LANSING — A patch of bare rocks studied last summer in Lake Michigan revealed threats to the invasive mussels that for years have disrupted the Great Lakes ecosystem.
These clamlike creatures brought to the Great Lakes by ships in their ballast water have killed native mussels, altered food dynamics and affected phosphorus cycling in Lake Michigan, according to a recent study in the journal Ecological Modelling by scientists and engineers from the University of Wisconsin-Milwaukee and the University of Maryland Center for Environmental Science.
The study focused on zebra and quagga mussels in nearshore and mid-depth areas of Lake Michigan.
These invasive mussels, native to eastern Europe, are members of a family of small freshwater mussels that attach to hard surfaces such as rocks, docks and boats.
In the summer of 2016, Harvey Bootsma, a professor in the School of Freshwater Science at the University of Wisconsin-Milwaukee, and his research team conducted an experiment to remove mussels in Lake Michigan.
Divers used paint scrapers to manually scrape and clear mussels from the rocks in an area about 48 square yards.
Still alive, the mussels were placed in mesh bags and moved far from the location.
They wanted to see how the algae in that area would respond without the mussels and how quickly the mussels would return.
Four years later in 2020, the mussels have not returned to that area, and Bootsma and his team think they know why.
They found that zebra and quagga mussels face their own threat from another invasive species: the round goby, a fish that eats small mussels.
If mussels are removed from an area, the only way they can recolonize there is for young mussels, also known as veliger, to settle on the rocks and grow.
Round gobies can easily crush the shells of smaller mussels and eat the young mussels before they can grow and recolonize.
“So, in this case, it’s almost like we’re fighting fire with fire in that one invasive species is actually helping to keep another invasive species under control,” Bootsma said.
Nathan Lucas, the aquatic invasive species coordinator at the Michigan Department of Natural Resources Fisheries Division, said it’s essential to communicate the threat of invasive species to the public. He coordinates response efforts that deal with existing species, identify next invaders and prevent them from being introduced into the Great Lakes.
“When new invaders come in, that is when you see a really big spike in our efforts trying to eradicate them from becoming a permanent fixture of our ecosystem,” Lucas said.
Bootsma said his research of invasive mussels began with a noticeable overgrowth of benthic algae in Lake Michigan.
Benthic algae grow on rocks at the bottom of lakes and are an important food source for some fish.
According to Bootsma, the algae break off when there is too much growth, wash up on shores, rots and create a horrible smell.
The algae also cause oxygen depletion when it decomposes, negatively affecting nearby organisms.
The benthic algae had been a problem in the 1960s but began to come back in the 2000s.
At the same time, his research team noticed that invasive mussels had become a problem in Lake Michigan and recognized a link between the two.
“We started focusing on the nuisance algae problem, but eventually realized we had to study mussels to understand the algae,” he said.
The team discovered that the mussels were helping the algae grow.
“The mussels were creating a perfect storm for the algae,” he said.
Mussels are “filter feeders,” meaning they feed on things that float in the water. Their filtering makes the water clearer and allows more sunlight to get to the bottom of the lake where the algae grows.
Bootsma said algae, like any other plant, needs light to grow.
He also noticed that the mussels were eating large amounts of plankton, which are the base of the lake food web.
Not only does that cause fish that rely on the plankton as a main food source to suffer, but when mussels consume plankton, they recycle digested nutrients into a fertilizer on the bottom of the lake with increased phosphorus that helps the algae survive.
Bootsma said phosphorus is an inorganic element that is needed to support plant growth.
“Ultimately, what is controlling the algal growth in Lake Michigan is phosphorus,” he said. “If you have less phosphorus in the system, algal growth will go down. If you have more phosphorus in the system, algal growth will go up.”
In 2019, he started a new experiment working with a chemical produced by bacteria called zequanox that can kill quagga mussels without harming other organisms.
The experiment involves putting enclosures on the bottom of the lake covered by quagga mussels and injecting zequanox into the enclosed space.
After sitting overnight, the enclosures are removed and the mussels in that area are monitored to see how they respond.
“And that’s an ongoing experiment now. We dove there a few times this summer, monitoring what’s happening to mussels there,” Bootsma said.
In Michigan, some prevention efforts are geared toward preventing the spread of invasives to inland bodies of water that aren’t already infested.
New state boating laws that took effect last year are aimed at reducing the spread of these mussels via boating pathways. They require boaters to drain all water from their boats and to safely dispose of water that might contain mussel veligers.
Boaters are also required to clean off plant material to prevent the spread of other invasive species.
The DNR’s Lucas said, “As we’ve seen in the Great Lakes and other water bodies that once they get in, they are very challenging to deal with. So, getting ahead of them is priority No. 1.”
Bootsma says one reason why it’s challenging to manage the Great Lakes and get the public concerned about conserving them is because they’re a mystery to many people.
“If we look at lakes, all we see is the surface,” he said. “They’re not thinking about everything that’s living under the surface because generally we can’t see that.”
Bootsma said part of his job as a scientist is to get people into the lakes physically or virtually to see what’s going on under the water.
“Hopefully that gets them more interested and more concerned about what’s happening to these systems,” he said.