A Minnesota ‘lake killer’ surfaced 11 years ago. There’s still no long-term fix.
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More than a decade after an aquatic invasive species called starry stonewort was first discovered in Minnesota, it’s taking an ecological and economic toll on a growing number of the state’s beloved lakes.
Nicknamed the “lake killer” early on, the fast-growing algae can form dense mats that choke out native plants and tangle around boat motors and swimmers’ legs.
Since Minnesota confirmed its first infestation in 2015, it has spread to 40 water bodies, including Six Mile Lake in Cass County, which was confirmed just last week.
If caught early, starry stonewort is no longer considered a death sentence for lakes. Authorities, though, say it’s never been eradicated from any U.S. lake or river. It remains a costly problem with no permanent cure.
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Researchers have found that chemical treatments or hand pulling can temporarily reduce the algae’s abundance, but don’t stop it from regenerating.
The problem is evident in Lake Koronis near Paynesville, the first Minnesota lake confirmed to have starry stonewort.
Initially, Minnesota Department of Natural Resources staff mistook it for a native species it resembles called chara. By the time starry stonewort was discovered at a public access in 2015, the invasive algae had already spread to 250 acres, an area about twice the size of the Mall of America.
“When it came in, it was winning the battle until we came along and started knocking it back,” said Kevin Farnum, aquatic invasive species manager for the Koronis Lake Association.
But Farnum acknowledged that the lake association’s efforts aren’t going to stop the invader permanently.
“Is it still finding areas where it can grow in the lake? Yeah, probably,” he said.
‘Don’t want to be known as that kind of lake’
Killing starry stonewort is challenging, because unlike plants, it doesn't have a vascular system that transports nutrients. Every stem or tiny branch is a single cell. So killing it requires bathing every cell in algaecide.
And even then, the treatment doesn’t wipe out the infestation. Starry stonewort can regenerate from tiny, star-shaped structures called bulbils left behind in the lake.
Researchers at the University of Minnesota’s Aquatic Invasive Species Research Center found that applying algaecide to starry stonewort can reduce its biomass. But it doesn't stop it from coming back the following year.
On a July morning, Farnum checked the lake to see how much starry stonewort had grown by tossing a metal rake attached to a long rope over the side of his boat.
Slowly, hand over hand, he pulled in the rake loaded with what looked like bushy, bright-green weeds. Except for a few strands of chara, “that’s all starry stonewort,” he said.
When the invasive algae was first found in Minnesota, the situation was grim. Farnum and other Lake Koronis property owners launched an effort to save their lake.
“We've had situations where people that should be very athletic have said, ‘I got tangled up, and it was kind of scary,’” he said. “We don't want to be known as that kind of a lake.”
They started with a mechanical harvester, cutting and removing dense mats of starry stonewort. It cleared some areas, but the algae quickly grew back.
So they tried another strategy: surveying the lake, identifying the most heavily infested spots and treating those with a copper-based algaecide. A contractor applies the chemical by dragging a weighted hose along the lake bottom.
“It's a little bit like the game of Whac-a-Mole,” Farnum said. “You just keep knocking it down wherever it's the highest. And you hope that you don't have to repeat it in the same year.”
It's a costly battle. Every year, Lake Koronis spends about $170,000 controlling starry stonewort. The money comes from a local watershed district, which has the authority to levy property owners, as well as a grant from the DNR. So far, residents haven’t pushed back much on the cost.
If left untreated, it’s possible that starry stonewort could eventually reach an equilibrium with native plants, Farnum said. But that could mean the algae spreads throughout the lake. “We don't think people are going to want that,” he added.
Early detection is critical
One positive finding from the past decade: Researchers have learned that if they find starry stonewort early, they have a better chance of keeping it contained.
“What we can do when it's found early in small isolated populations is use hand pulling to contain it, and that's proven to be really effective,” said Dan Larkin, an aquatic plant researcher at the University of Minnesota.
That’s been the strategy on Rush Lake, part of the Whitefish Chain of Lakes north of Brainerd.
In early July, divers from the Leech Lake Band of Ojibwe combed the bottom of a channel where starry stonewort was first discovered two years ago.
They used a method called diver-assisted suction harvesting, or DASH. It doesn’t use chemicals. Instead, the divers pull the algae from the lake bottom with long suction tubes and send it up to a floating barge.
Workers on the barge filter out the plant material into mesh bags for disposal, and release the filtered water back into the lake.
Early detection and quick action helped prevent a catastrophe, said Tony Coffey, president of the Whitefish Area Property Owners Association. If starry stonewort had escaped into the main part of the channel, it could have spread across Rush Lake — or the entire Whitefish chain, he said.
