Cornell scientists warn of dire threats to native fish habitat from warming temperatures, browning water
By Zachary Matson
Brook trout habitat in all but a small portion of Adirondack lakes could disappear as climate change warms lakes and summertime oxygen loss is exacerbated by lake browning associated with the region’s recovery from acid rain, according to a new study by Cornell University researchers.
The researchers estimated that only 5% of Adirondack lakes may sustain suitable habitat for cold-water fish like Adirondack brook trout and Atlantic salmon when water temperatures and oxygen depletion peak in the summer months.
The most resilient lakes are those deeper than 30 meters, far fewer than 1% of the region’s lakes, and the declining number of shallower lakes that are still relatively clear, less than 5% of Adirondack lakes, according to the study.
Stephen Jane, who led the research as a postdoctoral fellow with the Cornell Atkinson Center for Sustainability, said the combined effects of lake browning and warming temperatures are pushing fish into smaller and smaller slices of the water column during the stressful summer season.
“They are squeezed from above by warm temperatures and squeezed from below by low oxygen,” Jane said.
Warming air temperatures mean earlier ice breaks and later ice formation, lengthening lake stratification – the separation of a warm top layer from a cold bottom layer. Typically, mixing events in spring and fall replenish nutrients and oxygen throughout a lake’s water column. But as stratification lasts longer, dissolved oxygen by late summer is falling to levels that pose harm to fish and other aquatic life.
When Adirondack lakes were highly acidified, organic matter did not readily dissolve into the water column, resulting in many clear lakes. As pH levels increased so did the solubility of organic matter, and water clarity declined. The lake “browning” was a sign of recovery from acid rain but is now an emerging threat.
The browning effect concentrates warming in a thin layer at the water’s surface, where Adirondack water temperatures are rising faster than other regions around the globe. The browning prevents light from penetrating deeper into the water column, leading to some cooling in deeper waters. Scientists are finding that temperatures in the deeper parts of many lakes are cooling.
In some cases that can increase trout habitat, but the new study outlines how many more lakes are past a threshold where the combined harms of browning and warming outweigh the potential benefits of expanding cool waters.
At the time of a massive survey of over 1,400 Adirondack lakes in the 1980s, about 23% of lakes were clear enough that habitat could benefit from some browning, according to the study.
Now, less than 5% of lakes would benefit from continued browning, researchers found using the historical data, while more browning will exacerbate habitat loss in the rest of the park’s lakes.
“Cold water fish, especially in the Northeast are no strangers to loss of habitat,” Jane said. “The difference now is in the Adirondacks you have major drivers acting across the entire region, even in the absence of obvious human alteration of the environment.”
More research needed
Cornell researchers in Ithaca and at a longstanding field station at the Adirondack League Club near Old Forge continue to study brook trout and how to protect them in the Adirondacks’ changing waters.
The field station is working with Department of Environmental Conservation scientists on a study of different brook trout genetic strains, attempting to find which are best adapted to the warming conditions.
Pete McIntyre, an aquatic biologist at Cornell and co-author of the habitat study, said the new research shows “a staggeringly large proportion of Adirondack lakes where there is very little habitat remaining for brook trout at the end of the summer.”
McIntyre, who is also a lead organizer of the new SCALE survey of hundreds of Adirondack lakes, said the habitat study demonstrates the importance of collecting data at varying intensity levels across the region.
The researchers used a handful of closely studied lakes to determine the patterns of warming, browning and oxygen loss and then relied on the 1980s study to project those trends across more than 1,400 Adirondack lakes. A new survey would enable scientists to verify those trends and develop a better understanding of where brook trout are present.
“There is no substitute for going out to large numbers of [lake] systems to get those biological assessments and verification of predictions,” McIntyre said. “Which lakes have lost fish species they definitely had in the 1980s?”
Tommy Detmer, another author of the study, splits time between Ithaca and the Adirondack field station, where he oversees ongoing research. Detmer said fisheries managers should work to protect the region’s most resilient lakes from other threats, like invasive smallmouth bass that prey on brook trout.
He said lakes that are buffered against the warming and browning threats that have not been invaded by competitor fish are critical to maintaining healthy trout populations in the region; many of those lakes are privately controlled.
“Those lakes that haven’t been invaded are of high-tier conservation value,” Detmer said.
Cornell researchers working at the field station this summer started tracking the minute movements of some brook trout, monitoring the fish as they move up and down the water column. Initial data demonstrates how the monitored fish refuse to enter a lake’s oxygen-deprived depths, mapping the brook trout’s squeezed habitat.
“There is a line that the fish won’t go below,” Detmer said. “We are already in future climate scenarios, it’s no longer something we are looking toward that might be a generation down the road.”