Lake Superior is cold, deep and clear. But it’s no longer the clearest of the Great Lakes.
Lakes Michigan and Huron have gotten clearer, bumping Lake Superior to number three.
Scientists have been able to figure how much clearer by using satellite imagery.
Gary Fahnenstiel is a research scientist at Michigan Technological University and the University of Michigan. He says that the main reason behind increasing water clarity in Lakes Michigan and Huron is the invasive quagga mussel.
“There aren’t any quagga mussels, any substantial number of quagga mussels in Lake Superior, but large numbers in Lake Michigan and Lake Huron,” says Fahnenstiel. “And they’re filtering out the particles and everything else in the water, so they’re making it clearer. It’s been estimated in Lake Michigan that the quagga mussels can filter the entire volume of Lake Michigan in four to six days. You know, there’s literally trillions and trillions of them.”
Fahnenstiel says reductions in phosphorus loading into Lake Huron, and climate change are also driving the changes.
“As a lifelong Michigander, it’s kind of built into the paradigm: Lake Superior is the clearest, most oligotrophic, or least productive, of the Great Lakes. And that’s no longer true. In fact, it’s not even second anymore, now it’s third. Lake Michigan has passed it.”
But Fahnenstiel points out the changes are relatively small.
“I mean, we’re talking five to ten percent changes. Because Lakes Michigan and Huron were always relatively clear; it’s just they’ve gotten clearer, and this allowed them to pass Lake Superior,” he explains.
The findings also don’t indicate that Lake Superior has gotten any murkier.
“Lake Superior’s water clarity does not appear to have changed since ‘98, and if we look at the amount of algae in the water, the productivity in the lake, that doesn’t appear to have changed significantly in the last 50 to 60 years,” says Fahnenstiel.
But the beauty of clear, blue water in the Great Lakes has a flip side: it can spell bad news for the food web.
“So it’s not only that Lake Huron is the clearest water right now, Lake Huron has the least amount of algae,” says Fahnenstiel. “And it’s been almost a 50% drop in the algae in water in the last 15 years, as well as Lake Michigan.”
He says the mussels filter water, and eat tiny algae called phytoplankton, that form the base of the food web in the lakes, and that's bad for fish.
"The mussels have either directly or indirectly been related to most fishery declines in the upper Great Lakes - Lakes Michigan and Huron," he says.
“You know, there’s a reason western Lake Erie is the most productive fishery in the Great Lakes. It’s because it has a lot of algae,” explains Fahnenstiel.
“The one thing we always talk about as limnologists - ecologists who study lakes - is it’s always funny what people want. They want their lakes to look like their swimming pool, but have the fishery production of a trout pond. And it’s just impossible.”
In case you're thinking.... hey, maybe mussels can solve Lake Erie's problems with toxic cyanobacterial blooms: nope. Quagga mussels are picky eaters.
A study a few years ago by the National Oceanic and Atmospheric Administration (NOAA) and others found quagga mussels don't like to eat the kind of cyanobacteria, Microcystis, that's been blooming in Lake Erie every year, and that led to Toledo's drinking water shutdown in 2014. (You can check out this article from The Atlantic to learn more about why mussels actually add to the problems).