For decades, the northeastern United States suffered from the scourge of acid rain, as sulfur dioxide and nitrogen dioxide from power plants in the Midwest turned to acid in the atmosphere and — carried east by the winds — poured down into eastern watersheds. Amendments to the Clean Air Act in 1990 helped resolve the issue by spurring use of low-sulfur coal and adding scrubbers to smokestacks that decreased the acidity in rain tenfold. “The problem is that it was decades of titrating acid across the landscape, and there is still a lot of residual sulfur in the soil,” says Randy Fuller, Russell Colgate Distinguished University Professor of biology and environmental studies.

The Adirondacks, in particular, have granitic geology with thin soils, providing very little buffering capacity to neutralize the acid that remains in the ground. “You can turn the sulfur taps off, but you still have these legacy effects,” Fuller says.

Fuller has been part of a team of scientists gauging the effects of that residual acid in the soil on the biology of the region, as well as examining how efforts to reduce that acid have paid off. He presents some of those findings in a recent paper published in Ecological Indicators in February 2018. In partnership with colleagues from the U.S. Geological Survey, Fuller has found that some species, at least, face a long road to recovery from the acid remaining in the region.

“You can turn the sulfur taps off, but you still have these legacy effects.”

Scientists have worked to mitigate the effects of acidification by adding lime — otherwise known as calcium carbonate — to lakes and streams to neutralize the lingering acid. The good news is that the liming of a few lakes and ponds, including streams Fuller is working in, has caused populations of native brook trout to bounce back. “I was up there last fall with a student, and we saw lots of large brook trout,” Fuller says.

Along with colleagues from the US Geological Survey, and biologists from Cornell and SUNY, Fuller has experimented with adding lime to three tributaries to Honnedaga Lake, a two-hour drive northeast from Hamilton. Starting in 2010, the scientists applied lime directly into two of the streams; in the third, they applied it to the watershed from above by helicopter. As the pH in the streams increased, they observed brook trout returning to the stream to breed over the course of three years.

That visible success, however, belies more persistent effects of acidification on the ecosystem. When leaves fall into the streams, they become breeding grounds for microbes, which in turn become food for macroinvertebrates — including the nymphs of aquatic insects such as mayflies, stoneflies, and caddis flies. To study the effects of liming on these species, Fuller and his fellow researchers put leaf packs into both treated and non-treated streams.

Mayfly sitting on a blade of grass

When leaves fall into the streams, they become breeding grounds for microbes, which in turn become food for macroinvertebrates — including the nymphs of aquatic insects such as stoneflies, caddisflies, and this mayfly.

They found that in the acidic streams, certain species that are more acid tolerant, including stoneflies and some caddis flies, became more abundant. Others that are less acid tolerant, such as mayflies and other caddis flies, declined. More surprisingly, they observed the same pattern in limed streams as well, showing that the treatment had little effect. “Once you have altered the macroinvertebrate community as a result of acidification, the tolerant species expand, and they are not going to leave just because the pH has gone up,” Fuller says. “If the acid-sensitive species try to come back, they find there is already another species filling their role.”

As a result, restoring those native macroinvertebrate communities could be a much slower process than restoring fish species — and in fact some species may never fully recover. Fuller and his team also found that the liming process itself could potentially harm macroinvertebrate species by creating a “toxic zone” of high concentration in the immediate area where it is applied to the stream. The watershed liming from the air could help to mitigate those effects by dispersing lime over the entire stream drainage basin; however, that process is much more costly. “If you can afford it, you get a much better response, but that’s expensive,” Fuller says. “We spent $100,000 for just one small drainage basin.”

“Once you have altered the macroinvertebrate community as a result of acidification, the tolerant species expand, and they are not going to leave just because the pH has gone up.”

Apart from the effects on the aquatic species themselves, Fuller is interested in studying how losing some species could change stream ecology in the long term. “It’s great when the big species do well, but it would also be nice to know what is actually going on with the functioning of the stream ecosystem,” Fuller says. For example, different species might decompose leaves at a different rate that could change the food chain in streams over time. Based on the results of this study, at least, it’s clear that overcoming the effects of acid rain may be a longer-term prospect than previously thought – and we may never see some stream macroinvertebrate communities return to their pre-acid numbers.