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Climate change is helping invasive plants spread through Maine's waterways

Alex Dyer from Friends of the Cobbossee Watershed inspects some Eurasian water milfoil plants removed from Cobbossee Lake in Winthrop in September 2023.
Charlie Eichacker
Maine Public
Alex Dyer from Friends of the Cobbossee Watershed inspects some Eurasian water milfoil plants removed from Cobbossee Lake in Winthrop in September 2023.

It’s a chilly morning in Winthrop. But that’s not stopping a group of divers that’s taken a motor boat to the center of Cobbossee Lake, a nine-mile stretch of water just outside Augusta.

They’re with a group called Friends of the Cobbossee Watershed, and their goal is to manually remove a patch of Eurasian water milfoil — an invasive plant that’s been spreading across the lake in recent years.

They anchor near a small cluster of islands, and Em Russell gets his scuba gear ready for the underwater removal, which is done by hand.

“It’s just like dandelions in your garden. If you leave a part of the root, it’s likely that the plant is going to come back," he says.

Russell drops off the boat and gets to work, diving about four feet down and placing each plant in a bag that’s delivered to the surface.

Back on the dive boat, conservation director Alex Dyer inspects the first haul. The milfoil has a long stringy stalk surrounded by feathery green leaves. This batch is tangled up with a native called water marigold. Eventually, the team will bring it to a woodlot where it will be burned.

Eurasian water milfoil.
New York Invasive Species Information
Eurasian water milfoil.

"Often when they’re so closely growing together, we can’t help but pull a couple of the natives with the invasives, just to make sure we get good margins on the invasive root balls," Dyer says.

This kind of in-depth work wasn’t happening on Cobbossee Lake just a few years ago. But three different invasive plants have been discovered on the lake since 2018, including Eurasian water milfoil and its better-known relative, variable leaf milfoil. Left unchecked, both plants can grow into dense mats that crowd out native species, reduce water quality and property values, and hinder boating and swimming.

While Eurasian water milfoil is still rare in Maine, Dyer warns that it may the bigger threat in this area: it breaks apart and spreads more easily, and now, it’s reached the stream that flows out of Cobbossee Lake, meaning it could potentially spread downstream into Pleasant Pond.

"The more water bodies that it gets into, the more quickly it’s going to spread," Dyer says. "It’s so aggressive. It’s a very aggressive invasive."

It’s hard, expensive work containing an invasive aquatic species like Eurasian water milfoil. Local groups must survey for it using boats and dive gear. When it’s found, they work with the state to remove it by hand, herbicide and other methods.

Even then, they might not be able to stop an infestation. Invasive milfoils have been detected in more than 50 waterways in Maine, but eradicated from just six, according to data collected by the Maine Department of Environmental Protection.

That’s why officials say that boaters must prevent these plants from spreading in the first place. Infestations generally start when fragments get stuck on boats, then come loose in new waters.

“It’s very difficult to eradicate any invasive aquatic plant. Once it’s established it’s really, really hard," says John McPhedran, an invasive plant expert at Maine DEP. “Prevention is the mantra that we need to continue to promote.”

Em Russell dives into Cobbossee Lake to remove a patch of Eurasian water milfoil on Oct. 10, 202as Jen Peasnall helps direct him and watch for stray fragments.
Susan Sharon
Maine Public
Em Russell dives into Cobbossee Lake to remove a patch of Eurasian water milfoil on Oct. 10, 2023 as Jen Peasnall helps direct him and watch for stray fragments. They both work for Friends of the Cobbossee Watershed.

Boaters are always urged to clean, drain and dry their watercraft between outings. Inspectors promote this practice at public boat launches, and state lawmakers recently expanded the requirements for doing so.

But McPhedran says that even with these rules, it's getting harder to keep invasive plants like milfoil in check. That's partly because a frigid climate has traditionally worked in Maine’s favor – limiting the window for new plants and algae to grow. But now, climate change is leaving Maine’s waterways ice-free for more of the year.

“We are going to see probably even longer growing seasons, good conditions for plants to grow, good conditions for invasive plants to become established and grow well. Also conditions for native plants to grow well and more prolifically," McPhedran says.

It’s not just milfoil that’s causing concern. The state recently created a position to focus on invasive aquatic wildlife, like the Zebra mussel that's been found in nearby Quebec and New Brunswick.

And there are at least five other invasive plants in Maine lakes. One of the more troubling, European naiad, can spread by releasing hundreds of small seeds.

In southern Maine, the community around Lake Arrowhead has been managing an infestation of variable leaf milfoil for two decades. And three years ago, it discovered that European naiad was also spreading

“It’s a seed producer and seeds can get into the ballasts of boat tanks, it can get into live wells, it’s much trickier to inspect," says Dave Sanfason, president of the Lake Arrowhead Community association.

The outlet of Cobbossee Lake is off-limits to recreational boaters due to an infestation of Eurasian water milfoil that has spread there.
Charlie Eichacker
Maine Public
The outlet of Cobbossee Lake is temporarily off-limits to recreational swimmers, fishermen and boaters due to an infestation of Eurasian water milfoil that has spread there.

Sanfason says state and local groups have ramped up their efforts to educate boaters and remove invasives. He's worried what could happen if they overrun the lake and deplete its oxygen.

"We've been able to prevent that from happening with the variable leaf milfoil, but because we’re early in the introduction of this European naiad, we want to see if we can either eradicate it or come close to eradicating it early," Sanfason says.

Back on Cobbossee Lake, local groups have been trying to eradicate invasive milfoil for the last five years. But it's still spreading across the nearly 6,000-acre lake, and Alex Dyer says that her team could still be pulling it out into November. That's a much longer window than when she first began working to remove invasive milfoils around central Maine.

"I would say that the growing season for variable leaf milfoil 10 year ago was much shorter," Dyer says. "It was a lot more of a tightly packed field season. But you knew that the plants weren't spreading in May and September. So now we’re contending with a much longer growing season, a much longer field season."

Even if they do eradicate it , Alex Dyer says that groups such as Friends of the Cobbossee Watershed won't be able to let their guard down. The growing season for invasive milfoil seems to be getting longer. And there's a looming threat from other invasive species like European naiad.

"We have a whole list that our surveyors are trained to identify, not just the ones that we know we have in our watershed," Dyer says. "We’re also looking for the ones that we don’t know to be in our watershed yet. So this is going to be long-term, sustaining effort to protect our lakes. It can’t – this effort can’t go away. It has to continue. And it has to increase across the board."

Susan Sharon contributed reporting.

Support for Deep Dive: Invasives is provided by Maine Audubon, Friends of Acadia and Coastal Maine Botanical Gardens.

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