Starry Stonewort

Star(ry) Trek

An unknown species showed up where none had been before, and researchers sprang into action

By Becky Beyers

starry stonewortIt started with a routine scoop of water and lake weeds.

A year later, the starry stonewort in the net that day on Lake Koronis has made waves across not just Minnesota’s lakes and ponds, but through the network of scientists and advocates who deal with non-native and invasive aquatic species.

Scientists at the Minnesota Department of Natural Resources had gotten a routine call from homeowners along the Paynesville, Minn., lake, asking for help removing large patches of plants that were impeding boats and swimmers. A crew member that day noticed the plant looked odd and sent photos to a plant ecologist at the agency. She, in turn, passed it along to a colleague in New York, who confirmed the diagnosis.

The plant was starry stonewort, a grassy algae that forms dense mats in lakes. Until last summer, it hadn’t been seen in Minnesota and researchers had no reason to expect it would arrive so soon. But it had leapfrogged many miles, likely by hitching a ride on a boat or trailer from an infested lake somewhere far to the east and south of Lake Koronis, which is in west-central Minnesota. The network of researchers and public officials who work to control and manage invasive species in the state’s waters were facing a new and unexpected challenge.

What it is

Starry stonewort looks similar to other native charophyte (green algae) species found in Minnesota lakes, says Dan Larkin, assistant professor in the Department of Fisheries, Wildlife and Conservation Biology, but it has distinctive small star-shaped bulbils, about the size of a lentil, from which the grassy part of the plant sprouts.

Dan Larkin“There’s very little research on this species as an invasive,” says Larkin, who was hired last summer—shortly before the Lake Koronis discovery —as a Minnesota Aquatic Invasive Species Research Center (MAISRC) faculty member. Starry stonewort was first found in the United States in 1978 in New York and gradually has spread to the Midwest.

Hundreds of papers have written about it as a model species in cell biology research—it’s native to and endangered in Japan, on conservation watch in Britain—but little research has been done about it as an invader, Larkin says. “It is largely a rare species in its native range. But it often happens that a species will behave differently when it arrives in a new place.”

There are many anecdotal accounts about how this species invades, but little data to draw upon, he says. “There is very high uncertainty. Will it spread to new lakes in Minnesota? What will its impacts be?” In Michigan, starry stonewort has been found in over 120 lakes and there are reports of it spreading quickly in some locations. In Minnesota, the state's second infestation was reported just a few weeks ago, in a lake near Bemidji.

“We didn’t expect the first place it would be found here is Lake Koronis,” says Chip Welling, who coordinates aquatic invasive species work for the DNR. “But boaters can drive, and they can bring it along. The worst-case scenario is that a non-native species fragment survives the trip” and shows up in a lake far from previous infestations, as it did in Lake Koronis. “But right now, we have more questions than answers,” he says. 

Even less clear is how the invader affects aquatic animals and whether it’s taking up space that otherwise would be occupied by native plants.  

Timing is everything

While starry stonewort was only discovered in a Minnesota lake last summer, it might not be a new arrival. “Nobody knows how long it’s been there—it could be that it’s been there for years and finally got to the point where it was noticeable, or it could be that it was recently introduced and spreads very fast,” Larkin says.

Larkin’s arrival to MAISRC was in some ways perfect timing. “It wasn’t something I had worked with before, but out of necessity became familiar with it quickly,” he says. “There’s a window of opportunity to respond to a new invasion.” That’s where research comes in.

This summer, researchers are exploring several questions:

  • Where starry stonewort might go next. 
    Post-doctoral researcher Luis Escobar, who specializes in ecological niche modeling, used global occurrence and climate data to predict areas of the U.S. that may be vulnerable to starry stonewort invasion. The next step will be to take a finer-grained look at characteristics of lakes where starry stonewort has persisted—water chemistry and nutrient availability, for example—to predict which lakes in Minnesota and elsewhere could be vulnerable to invasion. 

  • How it grows and spreads. 
    Carli WagnerUndergraduate student Carli Wagner will be collecting plants, establishing cultures and doing controlled experiments in the new MAISRC lab, which has plant growth facilities for this kind of work. One question is to see how long starry stonewort can stay alive out of water, Larkin says. Can it stay alive on a boat for an hour, a day, or a week and survive to produce new plants if returned to water? “That will increase our understanding of the risk of spread by boats and other activity,” Larkin says. 

  • How to control it. 
    Killing off starry stonewort using herbicides or mechanical removal is difficult, because small fragments can give rise to new plants. Even if 90 percent of the plant dies, the rest of it can regrow. “And we don’t know how sensitive the bulbils are to control efforts,” Larkin says. “Even if you got the plants out, would bulbils left in the sediment sprout new plants?” Experiments in the MAISRC lab will be used to test the efficacy of different algaecides for starry stonewort control—and the potential for non-target impacts to native species.

Two citizen science efforts are also in development to help address future invasions and management: the “Aquatic Invasive Species Detectors” program will train volunteers to identify and report new invasive species occurrences in state waters, and the “Aquatic Invasive Species Trackers” program will engage citizen scientists in monitoring the outcomes of management efforts.

“We have large gaps in scientific information about this plant,” Larkin says. A gathering of an international group of starry stonewort experts on campus in June was aimed at synthesizing what is known about the ecology and effects of this species and identifying key research needs to support management responses.

For the DNR, which is responsible for preserving Minnesota’s natural resources, the establishment of MAISRC has been “invaluable,” Welling says. When invaders show up, property owners and lake associations sometimes hear about untried approaches, “and they may be oversold, or incompletely represented in terms of the effects on plants and water. Having a third-party is important to our understanding of these situations and possible research solutions.”

Long-term answers

Starry stonewort’s arrival in Minnesota is following a pattern common to non-native species, Larkin says. “Invasions take time,” and often follow a long pattern of dormancy before population increases.

This “lag phase” can take 50 years or more. “We tend to focus on the outcomes, versus the process of what happens with an invasive species, but to get from A to B can take decades,” he says. “What that means is early on we have a lot of uncertainty where we don’t know what the arrival of this species means. Sometimes a non-native arrives and does not cause harm, but when species are invasive and do cause harm to ecosystems or human uses, that’s bad.

“At different stages in the process, there are different needs and priorities; for example, curly-leaf pondweed is long established in Minnesota and here to stay, so the goal there is managing nuisance growth. But in the early stages, like this, the goal is to prevent further spread and potential impacts."

With starry stonewort, “we have the opportunity at this stage to limit damage.”