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It was a sort of scavenger hunt, only the participants were hoping to come home empty-handed.

In an effort to curb the spread of starry stonewort, Minnesota's newest aquatic invasive species, scientists at the University of Minnesota and two partner organizations put out the call to citizen volunteers who want to help save the state's lakes — or at least learn more about icky weeds. On Saturday, more than 220 volunteers turned up, inspecting dozens of lakes and collecting samples of anything that looked suspicious.

"We've found that people are eager to get involved," said Dan Larkin of the Minnesota Aquatic Invasive Species Research Center, which coordinated the daylong survey in partnership with the U's Extension and the Minnesota Department of Natural Resources (DNR). "It's just a matter of giving them the tools."

The DNR employs eight invasive species specialists who, along with county inspectors, monitor the state's beloved 10,000-plus lakes. But officials say the state simply lacks the time and resources to examine every lake regularly.

Saturday's search marked the first of its kind for citizen scientists, who answered that call in 20 counties. Volunteers — mostly environmentally conscious retirees and college students — were shown how to identify Minnesota's lesser-known invader, then sent out to public access points.

Like tenacious Eurasian milfoil, starry stonewort grows into dense mats that can shroud shallow waters, choke out native plants and create a wall between fish and their spawning grounds. Named for the tiny star-shaped bulbils on its root system, it was first detected in Minnesota in 2015. It has spread to nine area lakes in the last two years alone.

Boaters may be culprit

At Lake Koronis near Paynesville, it has grown to cover more than 50 acres of a shallow area near the southwest shore, and spread into the main basin at neighboring Mud Lake.

Within a year, researchers tracked the algae to Turtle Lake near Bemidji, some three hours to the north. Boaters are thought to be the culprit, because nearly all infestations are found near public access sites.

Aquatic scientists fear starry stonewort's long-term effects on native species and the surrounding ecosystem, but say they have little firm data on its impact.

"Until recently, no one had been looking for it," said Larkin, an associate professor at the U. "These are things that are growing underwater, and people tend not to pay attention to them."

Ranjan Muthukrishnan, a postdoctoral research associate at the U, is working to better understand starry stonewort and its preferred environment. Through meticulous risk-mapping, he has pulled environmental data from infected lakes in New York — one of the first states to be invaded — and created a model to predict which Minnesota lakes might have suitable conditions.

So far, research indicates the algae may thrive in bodies with lower water clarity and higher levels of calcium and nutrients. When matched with the state's extensive water quality reports, Muthukrishnan said, annual monitoring reports for starry stonewort will provide a much deeper understanding of its conditions.

'We'd better bag it'

In Dakota County, Kelsey White traveled to West St. Paul's Thompson Lake in a group of three volunteers. They took turns tossing a rake off the fishing pier into algae-ridden water, then yanked up clumps of mucky plants. Sitting cross-legged on the dock, they sifted through each strand for the telltale signs of starry stonewort or any other unidentifiable weeds.

The trio confidently cataloged native species like chara and curly-leaf pondweed, but didn't spot a single starry stonewort. After collecting five different samples, only one stringy plant warranted closer examination.

"If I can't name it, I guess we better bag it," said White, 28, of Inver Grove Heights.

To the organizers' delight, not a single group brought back samples determined to be non-natives on Saturday. Though results are still trickling in from other counties, DNR officials said there were no new confirmations of starry stonewort at the surveyed sites.

State officials anticipate the volunteer hunt will be held annually to increase public awareness, according to Tina Wolbers, a DNR aquatic invasive species prevention planner. With no known eradication methods, that's the best they can hope for.

In Beltrami County, where starry stonewort infected four lakes last year, labor-intensive techniques like mechanical harvesting, dredging and copper-based herbicides have all been tried. Bruce Anspach, a lake technician for the county, said it's up to boaters to be vigilant about not transferring plant fragments between lakes.

"If we want to keep our waters public, then we all need to do our part," he said.

Staff writer Josephine Marcotty contributed to this report.