As Ice Coverage of Lakes Decreases, Scientists Work to Understand What Happens Under Water in Winter

During the summer months, much of life in lakes is limited to areas near the shore where light touches the bottom. But on the first day of ice-over in winter, a fish could look up and see a vast and temporary micro-ecosystem containing mats of algae and dozens of microorganisms buzzing around. For Stephanie Hampton, the deputy director of the Division of Biosphere Sciences and Engineering at Carnegie Science, this is the most magical day of the year. 

“It’s like waking up and it’s a snow day, but it’s going to last for six months,” Hampton said. 

Researchers who study lakes, known as limnologists, had long and incorrectly assumed that winter is a relatively unimportant season compared to the more active summer months. But as temperatures rise across the globe and the duration of ice cover on freshwater lakes rapidly declines, many limnologists are confronting, and rushing to address, a serious knowledge gap on the role winter ice cover plays on lake cycles and functioning. 

Over half of the world’s 117 million lakes freeze during the winter. But as temperatures rise, the average length of ice cover on lakes has fallen by over 30 days in the past century and a half. Thousands of lakes that used to freeze for part of the year no longer have any ice at all, according to a recent review in Science. As winter warms faster than any other season across much of the United States, researchers are grappling with what that means for lakes and the communities and creatures that depend on them.  

Records of ice cover itself are, for a select few lakes, some of the longest and largest climate data sets collected by people. The oldest, collected by Shinto monks in Japan, go back to 1443. But investigations into what happens under the ice, and its impacts on the lake year round, are considered lacking by many in the field. 

“It’s sort of a feedback loop of ignorance,” said Ted Ozersky, an associate professor at the University of Minnesota-Duluth who studies winter limnology. “You don’t know much about what’s happening there and you don’t read a lot about winter, so you implicitly assume nothing interesting or important goes on … and that discourages people from studying winter.”

Challenging Assumptions

It was on Siberia’s Lake Baikal that Hampton began to realize the damage these assumptions have caused. In most lakes, the amount of algae that are photosynthesizing in the winter is lower than in the summer. But the ice on Lake Baikal is largely quite clear, and it supports large mats of algae that grow underneath. When the ice breaks up, the algae tumbles toward the bottom for other organisms to feed on. 

It became obvious to Hampton that winter was an important time for lakes. When she started discussing the issue at conferences, many other researchers said the same thing. At the time, only two percent of literature on freshwater lakes examined lake processes that take place under the ice. 

She and her colleagues eventually synthesized the existing data, and in 2016 published a paper that found that the amount of plankton under lake ice is higher than expected, on average about half as much as the summer. Some lakes, such as Lake Baikal, have clearer ice which allows more light through and more algae growth in the winter compared to the summer. 

Today, that literature is still sparse compared to summer months. Sapna Sharma, a professor at York University in Toronto who studies environmental stressors on lakes, said there was a concentrated interest in winter limnology during the 1960s and ’70s. Then for some reason, there was a pause before the work slowly began again about a decade ago.

Hampton helped author the recent review in Science that examines what researchers do know about the effects of lake ice and its decline on lake ecosystems. Scientists have now started to understand how ice cover plays a critical role in moderating water quality and research suggests ice may help lakes retain carbon. The loss of lake ice also threatens fisheries by creating changes in the food web that can impact the nutritional quality of cold-water fish species and limit their reproduction. Lake ice also creates a very specific set of environmental conditions that can limit the spread of invasive species.

These advances in winter lake research are made when much of the theory regarding the relationships between algae and nutrients in lake systems are built around research that occurs during the open water season, Hampton said. 

When it comes to determining the role that lakes play in global carbon cycling, those estimates are often drawn from summer data. Just using that small subset of data creates errors in estimates of atmospheric interactions and other downstream effects, said Hilary Dugan, an associate professor at the University of Wisconsin Department of Limnology. 

“One of the things that we need is expanded theory,” Hampton said. “Because theory guides the experiments and the observations that we make.”

Getting Uncomfortable for the Sake of Research

Hampton believes most limnologists now understand that winter is an important time of year to study, but that realization hasn’t exactly translated into more winter research. Beyond the challenges of doing field work during the academic year, many researchers aren’t equipped to enter the field in the winter. 

Much of the gear that researchers use for winter limnology research is the same as what’s used in the summer, which can create challenges. Ropes and nets that are used to lower equipment into the ice can freeze into a “solid pretzel,” Ozersky said. Some sensors can shatter in sub-zero temperatures. Batteries don’t work as well in the cold. Getting nets for fish surveys set up under the ice is doable with the right equipment, but challenging. 

Additionally, because cutting a hole in the ice opens it up to light and the open air, the water in the surrounding area isn’t necessarily representative of under-ice conditions. Using an extension pole can help move sensors away from the hole. Divers trained to handle the cold conditions can also swim under the ice to take those samples. 

There are certainly solutions to these issues, like setting up a shelter and bringing hot water packs out onto the ice, but the challenges can be intimidating for limnologists who are only trained for summer field work. But for well-trained scientists, research on the ice when conditions are safe is in many ways easier than on the open water, because they aren’t limited by access to a boat. 

There is no foolproof method to determine if ice is safe to venture out on. The judgment depends on understanding the ice thickness and quality, Sharma said. Anyone who goes out onto the ice needs to be prepared to fall through it. Having specialized floating suits that provide warmth is critical—normal winter clothes and an ice jacket work in a pinch. Scientists should also know how to self rescue and pull themselves back onto the ice should they fall through. 

All of these issues can be alleviated with training. Ozersky, Hampton and Dugan helped organize a “winter school” at UW’s Trout Lake Station, where early career researchers were trained on sampling techniques for winter research. Professional ice safety instructors that typically work with first responders trained students on self rescue and assessing ice safety. It was the first time these particular instructors worked with researchers, and the scientists were thrilled to teach them the word “limnology.”

Ice safety training is important, but as ice cover declines so does the window of time when ice is safe enough to support research. There is only so much data that researchers can collect without being out on the ice. Airboats like the ones used by first responders for ice rescue are a viable option for moving on ice and water, Dugan said, but they require a serious investment and specially trained pilots.

A remarkably warm winter last year wreaked havoc on research plans for Sharma, who couldn’t sample certain lakes due to the thin ice. At some of her research sites, police told her team to stay off the ice due to safety concerns. Ozersky had similar experiences. 

“We certainly had to be more careful when we were out there,” Ozersky said. “But in a lot of lakes that are usually quite heavily fished, we were sometimes the only people out on the ice, which was not always comfortable.”