NAPLES — The S.S. Libra may not have the elegant lines of a cabin cruiser, and its plodding pace is not much faster than an aquatic turtle’s. But the vessel is the first line of defense in the war against the variable leaf milfoil on the Songo River.

This summer, the boat and its four-man crew are stationed just south of Songo Lock, engaged in a fight to keep the aquatic plant pest from invading recently won territory to the north. Although the crew is highly experienced in milfoil eradication, the stakes are high and the outcome far from certain.

Keeping Maine’s fresh waters free of invasive weeds is crucial, not only to the health of native plants and animals, but also to the state’s economy, say those on the front lines. Maine’s lakes generate about $3.5 billion annually in tourism spending, increased property values and recreational boating activity.

“This impacts every lake in Maine,” said Peter Lowell, executive director of the nonprofit Lakes Environmental Association in Bridgton.

The southern Songo River is the latest battleground in a 10-year campaign to beat back milfoil from its northern reaches and the waters of Brandy Pond, part of a roughly 40-mile recreational water that stretches from Harrison on the north of Long Lake to Standish on Sebago Lake’s southern tip.

Led by the lake association, efforts to clean out the weed north of the lock have been successful. But milfoil continues to proliferate along the mile-long section of the river south of the lock.

Last summer’s warm, dry weather triggered both an explosion of milfoil below the lock and a surge in boat traffic. Inspectors at the lock were removing armfuls of milfoil that was entangled in the propellers of passing boats.

Alarmed that milfoil would regain a foothold north of the lock, the association launched a campaign to encourage boaters to stay south of it. Although state officials denied a request to close the lock, they installed more channel markers in the river to keep boaters away from the worst of the milfoil and agreed to pick up the cost of more boat inspections.

Similar battles to control or eradicate invasive weeds are taking place on lakes, ponds and rivers across southern Maine where infestations have been found. Compared with other states, Maine remains relatively free of invasive aquatic plants, which have been discovered in only 33 lakes.

But the race is on to keep the half dozen species of invasives seen so far from spreading to Maine’s 5,700 other lakes.

Public funding doesn’t begin to cover the costs of controlling the problem, said Lowell. The major federal source is the U.S. Fish and Wildlife Service, which grants eligible states, including Maine, up to $30,000 a year for invasive plant measures.

Maine raises about $1.1 million annually through milfoil stickers — $10 for residents and $20 for nonresidents — required on all power boats on fresh water. The money covers the cost of three Department of Environmental Protection workers who coordinate training and education.

It also funds small grants for boat inspectors and other efforts.

But the work is largely performed by thousands of volunteers and lake associations.

“We depend on the many eyes out there,” said Paul Gregory, environmental specialist at the Maine DEP’s invasive aquatic plants program.

The most damaging invasive species, the Eurasian milfoil, is a plant that grows in water up to 20 feet deep. Its tendrils rise to the surface and form thick mats that choke out native plants.

Milfoil ruins swimming and drives out cold-water fish such as trout and landlocked salmon. It can also cause problems for public drinking-water systems that rely on lakes.

The plant is spread chiefly by boats and trailers that are hauled out of waters with milfoil infestations and then put into other lakes. Only a small fragment of the plant is needed to establish a new colony.

The Little Sebago Lake Association has been working to remove milfoil on the lake for five years. The association operates two suction dredges, employs a seasonal staff of 15 supported by as many volunteers, and spends $60,000 annually, most of it from association members.

The result has been a dramatic reduction in milfoil in the upper part of the 2,009-acre lake in Gray and Windham, where efforts have been focused, said Pam Wilkinson, association president.

“We haven’t had a bloom in the upper basin for three years,” she said.

Volunteers and lake associations have set up rapid-response networks to deal with reports of new infestations. Late last month, a boat inspector on Kezar Lake in Fryeburg discovered Eurasian milfoil on a boat about to enter the water.

Within hours, a plant patrol team of 12 people from the Maine Volunteer Lake Monitoring Program, the oldest citizen lake monitoring association in the country, was combing for signs of Eurasian milfoil on Tripp Lake in Poland — where the boat had last been. They put in more than 130 hours surveying the lake by boat and diving.

“So far, all reports are coming clean,” said Roberta Hill, aquatic ecologist at the program.

The number of boat inspections conducted, by both paid inspectors and volunteers, has climbed steadily each year, from 2,848 in 2001 to 72,428 in 2010. Last year, the inspectors managed to intercept 281 invasive plants on boats, up from 254 the year before.

Education efforts are now focused on encouraging boat owners to make their own inspections. But it is slow going, said Gregory.

“We have observed people on boat ramps,” he said. “Fewer than 20 percent are inspecting equipment on their own.”

Back on the Songo River, Lowell said the milfoil war is one of advances and setbacks.

LEA’s proposal to close the lock this summer – a move opposed by marinas — fell on deaf ears in the Legislature this year. But marinas, other businesses, municipalities and Lowell’s organization banded together to ask the Department of Conservation to raise the fee at the lock from $6 to $10 a boat to pay for dredging and other milfoil eradication. So far there has been no response to their request.

Divers are laying barriers on the river bed, fashioned from shrink wrap donated by local marinas, to block the light and prevent the regrowth of milfoil. The 4-H Club at Lake Region High School in Naples is helping to make the barriers.

Lowell is hoping that more boaters will honor a voluntary ban on traveling through the lock this summer. Boaters generally are respectful of eradication efforts.

“I absolutely support inspections. We did our own check,” said Erik Ousback of Gorham, as he motored north through the lock last week.

As the summer unfolds, the S.S. Libra and its crew will continue the muddy work.

Graduate students Adam Perron of Naples and Dan Bishop of Bridgton, and college-student brothers Tyler and Christian Oren of Casco are all multi-year veterans of the Songo milfoil campaign. They spend 10 hours, four days a week vacuuming the weed into onion sacks and hoisting themashore, where the pesky plants are composted.

“It takes some practice at first, but we have it down good,” said Bishop.

Beth Quimby — 791-6363

bquimby@pressherald.com