Zebra mussels have invaded the state's 10th-largest lake and threaten to radically change its character within a few years.
Zebra mussels have invaded Lake Minnetonka, a breach of the state's defenses against invasive species that threatens to dramatically change the character of Minnesota's 10th-largest lake within just a few years.
Department of Natural Resources biologists confirmed Wednesday that a small number of mussels are attached to rocks along the shore, and their size suggests that a reproducing population has been in the lake for at least a year.
In places where they've become established, the fingernail-sized mussels proliferate by the millions, consume food needed by fish, clog water intake pipes, ruin fish spawning beds and litter beaches and shallow areas with razor-sharp shells.
The mussels were found on the east side of Wayzata Bay near Hwy. 101. That's not far from the lake's outlet to Minnehaha Creek, raising fears that the mussels may spread into that waterway, or may have done so already. Minnehaha Creek is connected to lakes Nokomis and Hiawatha in Minneapolis.
For years DNR officials have worked with the Lake Minnetonka Conservation District and others to educate boaters and anglers to prevent the spread of zebra mussels. They also inspected boats and trailers and directed owners to remove plants, mussels and water from bait buckets and vessels that traveled in infested lakes and rivers. The efforts may have bought some time, but they didn't stop the mussels' entry into Minnetonka.
"Unfortunately, zebra mussels still found their way to the lake," said Luke Skinner, supervisor of DNR's invasive species program.
The discovery was dreaded news for Dick Osgood, president of the Lake Minnetonka Association, which represents about 600 lakeshore owners and businesses.
"This has been our fear all along, and keeping them out has been our top priority for the last ten years," said Osgood.
Lake Minnetonka is the most heavily used lake in the state, he said, with an estimated 200,000 boats plying its channels, bays and open water annually.
With that amount of exposure, the discovery of mussels was not unexpected, said Osgood, but it was still a major disappointment.
"Bottom line is, I think they're here to stay," he said. "Not that we won't do everything possible in rapid response, but I think it'll change the lake forever."
Osgood and representatives of the Minnehaha Creek Watershed District plan to do a quick assessment of the lake to see if they can find mussels in other places. There's a chance, he said, that if they are only in one area, the mussels could be removed or killed before they spread further.
DNR officials said that the number of zebra mussels found was very low and that they would investigate the situation this week and beyond, including an extensive survey of the lake later this summer.
Osgood advocates limiting boat traffic in the infested waters, at least until the extent of the invasion is better understood.
In other places, he said, the discovery of zebra mussels is usually followed by 1 to 3 years of "lag time" in which a few more infested areas are found. At some point, usually about five years after the initial discovery, he said, the populations explode and the lakes start to change, sometimes unpredictably.
In some areas the numbers of different fish species increase or decrease, seeking a new balance as habitat and food sources change. Native mussels usually die out. And because zebra mussels constantly filter sediment and nutrients, water often becomes noticeably clearer.
That may please some, said Osgood, but it also means that light will penetrate deeper, boosting the growth of plants such as Eurasian water milfoil, an invasive that's already a major nuisance and expense to control in Lake Minnetonka.
Resident spotted mussels
A local resident found the mussels in the lake early this week and reported them to the DNR. Skinner said that anyone else who finds mussels should contact the agency.
Zebra mussels are native to Eastern Europe and western Russia, and likely came to the Great Lakes in ballast water of ocean-going ships that traveled up the St. Lawrence Seaway. They were discovered near Detroit in 1988. Their first appearance in Minnesota was in 1989 in Duluth harbor, and they subsequently spread to 17 inland lakes, including Mille Lacs, Prior, and Le Homme Dieu and to portions of the Mississippi, St. Croix and Zumbro rivers.
Udai Singh, senior hydrologist and water quality specialist for the Minnehaha Creek Watershed District, said plans for what to do next are still very preliminary. If the mussels are firmly in the lake, he said, the district may install equipment at various locations downstream in Minnehaha Creek to check for them. The discovery in Minnetonka is "really unfortunate," Singh said, and has already jump-started a new array of activities.
"Now since prevention is out the window, we will be more working in terms of control and management of them," he said.
Tom Meersman • 612-673-7388