Tackling Invasive Eelgrass

About 12 years ago, the Tennessee Valley Authority aquatic plant management team noticed a new, unusual species spreading at Guntersville and Nickajack reservoirs. The non-native eelgrass was overtaking the native plant habitat.

“We saw impacts on electricity generation and recreation, and for stakeholders – homeowners and property owners throughout the reservoirs, too,” Stephen Turner, TVA aquatic plant management program manager, said.

The rapidly growing plant—a blend of two non-native species of eelgrass—has since spread throughout Guntersville, Nickajack, and Wheeler reservoirs. And it’s a headache for many.

Native eelgrass has always grown in the Southeast. Still, the non-native version has required careful management of the reservoirs to ensure the best possible outcomes for the ecosystem, recreation, navigation, power generation, and more.

“It’s not like the other plants that have been there before,” Lauren Pelto, TVA’s South Region government and community relations director, said.

TVA and its partners, including researchers and the public, work together to manage it.

About 12 years ago, TVA’s aquatic plant management team noticed the new, unusual species of eelgrass spreading at Guntersville and Nickajack reservoirs.

Powering Partnerships
Eelgrass grows underwater, its bright green strands waving like hundreds of hands in the current.

While underwater plants provide shady cover for some fish, often a benefit to those who fish the reservoirs, thick mats of invasive eelgrass can wind around boat propellers, wash up on shore, or impact.

TVA’s hydroelectric dams
TVA works closely with My Lake Guntersville – a group of community, government, and economic partners from Jackson and Marshall counties in Alabama – to support treatment efforts on private residential and commercial areas on Guntersville Reservoir shorelines.

And now, TVA is coordinating with Mississippi State University researchers to test safe, effective herbicide combinations that will kill eelgrass. “This partnership will help inform TVA’s management strategy in the future,” Pelto said.

Eelgrass with a stem
Non-native eelgrass has a stem that acts like a scaffolding, allowing the plant to grow and spread easily.

Insidious Invader
To figure out how best to treat the eelgrass, it helps to understand the plant itself. And that’s been a challenge.

The invasive eelgrass is a cross between two Eurasian species of eelgrass, and it wasn’t until 2016 that Japanese researchers described it as an official hybrid. Then, this aquarium plant showed up here.

“When the bell was rung in the U.S. about this species being present, it was by a group in Florida,” said Maxwell Gebhart, research associate at Mississippi State University.

Eelgrass had appeared all over Florida around the same time, Gebhart said, which means people introduced it in several places, possibly by dumping their aquariums into water outside. From there, it traveled to and between TVA’s reservoirs.

Meso- What?
To tackle this invader, TVA’s experts had to think beyond what had worked before.

Through a partnership with Mid-South Aquatic Plant Management Society, TVA joined forces with Mississippi State University.

“They’re one of the premier aquatic plant management universities and research facilities in the Southeast,” Turner said.

“We supported a four-year research project, including studying the species of eelgrass itself and treatment technologies,” said Michelle Cagley, senior program manager in environmental sustainability research and development at TVA. They build replicas of reservoirs in an outdoor lab called a mesocosm.

“You can think of it kind of like a miniature ecosystem,” Gebhart said. “We try to replicate the natural conditions, and then we can test some ideas using that small natural system to see herbicide efficacy.” “Then, we move to the reservoir when we’ve identified which treatments to try,” Cagley said.

Based on what they’ve seen so far, Gebhart said, they’re aiming to field test herbicides in the coming years. TVA’s managers and the researchers pointed out that every herbicide they use is tested to be safe for the water, aquatic life, and people.

“Anything we use at TVA is an EPA-approved herbicide for aquatic use, and we have done additional NEPA reviews on all the herbicides that we use, too,” Turner said.

“A lot of the introductions in our system have come from boat traffic,” Turner said. “One plant gets stuck on a boat, and that’s all it takes, especially as aggressive as this species is,” Gebhart noted.

“It grows incredibly fast,” he said. “The plants can float for weeks and weeks and weeks at a time and not die.” And they spread unlike other kinds of eelgrass, too.

“The plants put out stolons for reproduction like strawberries do,” Gebhart said. “We’ve seen the formation of a stem structure, which is really unlike both parent species. That stem is acting almost like a scaffolding, where the plants are getting really, really high up.”

The stems snap off and become new plants, but meanwhile, the stem sends out roots and keeps producing new little eelgrass offshoots.

“Chains of individual plants can extend upwards of 45 plants,” Gebhart said. “This is just like a machine when it comes to cloning itself.”

In the future, TVA plans to continue using harvesters to remove eelgrass from open water, but it will use the most effective herbicide to target near-shore eelgrass.

The goals? Keep reservoirs open for recreation and navigation, and ensure people can use TVA’s public access areas. “The big picture is preserving and protecting the environment,” Daniel Sipe, TVA generation research manager, said. “This work demonstrates the importance of environmental stewardship and mitigating risk to TVA generation assets.”

Along with TVA and researchers, the third partner in successful eelgrass control is property owners, who can take steps of their own to help.

Pelto stressed that it will take an all-hands-on-deck approach to knock back such an invasive plant, one that spreads almost as fast as TVA works to remove it.

Research and collaboration remain front and center in tackling invasive eelgrass.

A Future Balance
So far, with at least another year of trials in the mesocosm, herbicide research is going well. “We’re coming back and seeing something new – here’s the next step to understanding this, or the next piece of something that we didn’t notice before,” Gebhart said. “We’re really happy to see these preliminary results. We’re learning a lot, and it’s very exciting to see.”

Eventually, the goal is to reach a balance – a balance of management between agencies and individuals who care about the reservoirs, as well as a balance of species.

Over time, with continued aggressive management, this invasive eelgrass may become part of a diverse mix of plants. That’s what Turner has seen already.

“When it first came in, it was so aggressive that it just wiped out a lot of areas,” Turner said. “But over time, we’ve seen the other plants become adjusted to its presence, so we’re starting to see other species growing in combination with it.”

The future will bring more effort, more research, and hopefully more coordination among all the groups that the invasive eelgrass impacts.

“We can use experience here to keep the eelgrass (in other reservoirs) from creating the situation that we had on Guntersville, Nickajack, and Wheeler, by treating it early,” Cagley said.

Pelto agreed with this hopeful outlook. “TVA employees are committed to evolving and learning, and they’re working hard with partners to find innovative solutions,” she said.

TVA’s aquatic management team manages the reservoirs to achieve the best possible outcomes for the ecosystem, power generation, recreation, navigation, and more.