BELGRADE — It was a clear, warm, still night on Great Pond, and in the darkness the late summer sky unfolded in a tapestry.

It was a perfect night for stargazing, but out on the lake members of a team from the Belgrade Lakes Conservation Alliance were training their eyes under the surface for an invader that could threaten to overtake parts of the largest water body in the watershed.

Joe Menyo of North Attleboro, Massachusetts, and Sebastien Dumont of Skowhegan, two recent University of Maine at Farmington graduates, operated the conservation alliance’s light boat, a rugged 21-foot flat-bottom fishing skiff retrofitted for nighttime operations against invasive variable leaf milfoil.

On Thursday night, Menyo and Dumont maneuvered the light boat away from the public landing on the south end of the lake. After getting out into deeper water, the two cut the motor and began setting up their equipment for a night survey. Standing on the bow, Menyo and Dumont clamped trolling motor mounts onto either side, then fit an extra long shaft attached with three high-powered LED lights into the mounts and tilted the poles into the water, until the lights were fully submerged.

A quick connection to a standard 12-volt car battery, and a cone of bright light poured out of the boat’s bow, illuminating the lake bottom. Menyo slowly directed the boat along the shoreline. In the bow, Dumont and Charlie Baeder, director of the conservation alliance, kept watch, turning the lights every so often to sweep across the lake bottom.

The beams brought an underwater world into focus, catching debris on the lake bottom, clumps of waving eelgrass, and hundreds of silvery minnows and larger fish, such as white perch and bass.

The team was hunting for frilly strands of milfoil, an invasive aquatic plant that has been discovered in the Belgrade Lakes watershed within the past decade. A single milfoil follicle can take root and grow rapidly in thick mats that can choke out the shoreline of lakes and rivers. A full-blown infestation can make swimming, fishing and boating almost impossible.

SHALLOW THREAT

In 2010, milfoil infestation was identified on Great Meadow Stream and North Bay, in the northeast end of Great Pond. The Belgrade Lakes Conservation Alliance started efforts to mitigate the infestation in 2012. Regular milfoil surveys and hand-picking by divers have kept the weed corralled, but experts worry about it spreading.

Milfoil grows in water up to 15 feet deep, and more than half of Great Pond is that shallow. If milfoil breaks out of North Bay, it could cause a much larger infestation.

“If we were to let it go, in any number of years it could be really bad,” Baeder said.

The light boat, which went into operation for the first time in July, adds to the arsenal in the campaign against milfoil.

The plant is typically identified through shoreline surveys from a kayak or a boat, or by divers and snorkelers. Both techniques are effective but time-consuming and take a lot of manpower, said Toni Pied, the conservation alliance’s milfoil director.

In theory, the light boat would allow a smaller team to survey a larger area, she said.

“The plan is that it is one tool we can use so we can cover a lot of ground quickly,” she said.

The light boat goes out about twice a week, working from sundown to midnight. One member of the crew pilots the boat at its lowest speed while others keep watch for the invasive weeds. If they find one, the crew drops a buoy near the plant and enters coordinates into a GPS so divers can come back the next day and clear it out.

So far, they have found least 20 plants during their night maneuvers, Dumont said. Surveys during the day usually turn up a lot more plants, but it helps to have teams looking around the clock.

Members of the conservation alliance got the idea for the light boat from the Lakes Environmental Association, headquartered in Bridgton. Peter Lowell, director of that group, said he got the idea for a light boat after watching biologists from the Department of Inland Fisheries and Wildlife use high-powered lights to find fish they stunned and tested. He thought the Lakes Environmental Association could put the same technique to use to find milfoil at night.

There are limitations. Unless the water is glass-still, light will reflect off the ripples, making it hard to see the bottom. The same problem comes up when water clarity is poor.

Despite the limits, the Lakes Environmental Association has been pleased with the way the boat has worked out and is retrofitting the lights onto a portable platform that can be attached to different boats.

“You can have three or four people looking at the same time,” Lowell said. “It is so efficient, it is pretty hard to beat.”

IN THE CLEAR

When the Belgrade Lakes Conservation Alliance designed its own light boat, members wanted to try to fix some of the problems the Lakes Environmental Association encountered, Pied said. For example, when milfoil field director Justin Leavitt designed the system, he opted to mount the lights underwater, instead of shining a spotlight through the surface.

Even that design has its limitations, Dumont and Menyo said. The surface still needs to be glass-clear to be able to pick out plants on the bottom, and the direction of the light could be stronger.

But as they pulled away from the public launch for a second survey round, the team already was talking about how to make the boat more effective for next season. It’s that type of experimentation and creative thinking that has driven a lot of the tactics, such as suction boats and burlap plant coverings, that have become common practice among groups tackling the milfoil problem in lakes across Maine.

“It’s about innovation,” Pied said. “It’s about trying new tools and seeing what works.”