(WXYZ) — Climate change is warming Lake Michigan's surface, but also its deep waters, which could have a significant impact on the lake's ecosystem and mixing process.
A long-term study from the National Oceanic and Atmospheric Association, published in Nature Communications, looked at 30 years of nearly hourly water temperature measurements from Lake Michigan spanning the entire water column, from the lake bottom to just below the surface.
The study's lead author, Eric J. Anderson, said the warming of the lake's deepwater has a number of implications, including on the lake's ecosystem.
Greg Dick, a professor of environmental sciences with an appointment in ecology and evolutionary biology at the University of Michigan, said Lake Michigan has already changed a lot in the last 40 years.
The lake's biggest challenge has been the invasion of mussels – specifically zebra mussels and quagga mussels, which have filtered out a lot of the lower food chain.
"Lake Michigan has already faced some big challenges with these invasive species, and this picture that we're talking about now is another big challenge on top of that," Dick said.
The warming deep waters of Lake Michigan will have a profound impact on the lake's food web, Dick said. He said they can influence the types of organisms that are present including shifts in Zooplankton, which are in the second-lowest rung of the food chain and reduce primary productivity.
"This is a concern because in the end, it will result in a reduced abundance of fish and larger organisms," Dick said.
Lake Michigan's food chain starts with Phytoplankton at the bottom. Phytoplankton are tiny photosynthetic organisms like algae that are doing photosynthesis.
"They take that energy from the sun and use that energy to fix carbon into food sources that the rest of us can eat," Dick said. "They're the ones bringing energy into the ecosystem and into the food."
Above the Phytoplankton are the Zooplankton, which are the organisms feeding on the Phytoplankton. According to Dick, they serve as a food source for small fish, and in some cases, the larvae of a bigger fish.
The mussels, which eat the Phytoplankton, are already removing the food sources of the Zooplankton and reducing the flow of energy up toward the fish, according to Dick. The warming deep waters are just going to make that interruption worse.
"This type of change in temperature can have a similar effect. It can reduce primary production, which eventually reduces the flowing up the food chain towards the organisms that we really care about, fish and so on," he said.
That could mean a reduced fish abundance, which so many Michiganders care about for sport fishing and commercial fisheries.
Lake Michigan is also a dimictic lake, which means the water mixes twice a year from the top to bottom, and the warming means the times of mixing are shifting.
The lake mixes once in the spring and once in the fall. In between those, the lake will have ice growth in the winter and a warm surface layer developed in the summer.
"In the study, we not only found that the lake is warming down to below 100 meters deep, but most of that warming is happening in the wintertime," Anderson said. "That really means the winter is getting shorter and less intense."
According to the study, the water has warmed about .11 degrees Fahrenheit each decade. That means less ice cover, which is already declining across the Great Lakes.
It also means that it could shift the way the lake mixes in the future.
"The projection is that Lake Michigan, with current climate trends, will turn into a lake that mixes only once per year within the next century," Anderson said.
What that means, according to Anderson, is that the winter collapses and we don't have the winter as we know it now. That can have a major implication on things living in the lake and disrupt the entire food chain.
"I think the most dramatic implication would be to the food web in the lake and our reliance on that. The fisheries we rely on, also the way we're affected by things like algal blooms and other things that occur in the lakes," Anderson said.
According to Anderson, though the data is specific to Lake Michigan, they expect it to represent generally what's happening in large lakes around the world, especially at the same latitude as Lake Michigan.
"We would expect this to be similar to what we'd see in the rest of the Great Lakes, with the exception of Lake Erie because it's so shallow, it's not going to have the same deep-water trends, just because there isn't the deep-water there," he said.