Edgartown will consider making it mandatory for hundreds of residents in the watershed of the Edgartown Great Pond to hook up to a new town sewer line, following recommendations of a study into pollution of the pond.

The final report of the Massachusetts Estuaries Project (MEP), released this week, finds that most of the 890-acre pond is moderately or significantly impaired by high levels of nitrogen, which poses a threat to eelgrass, shellfish and fish.

The most effective way to reduce nitrogen to acceptable levels, the report says, is to cut by 30 per cent the amount entering the watershed in wastewater, mostly coming from septic systems.

That change does not come cheap. This week at a special town meeting voters approved spending $70,000 for the design and permitting of connecting 149 lots in the Island Grove subdivision to the town wastewater treatment plant.

The town will come back in time for the April annual town meeting with projected costs, expected to run somewhere between $800,000 and $850,000, according to wastewater superintendent Joe Alosso.

Island Grove residents would meet their share of the cost through a betterment fee of $200 to $300 a year for 20 years. Plus they would pay about $10,000 to tie into the sewer line, Mr. Alosso said.

There were concerns expressed at town meeting: about the cost in these straitened times without tax increases, about the public-private funding of the plan, about whether this would set a precedent for other areas, about whether everyone who could tie in, would tie in.

Mr. Alosso said the town was meeting half the cost because everyone would benefit. He said the circumstances were unique.

As for whether people would opt to spend the many thousands of dollars to tie in, Mr. Alosso said a survey of Island Grove residents, taken in the summer of 2007, showed 80 per cent wanted the sewer and wanted to tie in.

Even so, he said, town officials had to begin talking about making it mandatory to tie in.

“You need a pretty solid reason to do that,” he said, “but I think this report [the final estuaries report] provides that.”

Edgartown health agent Matt Poole said he believed the board of health was prepared to make the tie-in mandatory. “Given that it is recommended by the MEP that sewering is the best way to improve the pond,” he said.

He too said he expected a regulation to be ready before the April town meeting.

The need for action to lower nitrogen levels in the pond is clearly set out in the estuaries report. Septic systems release about 5.5 tons of nitrogen into the watershed each year, which eventually enters the pond, the report found. Prepared by marine scientists at the University of Massachusetts at Dartmouth, the estuaries study is based on an intensive sampling program and later work using sophisticated computer modeling to precisely track the source of pollutants. The study involves coastal ponds and embayments in all of southeastern Massachusetts, including many other Vineyard ponds. The Edgartown Great Pond is the first Island pond to have a completed estuaries report. A draft report was obtained by the Gazette and published on its Web site late last year; the final version is virtually unchanged from the draft.

Nitrogen works as a plant fertilizer. Thus when nitrogen levels increase, so does aquatic plant life, particularly algae.

At night, when these plants take oxygen from the water and release carbon dioxide, oxygen levels in deeper water can fall below those needed to sustain animal life, like fish and shellfish.

“The fish can swim away, the shellfish die out,” the report says.

And when the plants die, they sink to the bottom and decay, also reducing oxygen. And as organic matter piles up, the bottom will no longer support shellfish.

Monitoring of the pond over a 45-day period after a breach found it contained too much organic matter, “with algae exceeding desirable levels about half the time.”

The excess plant matter also damaged eelgrass beds by growing on the eelgrass and robbing it of light. Eelgrass had come close to disappearing from the pond several times in the past 10 years.

Overall, the pond habitat was found to be moderately to significantly impaired throughout. Returning the pond to acceptable health would mean not only cutting by about 30 per cent of the nitrogen coming from wastewater, but also more frequent openings of the pond to the sea, and regular excavating of matter from pond inlets.

And this radical action is required, the report says, just to meet present conditions.

The report notes that if all the remaining buildable land in the watershed area was built on, nitrogen loading would go up a potential 62 per cent.

A meeting is scheduled to discuss the report with its authors on Monday at 4 p.m. at the Dr. Daniel Fisher house in Edgartown.

Gazette reporter Sam Bungey contributed to this story.