At around 8:15 on Tuesday morning, Steve Handy revved up his backhoe and began digging a five-foot channel through the barrier beach that separates Edgartown Great Pond from the ocean. By the afternoon, the channel was open and water began rushing out through the incoming surf.

Throughout the day, Mr. Handy continued widening the channel. Sand occasionally slumped down from tall sand piles into the rushing water to be carried out to sea.

“Once you create the small channel with the backhoe, then you get the hell out of the way and let Mother Nature remove the rest,” said Edgartown shellfish constable Paul Bagnall, who orchestrates the pond openings throughout the year. “I almost didn’t call this one,” he added, but a southeast wind on Tuesday was a good sign that the cut would stay open.

Water rushes in after opening the cut from Edgartown Great Pond to the sea. — Alex Elvin

Three or four openings each year help to maintain salinity for shellfish and eelgrass in the pond. They also flush out the nitrogen that accumulates mostly from septic tanks and runoff in the 4,500-acre watershed. Across the Island, increased circulation is considered one remedy to the problem of nitrogen loading, which has led to algae blooms and impaired most coastal ponds in the region.

With careful management by the Great Pond Foundation, local boards and other groups, the pond has rebounded since the 1990s, setting the pace for other Island towns facing similar problems.

“This year was a significant benchmark,” Great Pond Foundation president Tom Wallace said this week. He cited the best water quality in decades and the biggest shellfish population in 10 years, thanks in large part to the regular openings and a long-term oyster restoration project by the Martha’s Vineyard Shellfish Group.

By Wednesday the sand piles along the channel had disappeared and the breach had become another feature of the remote south-shore landscape. Eventually, sand carried east on ocean currents will rebuild the barrier beach, but Mr. Bagnall was optimistic the opening would last at least two weeks.

“The opening of the ponds used to be like a barn-raising event,” said Elizabeth Ricketts of the Great Pond Foundation. “People would come out with their shovels and everybody does it.”

Now it’s a much quieter and more calculated event. Only two or three people witnessed the first exchange of water on Tuesday.

The town and the Great Pond Foundation have worked to decrease the amount of nitrogen in the pond. — Alex Elvin

“It’s literally a game of inches out there,” said Mr. Bagnall, explaining that the surface of the pond must be high enough for the force of water to overcome waves and tides. That requires a minimum of 2.5 feet above sea level. “Three is better,” Mr. Bagnall said.

Tisbury Great Pond and Chilmark Pond, also on the south shore, benefit from manual openings throughout the year too. But those events are determined by property owners, rather than shellfish departments. Jo-Ann Taylor, coastal planner for the Martha’s Vineyard Commission, said those openings were likely aimed more at avoiding residential flooding than protecting the natural resource.

Mr. Bagnall aims for four openings throughout the year, but that isn’t always possible.

Drought-like weather this summer meant less groundwater entering the pond, so the water level never got high enough to maintain a breach.

“It wouldn’t be flowing like this,” Ms. Ricketts said Tuesday, looking out over the new channel. “It would just close back up when high tide came. You can see it’s running pretty good.”

In the 1990s, the foundation purchased a small dredge and began regularly clearing the tidal delta in the pond to enhance the periodic breaches. Last year, the town and foundation teamed up to remove sand in the delta that had accumulated during Hurricane Sandy in 2012.

A thick ice cover last winter prevented the annual dredging of the pond, but also helped preserve the work from two years ago, allowing for a strong 23-day opening in the spring. Dredging is only allowed in the winter to protect piping plovers, who nest on the beach.

On Tuesday evening, sanderlings skittered along the shore near the opening, feeding on insects in the sand. A lone fisherman reeled in a sizable striped bass, just east of the opening, where large schools of herring had been leaving the pond during the day.

Mr. Bagnall reported a group of seven seals playing around near the opening on Wednesday. One floated through the channel on the incoming tide, which he saw as a good sign. “That means the pond is getting a good return. It’s not always that quick. Sometimes it drains down for another day or so.”

Nature ultimately decides how long the breach will stay open. — Alex Elvin

Prior to the opening, salinity in the pond was about 17 parts per thousand, or about half as salty as the ocean. In a week, that will jump to about 25 parts per thousand, suitable for oysters and the eelgrass they attach to as juveniles.

Mr. Bagnall said if he had his way, he would make the pond salty enough even for bay scallops, but he was aware of only one other time — after a hurricane in the 1930s — when those conditions occurred and bay scallops appeared in the pond.

The Woods Hole Oceanographic Institution estimates that in 12 days, about 70 per cent of the pond water will have been replaced through the opening. "Perhaps a billion gallons of water would eventually leave the pond," Mr. Bagnall said.  

The town and the foundation have worked to decrease the amount of nitrogen flowing into the pond — mostly from septic tanks and runoff — but nothing compares to the benefits of rising and falling tides. Mr. Wallace said the openings were “probably the most significant item that helps maintain the quality in the pond.”

Edgartown Great Pond was the first on the Island to be studied by the Massachusetts Estuaries Project, which is overseen by the Department of Environmental Protection and established a daily limit for the amount nitrogen that should be allowed into the pond. Thanks to the regular openings, along with a plan in 2006 to sewer 300 homes in the watershed, the pond is among the healthiest on the Island. “We are pretty much hitting our target numbers,” Mr. Bagnall said. But that wasn’t always the case.

“We probably don’t want to go back to 1993,” he said, recalling an extremely hot summer when nitrogen led to an explosion of algae in the pond. After a hurricane in September, the water cooled and the algae died overnight, consuming oxygen and choking out other pond organisms.

“It killed almost all the shellfish,” Mr. Bagnall said. “There was a big outcry.”

That same year, the town voted unanimously to approve a major upgrade to the Edgartown wastewater facility to meet state and federal requirements. Even though nitrogen in groundwater would continue seeping into the pond for decades, Mr. Bagnall said, “Everybody’s feeling was, we’ve got to save Great Pond. It’s in serious trouble.”

“It took 12 to 15 years,” he added. “But we are seeing that benefit today.”

Mr. Wallace said the efforts to protect the pond were unique. He noted the “landmark decisions” such as the new fertilizer regulations that resulted from the MEP, along with septic system upgrades in the watershed. He also pointed to the foundation’s eight-year relationship with the shellfish group and the dramatic rebound in cultured oysters in the pond. The shellfish group seeded the pond with about 50 million baby oysters this year.

Every neighborhood around the pond now donates to the Great Pond Foundation, which has an annual budget of around $200,000. Those contributions have funded the purchase and maintenance of the dredge (known affectionately as Nessie), along with science and education programs, including monitoring efforts throughout the year.

Citing the Cape Cod Commission, Ms. Ricketts noted that waterfront property values on the Cape have declined along with the decline in water quality. But she did not believe money was the only motivating factor in saving the great pond.

“The main concern is the water quality,” she said. “The water’s warm, it’s clean, people can go fishing. You don’t have to worry about getting some kind of weird illness. And the shellfish are good.” She also noted the MEP study, which in some sense has given the town a head start in addressing the nitrogen issue.

Other ponds may be worse off, she said. “But they can get back. We got back.”

As of Wednesday, the breach had widened considerably. Mr. Bagnall had hoped that a steady southeast wind during the week would keep sand from rebuilding the beach, and he was optimistic the breach would hold.

“When it’s 200 feet across, it takes a lot of saltwater flowing back into the pond to close it,” he said.