "That's going to annoy a lot of people; reductions are expensive."
Calcium leak
The soil around these lakes has simply been overburdened, Watmough explains.
Hydrogen ions formed when sulphates and nitrates dissolve in the rainwater, are generally buffered by calcium ions from the soil, curbing acidity. But so much acid rain has fallen that there is not enough calcium available to do this, Watmough says.
"It may take thousands of years for the soils to recover," he says.
Many acid-damaged soils in Europe are treated with lime to replace lost calcium. But this would be expensive to do in Canada's vast wilderness, and harmful if overdone, Watmough says. Another option might be to burn trees to release stored calcium, although this has never been tested.
The only practical solution is to cut industrial emissions further, argues Watmough.
Long recovery
The lakes' plight is a reminder that it often takes longer to recover from pollution than it did to pollute, said John Gunn, a fish ecologist at Laurentian University in Sudbury, Ontario, who studies Canada's eastern lakes. He has found that many species find it harder to return to ecosystems disturbed by acidification.
"Lakes are resilient," says Gunn. "But the timeline of recovery is longer than we expected."
The situation also shows that the Canada-US Air Quality agreement, which aims to control levels of atmospheric pollutants across the countries' border, is still a work in progress, said Peggy Farnsworth of Canada's Environmental Protection Service. Further cooperation to cut emissions is likely to be necessary, she adds.
来源：www.nature.com
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