NAPLES – When it first putters up the Songo River, the SS Libra looks like just another boat enjoying a sunny morning.

Give it a few minutes. Once the crew of young workers reaches a designated spot, everything changes. Two men in wet suits hop into the water and drag a large hose in after them. Motors roar to life. Water gushes through a metal sluiceway and pours onto the deck, draining away over the side and through grates in the floor.

The Libra is a suction harvester belonging to the Lakes Environmental Association, designed to eradicate invasive plant species from the waterways of southwestern Maine. It recently finished its first summer of work between Songo Lock and Brandy Pond.

Specifically, the crew targets two varieties of milfoil that are nonnative to Maine ecosystems. According to the LEA Web site, the lack of a natural predator against this milfoil can lead the plant to spread, inhibiting recreational use, driving down property values and depriving aquatic life of oxygen.

While this isn’t the first summer the LEA has combated milfoil on the Songo River with a suction harvester, it’s the first time it has had one to call its own. Last year, the boat was donated by Jim Flick of Bridgton and converted to harvesting use by volunteers and local businesses.

“The remarkable thing was we started up this summer with no experience on it,” said Peter Lowell, executive director of the LEA.

The Libra has been docked at the residence of Terry Plum and Barb McDonough, who have allowed the LEA to use their property on the river for three years. The Naples Marina has donated winter storage of the watercraft.

It gets its name from the Libra Foundation, which gave funds to the LEA to help with the pontoon boat’s conversion. The foundation has also helped the LEA in the past, awarding $63,000 to the organization in 2004.

This summer, the Libra has been crewed by Adam Perron, 21, of Bridgton; Daniel Bishop, 23, of Denmark; and brothers Thayer and Trevor McKeith, 18 and 14, from Sweden.

The crew has spent six hours of an eight-hour workday on the water, four days a week. Last Tuesday, Bishop and Thayer were equipped with weighted wet suits for a morning underwater shift.

A foot pump on the boat allows a hose to suck water through the sluiceway, which funnels the water through four netted bags. In the water, a piece of equipment floats on an inner tube and provides air for the divers’ regulators. It is officially known as the “Third Lung,” and unofficially known as the “hookah.”

Having focused their efforts on the river for most of the summer, the Libra heads for blue buoys that mark minor patches of milfoil.

“We’ve already cleared out most of the large patches,” Perron said.

“We were changing the bags all the time,” Trevor added.

Removing bags full of milfoil is one of the key responsibilities of the crew that remains on the boat. In addition, those on deck will insure the safety of the divers and use pool skimmers to scoop up plant fragments on the surface.

This crew also signals to boats on the river to give the divers a wide berth. By the last week, most were doing so of their own accord.

“Almost every person on the river knows what we’re doing,” Trevor said.

About midway through the morning, with work on deck moving slowly, Trevor dons a snorkel and joins the divers. Adam said Trevor will be able to help the divers by spotting milfoil from above.

“He’ll see things they can’t see,” Adam said.

In addition to the milfoil, the crew helps clean up the river by feeding trash on it into the suction tube. Cans and golf balls are the most prominent items, but last week also saw the discovery of a squirt gun, the second of the season.

Perron recently graduated from the University of Maine at Orono, and hopes to return to work with the LEA next summer. Bishop, beginning his senior year at the University of Maine, is an environmental management major. Thayer hopes to go into a field related to scuba diving, perhaps underwater archeology.

Bishop said that while the job is physically demanding and potentially stressful, the crew doesn’t let it wear them down.

“Even the bad days are pretty good, because we’re outside and on the water,” he said.

“I’d say the downside of the job is that it’s ruined recreational swimming for me,” Perron joked.

Lowell praised the crew’s hard work, and said they received a standing ovation at the LEA’s annual meeting.

“I never imagined we would have the river so clean in one season,” he said. “They’ve just done an incredible job.”

Lowell said the LEA will continue milfoil-eradication efforts through boat inspections and work with local marinas. However, given that milfoil reproduces by fragmentation, it is difficult to banish it completely.

“I think we’ve accomplished more than we set out to do this year,” he said. “But that doesn’t mean that the challenge isn’t very serious. This stuff is very tenacious.”