Over the past 15 years, scientists and park rangers have fanned out across more than two dozen remote lakes at National Park sites around the western Great Lakes to see how global forces are affecting these bodies of water. Hiking, paddling, and boating through the backcountry, they collected water and sediment not only to understand the lakes’ past and present, but also to predict what their future might hold.

They found that the complex ecosystems of some of the most pristine lakes in Minnesota, Wisconsin, and Michigan are already being disrupted by climate change and other distant human impacts.

Some of the most significant changes have been seen in the most remote places, says David VanderMeulen of the National Park Service. Fifteen years ago, the agency started getting reports of harmful algae blooms in Lake Richie on Isle Royale.

“It’s in a wilderness area on an island in the middle of Lake Superior,” VanderMeulen says. “Yet we see all these changes.”

In the absence of human activities in the surrounding landscape, there must be other causes. The National Park Service enlisted the help of the Science Museum’s St. Croix Watershed Research Station to find them.

Protected but not safe

The team picked study sites far removed from any development, scattered from the eastern shore of Lake Michigan to the Minnesota-Ontario border. “This helps peg changes to climate and broader global processes, like nitrogen and dust in the atmosphere,” says Mark Edlund of the Science Museum.

They worked on lakes at Voyageurs and Isle Royale National Parks, Apostle Islands, Pictured Rocks, and Sleeping Bear Dunes National Lakeshores, Grand Portage Reservation, the Boundary Waters Canoe Area Wilderness, and several lakes in northern Minnesota state parks and national forests.

“The lakes were specifically chosen because they haven’t had a lot of other impacts to them. Things that are happening to them aren’t driven by local cabin development and things like that,” says Edlund.

They also chose lakes that would contribute to a diverse database. There were big lakes and small lakes, deep and shallow lakes, lakes with large and small watersheds. This helped them tease apart the different ways climate and similar global forces affect these ecosystems.

What they found was worrisome. With no interference from local disruptions such as development or agriculture, changes in the lakes were still dramatic. As the average global temperature has risen in recent decades, the foundation of the lakes’ food web has been affected.

In the past 30 years, waters have warmed, with less ice and longer growing seasons, especially in the last five to six years. Cycles of stratification into layers of warm and cold water have been disrupted, upsetting the oxygen balance. Precipitation and wind patterns have shifted.

The kinds of algae called diatoms growing in the lakes also changed. These single-celled plants form the base of each lake’s food web and are closely connected to water quality. Observation of diatoms showed how ecosystems are being altered. Combined with other information, it pointed to a key culprit.

“The fact is, climate change is affecting these lakes,” says Joy Ramstack Hobbs of the Science Museum.

Depending on diatoms

Diatom photos courtesy Mark Edlund and Joy Ramstack Hobbs

The same thing that makes the lakes valuable research sites, their remote locations, also makes them difficult to access, and traditional measurements, taken during three to four visits each year, can’t capture all of the changes. But analyzing diatoms can give a picture of a lake’s overall condition, not just a snapshot of the day when it was sampled. When these diatoms die, they sink into the sediment and can tell a powerful story.

“They can integrate what has happened in a lake over a number of years, so we can learn a lot from them,” says Edlund, who has been studying the organisms for 35 years.

There are many species of diatoms in any given lake, and the different kinds are connected to different characteristics of water, like the temperature or amount of food or sunlight. When those variables change, some species multiply, and others decrease. Conveniently, each kind leaves behind unique fossils, which are preserved in sediment on the bottom of the lake. With a microscope, trained eyes can count the different species, based on their fossils or living forms, and begin to unravel how diatom populations have shifted. That can reveal past and present conditions in a lake or its watershed, and how and why changes have occurred.

“Diatoms allow you to see how things like water chemistry are affecting lake ecology,” Ramstack Hobbs says.

Paired with weather records and traditional measurements like clarity and acidity, the changing diatom populations showed how the lake ecosystems are being disrupted, in much more detail than traditional monitoring could detect.

“Without doing the kind of monitoring we’re doing with diatoms, we might not have noticed the changes at all,” says VanderMeulen.

One of the most noticeable changes in diatoms over the past 100-plus years was simply the increase in their abundance. The number of diatoms found in sediment cores grew, along with other signs that more algae are growing than before. The species Fragilaria crotonensis—while not widespread—is found primarily in lakes with higher levels of nutrients. Where it was found in the study lakes, its population was rapidly increasing, likely responding to more food.

