Warming temperatures to blame; fish habitat at risk
By Zachary Matson
Dissolved oxygen levels have been declining in Adirondack lakes in recent decades and new research shows that could be a big problem for fish.
Warming temperatures mean earlier ice breaks and later ice formation. Which causes lake stratification – the separation of a warm top layer from a cold bottom layer – to strengthen and last longer.
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Typically, mixing events in spring and fall replenish nutrients and oxygen throughout a lake’s water column. But as stratification lengthens, dissolved oxygen by late summer is falling to levels that pose harm to fish and other aquatic life.
A new research paper published this month analyzed deoxygenation trends in around 400 lakes worldwide, including over a dozen in the Adirondacks, and examined the volume of water and habitat where dissolved oxygen levels dropped below 5 mg/l. Below that level coldwater fish like trout and salmon can struggle to find suitable habitat.
The researchers from Cornell University and Rensselaer Polytechnic Institute estimated that across the studied lakes the proportion of the water column falling below the critical oxygen threshold had increased between 0.9% and 1.7% per decade.
“Assuming this applies to lakes generally, this is a massive amount of water,” said Stephen Jane, a postdoctoral fellow at Cornell’s Atkinson Center for Sustainability, the paper’s lead author.
Jane said some lakes will be less sensitive to the oxygen declines but that those with lower oxygen concentrations are likely to see more of the water column fall into low-oxygen conditions.
“Lakes are definitely having increases in the amount of water that is not suitable for coldwater species,” Jane said.
Paul Bukaveckas, a Virginia Commonwealth University professor who has studied Adirondack lakes since the 1980s, is finishing up a paper that uses data from the Hamilton County Water and Soil Conservation District. The data included temperature and oxygen measurements at different depths on 17 Adirondack lakes, known as profiles.
Bukaveckas found that the shape and size of lakes determined how harmed they will be by the declining oxygen levels. Large, deep lakes will retain more oxygen-rich waters than smaller, shallower ones. Imagine a trout searching for suitable habitat in a lake: surface waters warm more than desired, so they move to deeper waters, but those deeper waters are losing the oxygen they need to survive. The lake’s “Goldilocks Zone” is shrinking.
“It’s an issue of habitat compression,” Bukaveckas said. “They have less and less habitat available in the lakes.”
Some shallow Adirondack lakes don’t stratify and have consistent oxygenation throughout, but they are also susceptible to warming.
The 25 years of Hamilton County data showed that dissolved oxygen levels had declined in a significant number of lakes. In the early 1990s, 73% of the studied lakes registered average summer dissolved oxygen levels above 5 mg/l. By 2021, 55% of the lakes exceeded the oxygen threshold. Bukaveckas projected that by 2045 around 45% of the studied lakes would meet the threshold.
That habitat compression will be more impactful on certain lakes. Bukaveckas compared two Hamilton County lakes to demonstrate the point: Piseco Lake and Lake Eaton. Piseco is around 130 feet deep, while Eaton reaches a maximum depth of 56 feet. Oxygen depletion is more acute in Lake Eaton, where the cold layer of water is much thinner than at Piseco. Average summer dissolved oxygen levels at Eaton dropped below 3 mg/l, according to findings Bukaveckas said in an interview.
The loss of oxygen can alter the chemical and biological characteristics of lakes and create an accumulation of methane, a powerful greenhouse gas. Oxygen depletion results in the formation of a toxic form of mercury, potentially enhancing the bioaccumulation of mercury in fish. It may also exacerbate harmful algae and cyanobacteria growth, which in turn can deplete oxygen.
The Adirondacks’ history of acid rain further complicates research. When 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 it also limited sunlight’s ability to warm deeper waters, potentially strengthening stratification and exacerbating oxygen losses.
“We think that is driving or contributing to faster than average rates of oxygen loss,” said Kevin Rose, an RPI researcher who runs the lab where as a doctoral student Jane studied stratification and deoxygenation.
In a 2021 paper published in the journal Nature, Jane, Rose and other researchers demonstrated that a decline in dissolved oxygen was “widespread in surface and deep-water habitats” across the world’s temperate lakes from 1941 to 2017.
Researchers at Rose’s lab compiled and published a dataset of 28 Adirondack lakes focused on the concurrent impacts of climate change and acidification, including the oxygen and temperature measurements. The data, which ended in 2012, showed that Adirondack lakes were losing oxygen at faster rates than global averages, Rose said.
Rose is also one of the lead scientists organizing a large survey of around 300 Adirondack lakes focused on climate change. As part of that survey, known as SCALE, researchers want to collect temperature and dissolved oxygen profiles, and on a subset of the lakes deploy probes to collect high-frequency data throughout the ice-free season.
Rose said an initial $500,000 appropriated to plan for the survey is supporting work at a variety of research institutions. RPI is mining datasets to determine a representative list of lakes to study. Cornell is working on methods to survey lake biology using environmental DNA technology. The City University of New York is using its satellite imagery expertise to maximize remote sensing of impacts to Adirondack lakes. He said they could start survey work in 2023 if the funding is available, estimating three years of sampling around 100 lakes each year.
A group of researchers and advocates met to discuss the survey plans in Saratoga Springs in 2021. Rose and Peter McIntyre, a Cornell scientist, summarized the four overarching research questions of the project. The questions focused on carbon cycling, lake biodiversity and harmful algal blooms.
“Oxygen and temperature was question No. 1,” Rose said.