On the water front: invasive lake species

Researchers like Gretchen Hansen are out to foil the spread of spiny water fleas and zebra mussels.

Spiny Water Flea laying laying on a finger

Some of the most destructive invasive species come in small packages. Few come smaller than the spiny water flea, a tiny crustacean with a long, spiked “tail.”

head shot of Gretchen Hansen
Gretchen Hansen, Assistant Professor, Fisheries, Wildlife and Conservation Biology

“Spinies” and the more famous zebra mussels change lake ecosystems far out of proportion to their size. At the University of Minnesota Twin Cities, Gretchen Hansen is untangling the web of biological and chemical impacts that these and other aquatic invasive species weave.

“A lot of our research focuses on documenting impacts while also identifying places that are more sensitive or more resilient,” says Hansen, an assistant professor in the Department of Fisheries, Wildlife and Conservation Biology. “We also study climate change and how lakes and fish respond.”

Spinies are part of the zooplankton—tiny animals that drift around with tiny plants called phytoplankton. So are native water fleas, which are eaten by many young fish and, unfortunately, spinies. Spinies afflict lakes large and small, including Lake Mendota in Madison, Wisconsin, and Minnesota’s Lake Mille Lacs.

Native water flea with its numerous eggs.
Daphnia, one of the largest native water fleas. Its body averages about 1/3 to 1/2 the core length of a spiny. Note its eggs (yellow spheres).
Invasive spiny water flea with a semi-detached egg sac and long spine.
Invasive spiny water flea with a semi-detached egg sac and long spine.

At the University of Minnesota Duluth, Donn Branstrator, a professor in the Swenson College of Science and Engineering, has done foundational work on spiny water fleas, their impacts, and what spreads them. And Valerie Brady, a senior research associate and aquatic ecologist in UMD’s Natural Resources Research Institute (NRRI), led a study that showed how they get caught on fishing lines and tackle. (see accompanying video).

For quick tips on how to stop the spread of spiny water fleas, view the 30-second video.

NRRI also sponsors projects that include:

  • A thorough testing of boat landing self service cleaning stations to find the best techniques for removing aquatic invasive species from the insides of boats; and
  • Testing international ship ballast tank treatment technologies to better understand how well they actually kill the hitchhiking critters. Testing is done in a lab near the Duluth-Superior harbor and on ships moving between ports.

Musseling in on native species

Zebra mussels are famous for clogging pipes, encrusting equipment, and carpeting shoreline lake bottoms with their razor-sharp shells. But figuring out how they affect species like walleye is complicated, due to their wide-reaching effects on lake ecosystems. Hansen is at the forefront of discovering those effects and their repercussions.

a single zebra mussel on the tips of two human fingers.
Zebra mussels, though small, have huge impacts on our lakes.
Hundreds of zebra mussels clumped together.
Don’t step on these shells—they’re razor-sharp. 

Much of zebra mussels’ mischief stems from their prodigious ability to filter and clear water. “They can quickly clear an entire small lake,” Hansen observes.

They do it largely by sucking in and eating phytoplankton, a major food source for native water fleas and other zooplankton.

“These changes lower on the food chain can cascade up to fish,” Hansen notes. “A few years ago, we published a study showing that young walleye and yellow perch grow more slowly and reach smaller sizes at the end of the year when spiny water fleas or zebra mussels are present.”

two Walleye fish swimming together in a lake.
Two yellow perch swimming in a deep part of a lake.
Yellow Perch

Like spiny water fleas, zebra mussels reduce the supply of zooplankton not only for young walleye, but for young yellow perch, an abundant prey fish.

As mature zebra mussels pull plankton from water all over a lake, they deposit much of that food energy as excrement in their relatively well-lighted, shallow water homes near shorelines. Energy from that unsavory source supports insects, algae, snails, rooted plants, and other life in the vicinity.

“Fish that can take advantage of that do well; others don’t,” says Hansen. “Walleye like darker, low light conditions. They don’t do as well in light—they lose their competitive edge. I think the water clarity effect is important for them.”

Three researchers kneeling on a shoreline examine the contents of a large net.

In studies, “We found that in zebra mussel-infested lakes, young walleye were smaller than in noninfected lakes. This has implications for how they survive if they’re not as big.”

Currently, Hansen is involved in follow-up research that will ask what the mussels mean for walleye survival, particularly whether the presence of mussels affects the growth or abundance of adult walleye.

She is also doing critical work on the process by which zebra mussels promote the production of methylmercury, perhaps the most toxic substance known. By consuming oxygen, the mussels create an environment where mud-dwelling bacteria can turn mercury into methylmercury that can move into the food chain.

Volunteers help detect interlopers

If zebra mussels are to be controlled, early detection is best. Hansen has been involved in tests of detection methods, including sampling water for environmental DNA (eDNA). To cast the widest net, citizen volunteers would be recruited to take water samples around the state and send them to researchers for analysis.

A recent study by the U of M’s Minnesota Aquatic Invasive Species Research Center (MAISRC) compared the abilities of citizen volunteers and professionals to take samples of lake water destined for eDNA analysis without contaminating or otherwise spoiling them. The results were encouraging.

two people in a boat are collecting water samples from a lake.
Collecting water samples on Big Sandy Lake, Aitkin County.
A woman thigh deep in a lake is netting for invasive species.
Netting for invasive species.

“Volunteers collectively did about as well as the professionals,” Hansen beams.

“Water is central to who we are as Minnesotans; it connects us all," says Nick Phelps, MAISRC director and professor, Department of Fisheries, Wildlife and Conservation Biology. “MAISRC provides online tools that allow local managers to prioritize intervention strategies, like boat inspections and washing. The tool is used by county, state, and Tribal natural resource managers."

Volunteers Ingrid Bey in Hubbard County, Kevin Zahler in Carver County, and Shelly Binsfeld in Sherburne County have all earned Aquatic Invasive Species Detector certification through MAISRC and University of Minnesota Extension in partnership with the Minnesota Department of Natural Resources (DNR). They have also taken part in MAISRC’s Starry Trek event, a statewide effort to detect starry stonewort, an invasive type of algae. One method is the “rake toss," where a volunteer throws a double-headed rake into a lake on a 30-foot rope, retrieves it, then looks for starry stonewort among any plant material caught in it.

Pop Quiz

spiny water flea

Which is a reason the spiny water flea is so harmful?

Expand all

A. They eat fish food

Yes! They eat the native water fleas, which are food for native fish.

Expand all

B. They clog up pipes

No, you're mistaking them for zebra mussels.

Expand all

C. They bite swimmers

No, unless you thought "swimmers" meant other zooplankton.

“We are having fun checking the lake for aquatic invasive species and learning more about the wonderful native vegetation in the lake,” says Bey. “We believe that prevention and education are key to keeping our lakes pristine. I have enjoyed every minute of it.”

“I have always had a strong concern for the environment and have tried to contribute where I can,” says Zahler. "The science is very interesting, and I like learning more.”

“My initial desire was to teach my children more about those ‘weeds’ they felt when swimming,” says Binsfeld. “Now, they know and appreciate aquatic plant life.”

With so many dedicated researchers of all stripes, the Land of Lakes is in good hands.


Funding for MAISRC is provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources, the State of Minnesota, and private donations. 

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