Algae growth is first documented biological impact of warmer Lake Superior
Scientists have known for years that Lake Superior and other Great Lakes have warmed rapidly in recent decades and now, for the first time, they have documented the first physical impacts of that warming — an explosion of a tiny algae called cyclotella.
That’s the finding of a new study published Friday in the journal Limnology and Oceanography.
“This is the first detection of a biological impact from climate change on the Great Lakes ecosystem,’’ said Euan Reavie, paleolimnology specialist for UMD’s Natural Resources Research Institute, one of nine scientists from six states and provinces who collaborated on the project.
Reavie studies the history of algae by taking samples out of the sediment at the bottom of the lakes. He and other researchers found cyclotella have been increasing for decades right along with the temperature of all five of the Great Lakes.
“The signal is consistent over a broad area, over all the Laurentian Great Lakes’’ that climate change is spurring the change in cyclotella, Craig Stow, scientist for the Great Lakes Environmental Research Laboratory of the National Oceanic and Atmospheric Administration, told the News Tribune. Stow was one of the principal researchers in the study.
“When you see something respond like this at the bottom (of the food chain), you know that’s there’s something larger going on out there,’’ Stow said. “We know what’s causing it. But we still don’t know what this means higher up’’ the food chain.
Stow and others said the broad expansion rules out other issues like pollution, invasive species or development.
The fact the algae are increasing from clear, cold, infertile Superior to shallow, warm and fertile Erie was surprising. But cyclotella also are increasing in other remote, cold-water lakes where humans have had little or no impact other than warming the climate — lakes without urban or agricultural runoff or any human-introduced invasive species.
“That’s telling us that there’s something else going on impacting all the lakes at one time,’’ Reavie said. “The only thing that can be is climate.”
Reavie believes the round, microscopic diatom is responding to stratification, or layering, of the water column that’s occurring more often and for longer periods as the lakes are warming.
Cyclotella favor the warm layer near the surface of the lake which has been more prevalent in recent years. Other larger algae are dying off and sinking and are disappearing from the ecosystem. That includes big algae like daphnia which have been a major food for small fish in the lakes.
“The big algae are dying off. They are still present, but not in nearly the numbers they used to be as cyclotella expands,’’ Reavie said.
Jay Austin of the University of Minnesota Duluth, chairman of the Physics Department and one of the UMD scientists who first discovered Lake Superior’s rapid warming, said Lake Superior and all Great Lakes are stratifying earlier and staying stratified, or layered, longer into the year.
“That’s giving the algae more time in their euphotic condition,’’ Austin said, namely lots of light near the surface where the warm water layers on top of cold water.
Each fall, the water gradually cools until at one point the cold water can layer on top of warm. Each spring, the water gradually warms, and starts to layer when it hits 39 degrees Fahrenheit, Austin said.
After that point water below the warm layer hardly changes temperatures at all, staying near 39 degrees. Water on the top warm layer can warm well into the 50s and even 60s at some points.
What this change at the very base of the food web might mean for zooplankton that eat algae — or the small fish that eat zooplankton or the herring, trout and salmon at the top of the food chain — no one knows yet.
“We need to find that out as quickly as possible,’’ Reavie said, noting the zooplankton that once ate larger algae will either transition to graze on cyclotella or they will die off. “I don’t expect people to be worried about more cyclotella, or to be scared of it. It’s not a problem on its own. What happens (in the food chain) above that may be the problem.”
Austin and other researchers studying historic temperature data from Lake Superior buoys discovered nearly a decade ago that that big lake has warmed rapidly at the surface in recent decades — faster than even on-land weather stations nearby. Later research found the same thing happening in other large lakes worldwide.
Other research has documented the marked decline in ice cover on Lake Superior over the last 40 years — corresponding with a global rise in temperatures that the vast majority of scientists who study the issue say can only be attributed to human-spurred greenhouse gasses like carbon dioxide and methane, mostly from burning fossil fuels like coal and oil.
In September, Reavie and his fellow scientists received a $2.5 million U.S. Environmental Protection Agency grant to keep studying algae patterns in the Great Lakes, including looking at algae near shore and in winter. Scientists will deploy robots to collect algae from under the ice.