As of February, the Minnesota Department of Natural Resources has officially designated Bde Maka Ska, Lake Harriet and Lake Nokomis as infested with zebra mussels. The DNR makes such declarations in an absolute fashion: If a single zebra mussel has ever been found in a body of water, it is declared infested.
But the true health of Minneapolis’ lakes is more complicated and optimistic, according to Mike Sorensen, the aquatic invasive species program administrator at the Minneapolis Park and Recreation Board. No zebra mussels have ever actually been found in Lake Nokomis, and while one adult zebra mussel has been discovered in Lake Harriet and two juveniles were recorded last fall in Bde Maka Ska, there’s no evidence of a breeding population in the Chain of Lakes.
“Although it’s declared infested, the jury’s still out on if we have a successful reproducing population,” Sorensen said.
In 2012, zebra mussels, an invasive species that came to North America from Europe and Asia via transoceanic cargo ships, were discovered in Lake Minnetonka. That news awakened the metro area to the threat of zebra mussels. Apart from damaging watercraft and docks and potentially clogging infrastructure, zebra mussels attach to and kill native mussels.
Lake Nokomis flows into Minnehaha Creek, which runs from Lake Minnetonka to the Mississippi River, and because water from the creek is sometimes pushed back into Lake Nokomis during flooding seasons, the DNR declared Nokomis infested with zebra mussels. But the Park Board has never found a zebra mussel there, despite searching for them every year since 2012.
Fast forward to the fall of 2017, when crews cleaning Lake Harriet found a mature, adult zebra mussel on a boat cover on the bottom of the lake, Sorensen said. That discovery led to a 60-hour search with scuba divers.
“They couldn’t find a single other mussel,” he said.
But once the discovery had been made, the DNR declared the lake infested. Additional searches of Lake Harriet last summer still turned up empty.
In September 2018, two young zebra mussels were found on the bottom of a sail boat being inspected as it left Bde Maka Ska. The boat had been in the lake all summer, Sorensen said.
Another large search was conducted. This one involved scuba divers and a cutting edge method known as environmental DNA sampling, in which researchers take water samples from throughout the lake and test them for zebra mussel DNA.
“We couldn’t find another zebra mussel,” he said.
Because it was so late in the year, the Park Board intends to conduct a more robust search for zebra mussels in Bde Maka Ska this summer. One mature zebra mussel is capable of producing half a million eggs, Sorensen said, so populations can escalate quickly.
Preventing the spread
To prevent the potential spread of zebra mussels into the waters of Lake of the Isles, Cedar Lake and Brownie Lake, the MPRB is considering a change to the way it deals with another common invasive species: Eurasian milfoil.
Harvesting machines that remove milfoil, which bunch up on water surfaces and prevent sunlight from reaching native plants below, will now start on the upper Chain of Lakes before moving south to Bde Maka Ska. The machines will be cleaned before restarting the process, instead of constantly moving up and down the Chain of Lakes.
Although some invasive species have entered city waters, other threats are still preventable.
“There are other things out there that we don’t have yet,” Sorenson said. “We don’t want people to think that it’s a lost cause.”
Two of those things the Park Board and DNR are on the lookout for in the metro are starry stonewort and spiny water fleas.
Spiny water fleas feed on the native zooplankton, which native species need as a food source, and their long tails make them hard for native species to feed on. The fleas are mostly found in the Boundary Waters and on Lake Mille Lacs in Northern Minnesota.
“We don’t have this here yet, we don’t want this,” Sorensen said.
Starry stonewort is an invasive algae species that forms a dense layer on the surface of the water. It is currently found in 12 lakes in Minnesota, including nearby Medicine Lake in Plymouth, where it was discovered last summer.
To get a better understanding of what is coming in and out of Minneapolis’ lakes, the MPRB inspects every water craft that comes in and out of a boat launch each year. In 2018 that amounted to 7,611 inspections. Those inspections found 13 boats with zebra mussels (all but one attempting to enter a lake).
Inspectors also asked boaters the last body of water they’d been in. The result was finding watercraft coming to Minneapolis from hundreds of bodies of water, from Lake Superior to Lake of the Woods. The inspections found 87 boats coming from bodies of water infested with spiny water fleas and 59 boats coming from lakes infested with starry stonewort.
The Park Board passed the 2019 Watercraft Inspections Policy, which maintains past standards of inspecting all watercraft, at its March 6 meeting and the organization has begun the hiring process for inspectors.
“Fortunately for all these [aquatic invasive species], the thing that works is to clean, drain, dry your watercraft,” Sorensen said.