“Containing it on one acre is one thing,” Coffey said. “Containing it on 14,000 (acres) is just not going to work."
The Whitefish association now combines hand pulling with chemical treatments to keep the starry stonewort contained to that one spot. At the end of last summer, divers found only a small amount, Coffey said.
Property owners pay for the effort through annual dues and donations. They even bought a remote-operated vehicle to search the lake bottom for signs the algae has spread.
Costly battle with no end in sight
There’s no clear statewide price tag for fighting starry stonewort and the problem continues to grow. And there’s still a lot researchers don’t know about starry stonewort’s effects on water quality, fish and native vegetation, including wild rice.
While the invasive algae has now been confirmed in 40 Minnesota water bodies, the size of the infestations varies from year to year. Lake conditions and weather can play a big role.
“Not every lake that gets starry stonewort is going to end up like Koronis,” Larkin said.
Once an invasive species is in a lake, the DNR typically does not treat it directly. Instead, they issue permits to lake associations, local governments or property owners to mechanically remove or chemically treat it.
Of the 482 permits issued last year, most were for curly-leaf pondweed or Eurasian water milfoil, according to a DNR report. Thirty-two permits — about 7 percent of the state total —were targeting starry stonewort, mostly in northwestern and central Minnesota.
The DNR also provides grants for invasive species management. In previous years, the grants totaled about $400,000 statewide.
This year, the total jumped to about $1.4 million, said Michael Verhoeven, a DNR natural resources consultant. The increase follows a legislative change that raised the surcharge on Minnesota boat licenses to fund aquatic invasive species work.
But local resources available to manage and control starry stonewort vary from lake to lake. Even when a lake has an association, it might not be as well resourced or financed as Whitefish, Larkin said.
“Other places aren't going to be able to have money in the bank and have the really engaged, knowledgeable people to be able to jump in,” he said.
Some Minnesota lake groups are turning to technology to keep starry stonewort from spreading to more lakes.
At a public landing on Grand Lake in Stearns County, a motion-activated speaker greets boaters, and reminds them to inspect their boats and trailers and remove any aquatic plants or animals. A video camera watches to see whether they’re dragging plant fragments.
"It'll pick up the license number on the boat, so you can track the boat, and it'll pick up detail where you can see a little teeny weed on the trailer,” said Jim Bartelme, who has lived on Grand Lake since 1952.
Bartelme has helped combat starry stonewort since it was discovered there in 2017. Divers hand-pull the plant four times a year and so far, have managed to keep it from spreading, he said.
"What we wanted to make sure is to contain it, and we didn't end up like Koronis with it all over the lake,” Bartelme said. “We've been able to do that.”
Last year, the lake improvement district added chemical treatments, too. But Bartelme said no matter what they do, starry stonewort ends up in the sediment at the lake bottom, where chemicals can't reach.
"By the end of summer, early September, we normally have it down to nothing almost,” he said. “Then we come back in July, and there it is."
‘If we stop, what’s going to happen?’
Grand Lake residents want to keep starry stonewort from spreading to other water bodies, Bartelme said. In addition to the camera system, they installed a self-serve power-washing station for boaters leaving the lake.
On a recent summer day, Bob Litfin of St. Cloud and his grandson, Jaydyn Johnson, sprayed their boat and trailer after a day of fishing, washing away any invasive species that might have hitched a ride.
“It's just one of those things that people either they didn't know about, or they didn't take care of it when they pulled their boat out,” Litfin said.
Every year, the Minnesota Aquatic Invasive Species Research Center hosts “Starry Trek,” a one-day statewide search for starry stonewort in Minnesota lakes. Volunteers receive training, then are sent to nearby lakes to bring back any suspicious-looking species.
About 1 in 4 new infestations of starry stonewort in Minnesota were discovered by Starry Trek volunteers, Larkin said.
He noted that the "lake killer" moniker used by the media when starry stonewort was first discovered was hyperbolic. Still, the algae has created ecological and recreational problems and made itself another invasive plant that Minnesotans need to manage.
Forty Minnesota lakes are now wrestling with the thorny question of how to keep this aggressive invasive species in check into the future.
Farnum’s encouraged that a younger generation is taking up the fight. His granddaughter, Ezri, was a teenager when starry stonewort was first discovered in the lake. Now 23, she’s president of the Koronis Lake Association.
“It's nice to have the younger people involved,” Farnum said. “They need to be involved because they're the ones that are going to have it in the future and have to deal with it."
At the same time, he acknowledges that keeping up the battle against starry stonewort is challenging. He doesn’t know how long they’ll be able to sustain the effort.
"Part of the problem,” he said, “is if we stop, what's going to happen?"
