Invasive aquatic plants spreading throughout state

By Nicole Brodzik
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University of Minnesota fisheries researcher Ray Newman and student on Christmas Lake in Hennepin County, Minnesota. He is looking for milfoil weevil, a promising biocontrol that eats invasive eurasian water milfoil. U of MN research of biological control of aquatic, invasive species. Minnesota Agricultural Experiment Station research project #41-074, “Trophic Relations in Freshwater Systems,” Principal Investigator Raymond M. Newman.

Eurasian watermilfoil is one of many invasive species that found itself in the United States during the mid 21st century. A native of Europe, the species grows in dense canopies made up of hundreds of stems, each covered in between 12 and 21 leaflets. It’s often confused with the native northern watermilfoil that has only 4 to 9 leaflets.

It’s one of the most pervasive invaders in Minnesota’s inland lakes. The floating mats can crowd out native plants and can cause problems for boaters, fishermen and jet skiers. It first showed up in Lake Waconia in 1989. In Lake Minnetonka, the plant was first located in Excelsior Bay in 1987. It has since become a widespread issue on the lake, though there are ways to manage the species from a recreational standpoint.

The Lake Minnetonka Association (LMA) uses mechanical harvesters to cut the top five to six feet of the invasive plant, making it easier for boaters until it grows out once again. Another option that has more lasting effects is use of a coordinated herbicide initiative. This method has helped tremendously on areas of the lake like Gray’s Bay, according to LMA Executive Director Eric Evenson. He said they’ve see between 80 and 90 percent rebounding from native populations once the herbicides took effect to bring down Eurasian watermilfoil numbers.

“I feel like we could have Eurasian watermilfoil completely eradicated from Lake Minnetonka with full lake treatment,” said Lake Minnetonka Association Executive Director Eric Evenson.

The herbicides that are used to kill off the invasive plant is one of the more selective treatments for invaders. The damage it causes to native species is minimal while it’s effects on the Eurasian strand are lethal. However, removing large portions of the plant with the use of chemical is expensive and time consuming. Because the species is found in around 300 Minnesota lakes, it’s likely to be reintroduced by waterfowl, and undoubtedly by boaters, and that means the treatments would have to continue indefinitely to maintain a Eurasian watermilfoil-free Lake Minnetonka.

Even if researchers were able to rid a body of water of Eurasian watermilfoil, its presence in those lakes at all has already created a potentially more serious problem.

New super strands of hybrid watermilfoil are now popping up that are much more resistant to treatment. These hybrid specimen are the result of crossbreeding between the invasive species and the native northern watermilfoil.
The MCWD, University of Minnesota and the University of Montana completed a study earlier this year on the new outbreak of hybrid strands in Lake Minnetonka and Christmas Lake. Using genetic screening, researchers found that in bays treated with herbicide, the hybrid milfoil was more prevalent. Researchers concluded that, “Large scale herbicide treatments could promote hybrid watermilfoil growth and some hybrids may show greater tolerance to treatment.”

The fear is that without an effective means of control, the invasive or hybrid milfoil species will crowd out and eventually eliminate native plants. Watermilfoil are far from the only threats to lake ecosystems in the west metro.

Curly-leaf pondweed, flowering rush and purple loostrife have also entered into Minnesota ecosystems and are spreading fast. Part of the invasive plants creep into new bodies of water on boats, trailers and fishing rods.

“Just make sure you clean your trailers as well as boats,” Evenson said. “Almost everything could be controlled by fully drying off your boat. It’s not too late. We haven’t seen anything irreversible happen yet.”

Starry Stonewort was first discovered in Minnesota on Lake Koronis in 2015 and had spread to 7 other lakes in 2016.
Its spread throughout North America in a way that feels like it came straight out of a science fiction movie. One small fragment of the plant found it’s way to North America in the ballast water of an international barge coming through the great lakes. Today, that small plant fragment has cloned itself over and over, creating established colonies in Vermont, Pennsylvania, New York, Indiana, Michigan, Wisconsin and Minnesota according to the U.S.
Geological Survey.

In Minnesota, the large mats of the grass-like algae are filled with thousands of clones from a plant that originally landed in Lake Koronis in 2015. With only male specimens of the invasive algae found in North America, the only way for Starry Stonewort to reproduce is asexually. This also means that it’s spread undoubtedly comes from human interaction and transportation on boats, trailers and other recreational equipment.

Dan Larkin is one of the top Starry Stonewort researchers in the state and works out of the Minnesota Aquatic Invasive Species Research Center.

“If the females were here, they would produce spores and could be spread pretty easily by water fowl,” Larkin said. “But we only have the males here, so we know it’s the actual plants themselves being moved around by boats.”

A major concern for researchers like Larkin is how difficult it is to treat Starry Stonewort once it arrives in an ecosystem. Unlike the herbicides used on invasives like Eurasian Watermilfoil, Starry Stonewort is technically not a plant which means it isn’t vascular and therefore, harder to treat.

“With plants like the watermilfoil, when you apply the herbicide, it moves through those veins to kill off large portions of the plant without having to treat the entire thing,” Larkin said. “You don’t have that with Starry Stonewort because its technically an algae.”

Right now, Starry Stonewort is being treated with copper-based compounds as a potential option to limit it’s spread within lakes. Larkin said the issue with treating the invasive algae is that they haven’t found really effective treatment options yet and the ones they are experimenting with have the potential to large the watershed communities as a whole. Larkin said his real hope is that it will be people who help manage the spread, and prevent it from taking over the lakes and ponds in Minnesota.