Changing Ecosystem Dynamics and the Fate of Brook Trout in the Adirondacks: How Concurrent Lake Browning and Warming Are Impacting an Iconic Cold Water Fish Species

As the state fish of New York, the brook trout has played a prominent role both in securing Upstate New York’s reputation as a destination for excellent freshwater fishing and in helping propel the region’s expansive outdoor recreation industry. Brook trout are especially important to the culture and economy of New York’s North Country, a constellation of fourteen Upstate counties that serve as home to Adirondack Park, the country’s largest “forever wild” forest preserve renowned for its unspoiled beauty. For generations, New York residents and tourists from across the globe have descended on Adirondack Park each summer to explore its wilderness, fish, and hunt.  Brook trout are the jewel in the crown of this unique natural area:  Adirondack Park is one of the few regions of the United States where the species thrives in lakes, underscoring its distinctiveness as an ecosystem and its stature as sacred ground for anglers and other outdoor enthusiasts. 

Despite the long standing centrality of brook trout to the regional identity and culture of New York’s North Country, the species faces a daunting challenge in Adirondack lakes, according to a recent study led by NE CASC researchers Stephen Jane and Peter McIntyre.  Their findings, published in the journal Proceedings of the National Academy of Sciences, reveal that Adirondack lakes are experiencing a shift in ecosystem dynamics that is dramatically reducing their suitability for brook trout and other cold-loving species such as whitefish and salmon. The researchers say this transformation is driven by the simultaneous warming and browning of the region’s lakes, conditions caused by a combination of climate change and the damaging legacy of acid rain. 

Jane, a former NE CASC postdoctoral fellow at Cornell University and lead author of the PNAS study, describes the problem this way: While brook trout depend on water that is both cold and well oxygenated for their habitat, Adirondack lakes have increasingly become either cold or well oxygenated–depending on water depth. Over the past several decades, for instance, Adirondack lake temperatures have increased near the surface–where oxygen levels are high–while lake temperatures have remained cold near the bottom–where oxygen levels have decreased. The result is what Jane calls an “oxythermal squeeze” in which surface waters become too warm, and deep water becomes oxygen-depleted, leaving brook trout in a precarious state of existence where they are restricted to a shrinking middle layer of hospitable water.

“For the time being, brook trout are hanging on as a species in the Adirondacks,” says Jane. “But there’s no question that these fish are going to be squeezed into ever smaller spaces–sometimes only a few feet thick from top to bottom–and that limitation will almost inevitably cause their populations to dwindle.” 

The inhospitably warm upper layer of water present in many Adirondack lakes is created by a process known as browning, which occurs when dissolved organic matter from plants turns water tea-brown. Browning water traps heat near the surfaces because the sun’s rays are absorbed by these organic compounds in the water. The tea-stained water also prevents sunlight from penetrating into lower lake depths, creating stratified water columns in which warm, oxygenated upper layers do not mix with the cold, deoxygenated lower layers. 

The pervasive browning of Adirondack lakes arises from the altered soil chemistry that is a product of acid rain, which plagued the region throughout much of the 20th century. Although the 1990 Clean Air Act amendments halted acid rain, its effects remain visible in the reduced capacity of Adirondack soils to absorb weak organic acids. “For more than 100 years, we acidified the region through smokestack emissions that catalyzed the deposition of nitric and sulfuric acids in the Adirondacks,” says Peter McIntyre, an NE CASC principal investigator, Cornell University faculty member, and the study’s senior author.  “That has fundamentally changed the ability of the soils to hold onto and process these organic acids before they are released by the region’s watersheds and into its lakes. And with decreasing acidity and increasing atmospheric CO2, forest ecosystem productivity is translating into more and more plant matter leaching into our lakes.” 

The problem of lake browning becomes even more troubling, McIntyre adds, when additional dimensions of climate change are considered. “On one hand, rising average temperatures due to climate change have lengthened the growing season, increasing the production of plant matter that can be transported into lakes,” McIntyre says. “On the other hand, the growing frequency of intense precipitation events that we are experiencing under climate change makes it more likely that dissolved plant matter will be flushed from the landscape into Adirondack lakes to further accelerate browning.” 

To conduct their study, Jane and the team placed data recorders in 15 Adirondack lakes, measuring dissolved oxygen and temperature throughout the water column in each lake. The devices collected measurements on an hourly basis from late spring until the early fall, enabling them to build high-resolution maps of potential brook trout habitat. After collecting this data, Jane compared his findings to 1980s records from almost 1500 Adirondack lakes to identify which ones are likely to present a stressful environment for brook trout during the summer months under the present-day climate. 

According to McIntyre, Jane’s findings were alarming. “Steve’s work shows that the scale of the problem is much larger than we expected,” McIntyre says. “In the coming decades, cold water fish species will be squeezed out of numerous Adirondack lakes if these warming and browning trends continue. Prior to these results, the assumption was that habitat degradation was relatively uncommon rather than ubiquitous across the park.” 

Although brook trout habitat in North Country lakes is being severely undermined, the study identifies two types of lakes that hold promise as refuges from the challenges of browning and warming. The first are deep lakes that extend to depths of thirty meters or more. These lakes have proven immune to the thermal squeeze because, as McIntyre says, “they contain so much water that their oxygen is difficult or impossible to deplete after it is refreshed by biannual mixing in spring and fall.” The second type includes shallower lakes that are entirely–or almost entirely–clear. As McIntyre explains, these water bodies have cold, oxygenated layers that expand more rapidly than their deoxygenated layers. “Brook trout in clear, shallow lakes may actually benefit from some degree of browning, which prevents solar energy from warming their lower layers,” he says. “Unfortunately, these favorable conditions are expected to arise in only five percent of Adirondack lakes.” 

Given the study’s revelations about brook trout habitat decline, Jane says that it will become increasingly important for conservation agencies to prioritize the management of lakes that have escaped the oxythermal squeeze. “First, we need to identify lakes that are most likely to provide cold-water fishes with places where they can thrive well into the future,” says Jane. “Second, we need to make sure that these locations are protected from invasive species, nutrient pollution, and other forms of environmental degradation. By taking these measures, we can hopefully help these iconic species navigate the immense obstacles that are impeding their ability to persist in North Country lakes.”