CORVALLIS, Ore. -- Lakes recovering from the impacts of acid rain in the northeastern United States may offer a buffer from the effects of climate change for an iconic recreational fishery.
Brook trout are sensitive to acidity and to water temperature. While recent reductions in acid rain have led to brook trout recovery in many mountain lakes, these fish are increasingly under threat from more frequent and severe hot summers.
Fortunately, trout may benefit from an unanticipated change in lake water chemistry. That's because changes in water clarity can affect the amount of deep, cold water habitat, and provide a key refuge for trout from increased warming at the lake surface.
Lakes recovering from acid rain tend to experience increases in the amount of plankton and dissolved organic matter in the water. As a result, sunlight can't penetrate as far into the lake. The darker water reduces light penetration and the amount of solar radiation that reaches deep water.
In a report published today in the journal Global Change Biology, scientists led by Dana Warren, an aquatic biologist at Oregon State University, described the changes that are unfolding in these lakes and the implications for brook trout. Co-authors were from Syracuse and Cornell universities
Since passage of federal Clean Air Act regulations in the 1980s and 90s, the researchers wrote, acid rain has been reduced, and lake-water chemistry has begun to return to pre-industrial conditions. That process is expected to take many more decades, but scientists are now seeing self-sustaining brook trout populations become re-established in lakes where they have been absent or in low abundance for three decades or more.
The changes in water chemistry may facilitate that trend.
"As lakes recover, they get darker. Darker water absorbs more light, and solar radiation doesn't go as deep," said Warren. "This means that warming is kept to the upper layers of the lake, which can lead to more cold-water refuge habitat in deeper water during hot summers, like the one we saw last year across the northeast."
This process is particularly important for trout in the numerous small lakes across eastern North America, he said, where the amount of cold-water refuge and the degree of lake stratification can be limited in hot summers.
Warren is affiliated with the colleges of Forestry and Agricultural Sciences at Oregon State. His research focuses on the interaction of chemical, physical and biological factors in lakes and streams.
In the Adirondack region of New York -- an area heavily impacted by acid rain -- a self-sustaining brook trout fishery has returned to a lake where Cornell researchers have worked for more than 50 years. Located on private land, the lake represents a well-studied example of native trout recovery in a historically acidified mountain lake ecosystem. Similar improvements have been seen in Brook Trout Lake in the southern Adirondacks and in a dozen other lakes in Adirondack Park that have been stocked with this iconic fish species.
Brook trout generally prefer temperatures below 61 degrees and become stressed when temperatures exceed 68 degrees.
In addition to changing the amount of organic matter leached from the landscape, chemical changes brought about by acid rain reductions can also increase the supply of phosphorus, a critical nutrient for plankton. As plankton multiply, they reduce light penetration into the water and cause the boundary between the layer of warm surface water and colder deep water to rise -- creating a thinner, warmer layer at the top and a larger, cooler area below.
Climate models for the Northeast suggest that summer temperatures in this region are likely to increase, and the top layers of stratified lakes are therefore also expected to become warmer and larger.
"It is important to recognize that this fortunate by-product of acid-rain recovery does not eliminate the direct and indirect threats to these populations from climate change," Warren added.
"It may afford the populations in these lakes greater resistance to impacts of climate change in the near future, but in the long term, climate change remains a major issue for trout, especially those in more southerly regions at the edge of their range."
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Dana Warren, 541-737-2244, firstname.lastname@example.org