Although lake biology can be monitored by measuring other types of life, like fish or insects, diatoms are simpler in some ways, and provide a great deal of precision. It was an innovative approach at the time work began in 2005, now used more widely.

Fundamental changes

One of the biggest changes the researchers observed is in how lakes stratify in summer into a warm layer of water on top and a cool layer below. Stratification is a natural cycle that occurs in many lakes, but not all. Some split into layers once in spring and then mix again in autumn, while others switch back and forth throughout the season. Meanwhile, many shallow lakes rarely stratify.

Stratification is driven by everything from air and water temperature to wind. Slight changes can affect what lives and thrives in a lake, including diatoms. In addition to affecting the temperature, oxygen levels in the lower layer tend to dwindle over time, creating conditions that influence its ecosystem.

But these cycles are being significantly altered by global warming. Lakes that were once kept from stratifying by wind mixing the waters have begun stratifying more as winds have declined on average. Ultimately, some lakes that did not historically stratify are now experiencing it regularly, while other lakes that would stratify once in the spring and mix in the autumn are now seeing much longer stratification seasons.

“Changes in diatom communities over the long- and short-term appear to be driven mainly by changes in the lake thermal regime,” says VanderMeulen.

Disrupting stratification can have cascading effects on lake ecosystems, especially combined with other effects of climate change. When lakes stratify, there is often little oxygen near the bottom, which can cause nutrients to be released from the sediments. Those additional nutrients can trigger algal blooms when the water mixes again. The lack of oxygen near the bottom can also trap fish in warm-water top layers just when they need to cool off. And earlier stratification in spring can prompt early growth of phytoplankton—those important algae that live in the water column—putting them out of sync with the creatures that rely on them for food.

While all the study lakes are seeing significant shifts in lake ecology, there was clearly one kind hardest hit: deep lakes that usually stratify once in the spring and mix again in late fall. More specifically, deep lakes with small watersheds, where there are fewer effects from runoff, and the water is more connected to global trends.

“Deep lakes are taking it on the chin,” Edlund says. In Voyageurs National Park, Cruiser Lake and Mukooda Lake—deep, clear, and cold—typify lakes that are experiencing the disruption of their natural conditions and the diatoms growing there.

It would be tempting to think these lakes could more easily stay cool all summer, and therefore are safer from global warming. But because of their dependence on stratification, they are actually more sensitive. And these same lakes are often the gems of National Parks, home to the clearest water and coldwater fish like lake trout and cisco.

Conclusions, and more questions

It can be frustrating for local park managers to face such challenges when the cause and solutions are global in scale.

“The challenge is to try to take the results of this study and translate that into any kind of management action that could be taken by the parks,” VanderMeulen says.

One way it will be useful is helping the National Park Service identify the most vulnerable lakes, and closely watch for changes. This in turn could help prioritize efforts to reduce human impacts like invasive species or nutrient-rich runoff. Those impacts can combine with climate change to accelerate change.

“Knowing where the greatest change is happening will allow us to direct limited resources, such as additional monitoring or buoy deployment, to the right systems,” the researchers say.

The next step could be to extrapolate thresholds for water quality and diatom populations that would help lake managers monitor for major changes.

Understanding the effects of climate change on these lakes could be critical for future generations. The landscape where most of the study lakes are located, the boreal forest, covers 15 percent of the planet and contains three million lakes adding up to more than 60 percent of Earth’s surface freshwater.

The project has led to new questions, and new research projects. This summer, the Science Museum is launching an ambitious new study of lakes in the Boundary Waters Canoe Area Wilderness. These are some of the most pristine waters in the conterminous United States, but they too have been exhibiting concerning trends. The study will add airborne dust collection to diatom and water monitoring. It may be that the wind is carrying soil and nutrients great distances from farm fields and other sources—another almost invisible process with potentially significant impacts to our treasured lakes.

References

Ramstack Hobbs, Joy M., Heathcote, Adam J., VanderMeulen, David D., and Edlund, Mark B.. 2022. “Integrating Water Quality Monitoring and Diatom Community Trends to Determine Landscape-Level Change in Protected Lakes.” Ecosphere 13( 8): e4199. https://doi.org/10.1002/ecs2.4199

Edlund, M.B., Ramstack Hobbs, J.M., Heathcote, A.J. et al. Physical characteristics of northern forested lakes predict sensitivity to climate change. Hydrobiologia 849, 2705–2729 (2022). https://doi.org/10.1007/s10750-022-04887-9