Lasting ‘lake heatwaves’ imperil ecosystems

January 20, 2021

Lakes will get and stay hotter for longer, leading to ecosystem troubles. (AP Photo/Dar Yasin)

As the climate changes, lakes are on track to experience lengthier and more severe periods of extreme warm surface temperatures by the end of the century, novel research published Wednesday shows, with some even projected to reach a “permanent heatwave state” that could alter entire ecosystems and imperil the economic benefits they provide.

Establishing the term “lake heatwave” in academic literature, the authors of the new modeling study, a collaboration by researchers in Ireland, Germany, Sweden and the U.K., say their findings should serve as a wake-up call for individuals and governments regarding the need to rein in greenhouse gas emissions.

The most drastic effects described in the study published in Nature, which shows the impact of heatwaves on 702 lakes between 1901 and 2099, would occur in a high-emission scenario that has been shown in other research to best represent the path down which the world is trending through mid-century and plausibly beyond.

Under that scenario, called Representative Concentration Pathway 8.5, the average temperature of lake heatwaves is expected to increase from approximately 3.7 to 5.4 degrees Celsius, with the events lasting more than three months by the end of this century, as opposed to about one week now.

That’s compared to increases reaching about 4 degrees Celsius and one month under the most conservative emissions scenario, known as RCP 2.6.

Co-author Eleanor Jennings, who directs the Centre for Freshwater and Environmental Studies at the Dundalk Institute of Technology in Ireland, told The Academic Times she was especially disconcerted by certain findings regarding lake stratification. 

Typically, stratification is a seasonal process by which the surface heats up while lower waters stay cooler and are often deoxygenated, effectively splitting up the body into separate layers. But some lakes could become so stratified under the most extreme warming circumstances that they may spend multiple seasons in this state or never even leave it, according to the study. 

“That would fundamentally alter that ecosystem and would have huge implications for biodiversity within those lakes,” Jennings said. For example, a recent local study at the Dundalk center found that colder-water-dwelling species like Arctic char could disappear from those lakes because of warming temperatures.

“The rate at which species adapt isn’t often as fast as how our climate is changing. Species are trying to play catch-up,” Jennings said. “So it’s not just like we can say, ‘Oh, our Irish lakes might become like a lake in Spain’ or something like that, because we don’t have those species here.”

In contrast, such warming could benefit types of phytoplankton like cyanobacteria that stay in warmer, calmer waters near the surface and can cause harmful toxic algal blooms that the Centers for Disease Control and Prevention says are increasingly menacing the U.S.

“A projection that these conditions are going to last longer and be warmer in lakes could potentially favor these species that have implications for water quality,” Jennings said.

That could keep humans from swimming in those lakes or using them as a water source, she noted. Other “ecosystem goods and services” potentially in peril from lake heatwaves include tourism and commercial fishing that is done in some larger freshwater lakes. 

Deeper lakes reaching up to nearly 200 feet will experience less intense but longer-lasting heating events, according to the study.

Besides being notable for focusing on a marine environment that is less frequently the subject of climate analyses, the study is a departure from previous work that has started by examining extremes in the weather and then looked to the lakes to see what happened.

“In this case, it’s using data from within the lake to define when those conditions are extreme,” Jennings said. “Which is what is most important if you’re something that lives in a lake.”

Jennings hosted the study’s first author, R. Iestyn Woolway, for two years while he undertook the project with support from the European Union’s Horizon 2020 research and innovation program under a Marie Sklodowska-Curie Action Individual Fellowship. The competitive funding sources are meant for scientists pursuing cutting-edge work, and Woolway has been involved in recent years in lake modeling on a global scale.

Now, Woolway is exploring the implications of temperature changes for other lake processes, particularly their methane-emitting abilities and role in the carbon cycle. While that’s an area that until the last five to 10 years was less understood, any aspirations to accurately measure and balance out carbon emissions worldwide must take lakes into account, Jennings said.

Jennings hopes the researchers’ increasing drumbeat will help drive individual and institutional efforts to curb greenhouse gas emissions.

“If you’re just talking about these lake heatwaves, the answer to managing them actually lies at the changes in the carbon footprint, either personally or at a kind of governmental level,” Jennings said. 

“It’s a bit hard to manage at a local level, except that you can maybe use some sort of modeling techniques to predict what might happen,” she said. For example, if someone is managing two lakes and one is likely to have longer stratification periods, that person could decide to use the other as a water source instead.

The study “Lake heatwaves under climate change,” published Jan. 20 in Nature, was authored by R. Iestyn Woolway, Dundalk Institute of Technology and the European Centre for Space Applications and Telecommunications; Eleanor Jennings, Dundalk Institute of Technology; Tom Shatwell, Helmholtz Centre for Environmental Research (UFZ); Malgorzata Golub and Don C. Pierson, Uppsala University; and Stephen C. Maberly, UK Centre for Ecology & Hydrology Lancaster Environment Centre.

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