DNR hoping 1,000 pounds of potash adds up to a victory against zebra mussel threat
By Dave Orrick / St. Paul Pioneer Press
Minnesota might have decontaminated its first lake from invasive zebra mussels Friday.
The potential milestone came after workers pumped potash, a naturally occurring salt compound commonly used in fertilizers, into an infested corner of Christmas Lake in Shorewood, killing every last zebra mussel in the area.
“It will – will – kill all the zebra mussels in the containment area,” said Keegan Lund, metro area invasive species specialist for the Department of Natural Resources. “I’m confident in that.”
However, if the infestation, discovered in August, is more widespread in the lake than officials think, Friday’s application of potash by a private contractor will not have fully cleansed the lake.
Crews from the DNR and Minnehaha Creek Watershed District will inspect the lake beginning in the spring.
Potash is a new arrival in the zebra mussel-killing arsenal. Friday’s procedure involved dissolving 1,000 pounds of potash, essentially potassium chloride, in water and pumping that water through holes in the ice.
It’s the third such use of the substance and the first in Minnesota, allowed only through an emergency exemption granted by the U.S. Environmental Protection Agency. The exemption also will allow Independence Lake in the west metro to be treated in the spring.
Although potash is an efficient killer of zebra mussels, it’s not seen as a silver bullet to the nonnative invader, which can alter the food chain of a body of water and wipe out native mollusks.
“We see this as a way to treat a localized area right now,” Lund said. In August, zebra mussels were discovered near the public boat launch on Christmas Lake, and the infestation was believed to be isolated to an area of less than an acre, an isolation that officials tried to ensure by erecting a barrier that prevented anything, from mollusks to fish to boats, from crossing.
“We definitely have hesitation about applying it to any lake on a widespread scale,” Lund said. “We would have hesitation about introducing that amount of chloride or nutrients into a lake. There would be ecological ramifications.”
In addition, potash kills all mollusks, including native species. The potassium interferes with the respiratory system of mollusks, preventing them from breathing. It doesn’t affect fish or vegetation, he said.
Still, potash, if approved for such uses in the future, could be a potent weapon for wildlife managers as they try to slow the spread of zebra mussels, which have spread to more than 200 bodies of water in the state. Once a water body becomes infested – often by hitching a ride along a boat trailer or dock being installed from another lake – the only thing preventing the mussels from proliferating has been the habitat of the lake itself, and most Minnesota waters are hospitable to zebra mussels.
Friday’s events suggest that decontamination is possible, and officials said the lake’s tough invasive species regimen, as well as extraordinary steps taken on Christmas Lake, provides a potential road map.
The Christmas Lake Homeowners Association has been among the most aggressive in the state at restricting boat ramp access without inspections of watercraft and boat trailers. The Minnehaha Creek Watershed District regularly inspects waters near boat ramps for lakes believed to be at higher risk of infestation. Christmas Lake’s proximity to neighboring Lake Minnetonka, which is infested with several invasive species, placed it on the high-risk list.
As a result, Craig Dawson, director of research and monitoring for the watershed district, said he’s confident his agency detected zebra mussels with two weeks of their arrival. The lake access was essentially closed, and the area was treated with Zequanox, a bacteria that target zebra mussels, and a copper-based compound that kills all animals. The next step was to dredge the entire area around the boat launch, physically removing any remaining mussels. However, the infested area proved too large to dredge, so potash surfaced as the potential knockout blow.
“Still,” Dawson said, “I would say we got lucky that we discovered the mussels right away.”
Zebra mussel killers
What: Potassium chloride
Pro: Kills all zebra mussels, but not fish or vegetation
Con: Kills native mollusks, loads lake with nutrients
What: Natural bacteria
Pro: Kills only invasive zebra mussels, not native mollusks or fish
Con: Might not kill 100 percent, expensive
What: Elemental poison
Pro: Kills all zebra mussels
Con: Indiscriminate; kills all animal life, including fish
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