A Shorewood lake could be the first in Minnesota in an effort to protect the state's lakes from fast-spreading zebra mussels.

Christmas Lake, near Lake Minnetonka, was the first in the country earlier this month to use Zequanox, a biological pesticide that kills off zebra mussels, and found that it worked in a small, closed-off area. Other lakes have experimented with the product, but the west metro lake was the first to use it not in an experiment, but to try to rid the lake of the aquatic invasive species that can damage boat motors and threaten fish populations.

Now, the state Department of Natural Resources is seeking federal approval to use liquid potash, or potassium chloride, for the first time in Minnesota.

"It's just a good opportunity to try what tools we have that are available," said Keegan Lund, an invasive species specialist with the DNR, which is working with the city of Shorewood and Minnehaha Creek Watershed District.

While crews said Thursday that Zequanox worked in the closed-off area, they also discovered 25 zebra mussels outside that area. The lake's water temperature is now too low for Zequanox to be effective, so instead, crews are doubling the enclosed area to where the mussels were found, hoping to use potash if state and federal officials sign off on it.

Lakes in Texas and Virginia, as well as Lake Winnipeg in Canada, have used potash to kill off 100 percent of zebra mussels, the DNR says. But it is toxic and kills both native and nonnative mussels.

Dead zebra mussels were displayed.

"The risk there is relatively small," Lund said of Christmas Lake. "It really is a pretty safe product. But we're all concerned about the nontarget effects it could have."

The DNR needs approval from the state Department of Agriculture and an emergency exemption from the U.S. Environmental Protection Agency to use potash on Christmas Lake. Lund said they hope to get approval soon and use potash this fall before the lake freezes.

Going on the offensive

Minnesota officials have been intensifying efforts in the past few years to slow the spread of numerous aquatic invasive species in the state like zebra mussels, which have infested nearly 200 waterways like Minnetonka. But it has focused on prevention and education through billboards, increased boat inspections and new boating restrictions.

Now, with Zequanox and solutions like potash, officials are going on the offensive.

However, for large lakes like Minnetonka that are fully infested, it probably is too difficult and costly to kill off zebra mussels with these products. On Christmas Lake, however, officials detected the zebra mussels soon after the lake was infested in August.

Crews applied Zequanox, which uses dead bacterial cells to kill the zebra mussels, on the lake on Sept. 8. It was just federally approved to be used on open water in July. Before then, the product had been tested only in small parts of lakes like neighboring Minnetonka or used to reduce zebra mussels in power plant water pipes.

It cost the DNR and Watershed District about $9,300 to treat a 50-by-60-foot area.

Since then, crews and University of Minnesota students have been monitoring the lake every day, logging more than 125 hours of scuba diving, looking for zebra mussels. Then, on Sept. 19, 25 zebra mussels were found about 50 feet away from the contained area.

"Everybody was disappointed," said Craig Dawson, director of the Minnehaha Creek Watershed District's aquatic invasive species program. "But also, realistically, it was always a possibility. We were never 100 percent [sure] we had 100 percent [of the zebra mussels] contained."

The lake's only public boat launch was shut down in August and will remain closed as crews now hope to use Christmas Lake as the testing ground for another possible solution in the fight against invasive species in Minnesota.

"Nothing," Dawson added, "is a guarantee with zebra mussels."

Kelly Smith • 612-673-4141

Twitter: @kellystrib