When common carp were purposely introduced to Minnesota lakes sometime before 1900, they apparently brought along another visitor that today is just as reviled as the big rough fish: a water plant called curly-leaf pondweed.
A century after the aggressive pondweed was discovered in state waters, agencies from cities to watershed districts to the state Department of Natural Resources (DNR) have declared war on the invader. Now, the results of contrasting eradication efforts in three conjoined lakes in Eden Prairie and Bloomington could help shape future efforts to contain curly-leaf pondweed.
Near the end of this month or early in May, herbicide will be applied to Southeast Anderson Lake in Bloomington in the first of four annual chemical treatments to kill pondweed. Two years of alum treatments will follow, binding phosphorus to the lake bottom. The hope is that with nutrients tied up in the lakebed, the lake will no longer see late-summer blooms of blue-green algae, which can be toxic to animals and people. Alum treatments are effective for up to 10 years.
That slow, steady effort to get rid of pondweed contrasts with the more dramatic approach taken last fall in Eden Prairie's two Anderson Lakes, just across Hwy. 169. The southwest and northwest lakes were pumped dry in the hope that allowing the exposed lake bottom to freeze would kill curly-leaf pondweed seeds. The lakes are naturally refilling with water now.
The Nine Mile Creek Watershed District as well as the DNR will be watching to see which treatment better allows native plants to reestablish themselves before curly-leaf pondweed tries to take over again. Samples will be taken from the three lake bottoms to check on the number of curly-leaf pondweed seeds. While eliminating pondweed is the ultimate goal, that probably isn't realistic, said Kevin Bigalke, Nine Mile's administrator.
"That's always your goal, though it may not be the case," he said. "We want near-100 percent removal so [native plants] can reestablish themselves and curly-leaf pondweed can't reestablish itself as it was."
Draining the 81-acre Bloomington lake without contaminating the drained Eden Prairie lakes would cost more than $1.1 million, according to an engineering report. The Watershed District also would have had to seek approval from 21 landowners, including homeowners, the Minnesota Department of Transportation and the city of Bloomington. In public meetings held on the project, some residents indicated they would not support a lake drawdown, Bigalke said. The six years of chemical treatments, at an estimated cost of $554,000, will be paid by the Watershed District.
Curly-leaf pondweed is a native of Eurasia, Africa and Australia. Some experts say it came to the United States in the 1880s as an aquarium plant; a DNR fact sheet says it may have come to Minnesota along with non-native carp that were stocked in lakes as game fish.
The plant has narrow, rippled leaves and begins growing when lakes are still covered with ice. It is thick and thriving by the time native plants are ready to sprout, inhibiting their growth. Curly-leaf pondweed dies at midsummer, leaving rotting masses in the water that deplete oxygen and release phosphorus that can spur algae blooms.
An herbicide called endothall will be used to kill the pondweed. Professionals will apply the chemical with a boom below the water surface, working from a boat when water temperatures are between 48 and 60 degrees. Bigalke said applying the chemical early means it won't affect native water plants that appear later. Residents will be notified when treatment begins, and warnings will be posted around the lake to keep pets and children out of the lake for a few days during and after application, Bigalke said.
At first, the lake may not look very different, he said.
"Our hope is that we see less curly-leaf and that the native aquatics establish themselves, but we might not see that right away," he said. "Hopefully there will be less algae each year and you will gradually see water quality improvement."
Scott Anderson, a senior civil engineer with the city of Bloomington who works with water resources, said the city wanted to tackle deteriorating water quality in Southeast Anderson Lake before it worsened. The lake doesn't violate water quality standards now.
"People swim and boat on it; it's more of a natural lake," Anderson said. "The water quality could be better. The City Council felt this was a good time to undertake a project like this, before water quality dipped."
Mary Jane Smetanka • 612-673-7380