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Great Lakes face new threat from invasive quagga mussels

The quagga mussel Kishore Gopalakrishnan held up to the springtime sun was about as big as his thumbnail, but the Wayne State University researcher cautioned against discounting the devastation such a diminutive creature can cause.

"This can filter one liter of water per day," he said, its shell between his fingers. "Can you believe that?"

The frenzied filter-feeding of invasive mussels can kick off a chain reaction in the Great Lakes that disrupts the food chain and leads to a buildup of a deadly toxin in the ecosystem.

And their impact might get worse. Researchers at Wayne State have found invasive mussels are reproducing for longer seasons each year, which could allow them to spread farther and faster through the basin and cause more damage.

"It's just compounding," said WSU doctoral candidate Katrina Lewandowski. "Invasive species like mussels, they're almost like an ecosystem engineer. They can change the ecosystem so much, that when you add on top things like climate change, longer seasons of mussel spawning and less ice cover in the winter, that can really change the lake structure."

Wayne State researchers started collecting invasive zebra and quagga mussels and their larval offspring known as veligers from the Detroit River in 2010. They typically scrape mussels from the shores of Belle Isle using a small net at the end of a long pole. 

They didn't intend to measure spawning seasons — they were trying to develop a product to stop the mussels from spawning, a project Gopalakrishnan continues to pursue, and needed to collect mussels and veligers to stock the laboratory.

"Serendipitously, five years into it, we're like 'Wait a minute. We're out here longer and longer into the season," said Donna Kashian, Wayne State professor and director of environmental sciences who oversees the invasive mussel research. "We didn't know we were studying (spawning seasons), and we were." 

After noticing the pattern, Kashian and the research team started officially collecting spawning season data in 2016. They found invasive mussels collected from the Detroit River usually start their spawning season in April but are extending their spawning seasons later as time goes on. 

"We didn't notice anything was going on until, all of a sudden, we're standing out in December collecting veligers," Kashian said. "At first, it's little by little, later and later, and you don't notice it. Then all of a sudden we're like 'Hey, wait a minute. It’s December.'"

This year, invasive mussels set a new reproduction record: They were still spawning into January. The two years before, the mussels stopped spawning near Christmas. In 2016, when the Wayne State researchers started officially tracking mussel spawning, they stopped spawning a bit before Thanksgiving.

How mussels impact Great Lakes

Invasive zebra and quagga mussels are native to eastern Europe. They were discovered in the Great Lakes in the late 1980s, likely arriving in the ballast water of international ships.

Quagga mussels now cover much of the bottom of lakes Erie, Ontario, Huron and Michigan, Lewandowski said. Lake Superior has largely been spared, although some invasive mussels have been detected there.

Zebra and quagga mussels over-filter the water, Lewandowski said. They eat lots of algae, which means that algae isn't available to feed the plankton that feed fish species like alewives and yellow perch, which in turn are food for fish such as trout and walleye.

Instead of plankton, fish sometimes eat veligers, or mussel larvae, Lewandowski said. She is in the second year of a study to determine whether fish do better when they eat plankton, veligers or a mix, to see if she can gauge mussels' impacts on fish nutrition.

The mussels' impact on the food web does not end there, Gopalakrishnan said. They appear to play a role in deadly outbreaks that can kill fish and birds.

By eating algae, zebra and quagga mussels make the water more transparent, which allows more sunlight to shine through, he said. That sunlight feeds plants that live in the water. When those plants grow fast, they die and decompose, which uses up oxygen in the water.

That's bad for plants and fish that need oxygen to survive, Gopalakrishnan said. But it also creates a condition for a bacterium that grows a deadly toxin, botulism, to flourish. Mussels eat that toxin, fish eat the mussels, birds eat those fish and die in droves. 

Federal scientists estimate botulism has killed 100,000 Great Lakes birds since 1999.

"We could see, yearly, a lot of birds dying due to botulism," Gopalakrishnan said. "Everything is interconnected."

That's why understanding the role of mussels in the ecosystem and their apparently lengthening spawning season is important, Kashian said. 

Kashian said the Wayne State lab is the first to document their reproduction occurring later into the year. Other researchers have found similar trends in other species, Kashian said, pointing to studies showing extended spawning seasons for yellow perch in Lake Michigan, walleye in Minnesota's inland waters and grayling in Switzerland. 

"You have to step back and ask what's causing it," Kashian said.

Behind longer spawning

There are a few possible explanations for the longer spawning season, Kashian said. 

First is a change in temperature. Climate change is driving warmer average temperatures in the Midwest, and warming air contributes to warming water temperatures. 

Average air temperatures have increased by 2.3 degrees Fahrenheit since 1951 in the Great Lakes region, and summer lake surface temperatures are increasing even faster, according to the Great Lakes Integrated Sciences and Assessments, a collaboration of scientists from the University of Michigan, Michigan State University and the University of Wisconsin.

Surface water temperatures on Lakes Michigan-Huron increased 5.2 degrees Fahrenheit between 1968 and 2002, 4.5 degrees on Lake Superior,  2.7 degrees on Lake Ontario and smaller increases on Lake Erie, according to data compiled in the federal government's Third National Climate Assessment published in 2014.

More recently, the Great Lakes experienced record-warmth throughout last fall, at times reaching 6 degrees above average, according to data collected by the National Oceanic and Atmospheric Administration.

More: Great Lakes' record warmth to fuel lake effect snow, may disrupt fish

Decreasing Great Lakes ice cover also could be driving the expansion of invasive mussels' spawning seasons, Kashian said.

On average, Great Lakes ice cover has declined consistently since 1973. Ice declines happened suddenly in the mid-1980s in lakes Michigan, Erie, and Ontario, then in the mid-1990s in lakes Superior and Huron, according to GLISA reports.

"You can see changes, variability around the polar vortex, but that minimum (average ice cover) has dropped," Kashian said.

Declining ice cover could change the amount of light that reaches the mussels below the surface.

"That may be our canary in the coal mine, where it's really showing that something is going on," Kashian said.

More: What happens under Great Lakes ice? Scientists want to know before it's too late

The third explanation involves the mussels themselves, which could be adapting to the Great Lakes region's weather. That possibility could occur alongside warming water temperatures and declining ice cover, Kashian said.

Even if the invasive mussels are adapting to handle the Midwest cold, she said climate change likely will benefit invasive mussels and harm the 43 native mussel species found in the Great Lakes.

"In order to be a good invader, you have characteristics already that make you a good invader," Kashian said. "You tend to have a wide range of environmental tolerances, you tend to be able to take advantage of new things. You're just very flexible.

"Whereas a lot of native species that are prone to one area, they've co-evolved in that habitat. They have their tight little niche. So when the environment changes around them, it’s harder for them to move outside of their niche."