Invasive aquatic weed cropping up across Great Lakes region

Starry stonewort, which forms dense surface mats in lakes, first turned up in North America in 1978 in the St. Lawrence River in New York state. Researchers think it probably arrived in ballast water from ships entering the Great Lakes.

MINNEAPOLIS — An aquatic weed is creeping across the Great Lakes region that grows really fast and is very hard to kill.

Scientists don’t know a lot yet about starry stonewort, but they’re hurrying to find out more. The plant, which forms dense surface mats in lakes, first turned up in North America in 1978 in the St. Lawrence River in New York state. Researchers think it probably arrived in ballast water from ships entering the Great Lakes.

It wasn’t a big concern for about 30 years, but then it took off. Now it’s widespread on Michigan’s Lower Peninsula, where it has infested more than 200 inland lakes, and parts of western New York. It was found in Wisconsin in 2014 and in Minnesota in 2015. It has also reached some lakes in Indiana, Pennsylvania, Vermont and Canada.

WHAT IS STARRY STONEWORT?

The name comes from a tiny, white six-pointed-star-shaped “bulbil” on the stems. The plant is considered beneficial and even endangered in its native Europe and Asia. But for some reason it turned aggressive in American waters.

It’s difficult to kill with herbicides because it doesn’t have a vascular system that could carry the poison to the entire plant, said University of Minnesota invasive species specialist Dan Larkin. Herbicide treatments often result in a “haircut effect” that burns off the top of the plant, but leaves the rest alive to regrow.

WHY IT’S A CONCERN

The weed mats can be a nuisance to boaters and anglers. Scientists are also concerned about the potential harm to native plants, fish habitats and other disruptions to aquatic ecosystems. But Larkin said there’s almost no peer-reviewed research documenting the adverse effects. For example, it isn’t known yet whether it crowds out wild rice. Still, experts wouldn’t be sounding the alarm if they weren’t worried.

Researchers are just starting to understand the conditions in which the plant thrives, and modeling suggests that large swaths of the U.S. could be highly suitable, including the mid-Atlantic states, the Great Plains and much of the West, Larkin said,

HOW IT SPREADS

It reproduces when plant fragments and bulbils break off, and they can hitchhike on contaminated boats and trailers. Recent discoveries have often been concentrated near public accesses such as boat ramps. Scientists don’t know yet how long those plant fragments can stay viable out of water, Larkin said.

Starry stonewort resembles other plants, and the distinctive bulbils don’t appear until late in the season, so it can go unidentified for a long time.

It was confirmed in September in Lake Winnibigoshish in northern Minnesota, but that infestation probably went unnoticed for several years. That finding is concerning because the Mississippi River flows through “Big Winnie,” a popular fishing destination and wild rice lake. No infestations have been found downstream so far, though monitoring continues, said Tim Plude, an invasive species specialist with the Minnesota Department of Natural Resources. He said the species doesn’t seem to like fast-moving water.

HOW TO CONTROL IT

It’s tough. Wisconsin has had some success with an expensive method known as DASH — Diver Assisted Suction Harvesting. Divers hand-pull plants and feed them into a big suction hose.

Researchers hopes to learn this summer whether a different approach worked on Lake Koronis, a popular recreational lake in central Minnesota where the weed was first discovered in the state. Crews used a mechanical harvester, aided by a diver, and followed up with herbicide.

The best strategy is prevention. Boaters are urged to thoroughly clean their watercraft, trailers and other equipment of plants, mud and other debris before moving them from one body of water to another.

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