More Questions Than Answers For Starry Stonewort In Minnesota

August 25, 2021

PAYNESVILLE – Last year’s discovery of starry stonewort in Lake Koronis has made the lake our starting point for learning how to manage this new aquatic invasive species in Minnesota.

And learn we must. “There are more questions than answers for starry stonewort,’’ said Dan Larkin, assistant professor with the University of Minnesota Aquatic Invasive Species Research Center in the Twin cities.

A native of Europe, starry stonewort was discovered in the Saint Lawrence Seaway in 1978. Yet it really didn’t attract much attention until the mid-2000’s, when it took hold in inland lakes in Michigan, said Larkin. “So it really wasn’t on a lot of radar screens,’’ he said.

At this point we do not know where it is most likely to spread, or what its ecological impact will be when it gets into a lake.  

Nor do we know how effective different management tactics are for controlling it, said Larkin.

Lake Koronis should help fill in some of the blanks. The Koronis Lake Association is working with the Minnesota Department of Natural Resources and other partners on what’s hoped will be a five year, $800,000 effort to manage it. As part of that effort, researchers will be looking closely at the effectiveness of various techniques and applying scientific controls to develop reliable data.

The Minnesota Department of Natural Resources applied an algaecide to the heaviest infestation of starry stonewort near the Highway 55 access earlier this season, according to Chip Welling, AIS management coordinator for the DNR. It’s difficult to answer whether the treatment was effective or not, said Welling.

Vascular plants such as Eurasian water milfoil will uptake the herbicide and the effects of treatment are usually pretty obvious. But starry stonewort is a plant like algae comprised of single cells. Each cell needs to be bathed or have contact with the algaecide.

And, algaecides are copper based. It’s more difficult to get good depth penetration with them, and the copper persists in a non-active form.

Welling said it seems the application in Koronis had some effect, but it was not dramatic.

A larger effort to remove the plant in the lake with a mechanical harvester will get underway in a couple of weeks, according to Kevin Farnum with the Koronis Lake Association.

Buoys have been placed in the channel at the busy Highway 55 access for boaters to follow so they can avoid churning and slicing the plant where it is growing most aggressively. Farnum said boaters have been doing a good job of staying in the marked waterway. “That’s a good thing,’’ he said.

The bad thing is that boat traffic on the lake appears to be as busy this season as any, meaning the chances remain high for transporting the algae, he added.

Unfortunately, the plant also looks as if it is growing well in the lake, Farnum added.

Time will tell just how well it will do in Lake Koronis. Larkin said research suggests it does best in alkaline waters with high phosphorus levels. It also seems to do best in backwaters with minimal flow.

A recent study published by the research center found that Minnesota’s climate may help limit its range to the southern part of the state. “Our climate suggests that Minnesota has some areas of moderate suitability, but not the best climate,’’ said Larkin.

All of the starry stonewort in the U.S. is male; the plant is not reproducing sexually but instead cloning itself.

All it takes to spread the plant is a small fragment, or the white, star-shaped bulbils that the plant produces. A bulbil will have as many as 50 cells, each capable of sprouting a new plant.

Larkin said we do not know how long fragments of the plants or bulbils can survive once outside of the water. Transport by humans is believed responsible for its spread, said Larkin.

While it was discovered in Lake Koronis only last year, it’s possible it was there earlier. It could easily have gone unnoticed, noted Larkin.

It will grow in thick mats and displace native vegetation. Just how greatly that will affect native plants, invertebrates and fish will be learned.

Welling said DNR staff is surveying lakes around Koronis and beyond for starry stonewort. So far none has been found. It’s more likely to reach the size and distribution density where it can be found in mid-to-late summer, he noted.

It was first discovered in Wisconsin in 2014, and now the number of infested lakes is six. The DNR is concerned about the potential for a similar spread here, making it all the more urgent that boaters using Koronis follow protocols to avoid carrying it from the lake.