Wastewater discharge from urbanization to the north and east of Edgartown Great Pond has been identified as the primary source of increased nitrogen loading in the pond, according to a new study by a scientist with the Marine Biological Laboratory in Woods Hole.

“Human development of the land is causing more and more nitrogen being put into . . . our coastal ponds and estuaries,” researcher Javier Lloret told a webinar audience last Thursday in the kickoff of a series sponsored by the Great Pond Foundation. His pilot study used non-radioactive atoms called stable isotopes to trace both the sources and the effects of excess nitrogen in the pond.

Stable isotope carbon-13 indicates the proportion of marine to land influences in a water sample, while nitrogen-15 reveals whether the nitrogen in a sample comes from septic discharge, fertilizer or atmospheric pollution, Mr. Lloret said.

Used together, the isotopes can help pinpoint the type of nitrogen entering a waterway, as well as where it is coming from, he said.

In the Edgartown Great Pond, a state study found that wastewater is responsible for about two-thirds of the nitrogen load, with atmospheric sources accounting for less than 25 per cent and the remainder coming from fertilizers, Mr. Lloret said.

His own sampling found several species of macroalgae — seaweeds that flourish in nitrogen-polluted water — that are growing in the pond, indicating an ecosystem that is degrading.

“There were certain locations where they were pretty abundant,” Mr. Lloret said. “That’s something to keep an eye on.”

The isotope study found wastewater-fed algae hotspots in the vicinity of multiple housing developments in the pond’s watershed, including Jane’s Cove, Wintucket, Mashacket, Turkeyland and Slough coves.

“The wastewater inputs are mostly occurring with the northern and eastern coves,” Mr. Lloret said. “The western side seems to be mostly clean . . . This matches well with what we know about housing and the nitrogen load.”

With nitrogen unchecked, weedy algae blooms encourage the rapid growth of microscopic plants called phytoplankton, leading to eutrophication — the choking-out of pond life, including fish, Mr. Lloret said.

Still, the pond remains healthy, despite nitrogen levels comparable to more seriously-affected waterways he has studied on Cape Cod, he said.

“Considering the amount of wastewater . . . that the pond still presents a very, very wide cover of eelgrass on the bottom, that was a little surprising,” he said.

“The pond is at a relatively high — I would say close to high — ecological status.”

Mr. Lloret’s study also found that opening the pond to the ocean serves to flush much of the human-caused nitrogen away, as shown by both isotopes. Following the August 7 opening, he said, tests revealed an immediate increase in the marine influence and a sharp drop in wastewater nitrogen.

“It was very, very interesting to see the isotopes immediately recognized that opening,” he said.

Following the presentation, Mr. Lloret joined Island water quality experts and the Martha’s Vineyard Commission cartographer in a discussion of the effects of excess nitrogen and changing land use on Vineyard ponds and estuaries.

Moderated by Edgartown health agent Matt Poole and West Tisbury health agent Omar Johnson, panel members backed Mr. Lloret’s recent findings with their own observations over time.

“As the coastal population goes up on the Island, the shellfish population goes down,” said longtime Edgartown shellfish constable Paul Bagnall, describing the loss of habitat in formerly bay scallop-rich waterways.

“We no longer have much eelgrass left in Katama Bay or Sengekontacket Pond,” Mr. Bagnall said. “We rely on Cape Pogue for probably 80 per cent of the [scallop] harvest now.”

Lagoon Pond is also suffering, said Melinda Loberg, a board member of Tisbury Waterways Inc. and former Tisbury select board member.

“Last year, on the opening day of scallop season . . . I was out there, and through my peep box all I saw was the surface of the moon. It was so devoid of any kind of marine life,” Ms. Loberg said. “I think that is the biggest concern that nitrogen has produced in all of us.”

Improvements, including enhanced sewer and septic systems to denitrify wastewater, need to begin immediately if the ponds are to be preserved for future generations, said Michael Loberg, a member of the Tisbury board of health.

“This problem occurred on our watch,” he said. “This needs urgent attention . . . Time is not our friend here.”

But even with advancements in treatment technology, reducing wastewater pollution may require limiting residential development, potentially through zoning changes, Mr. Loberg said.

“Nitrogen comes from food. We eat it. We excrete it,” he said. “If our population isn’t controlled a bit . . . we may create systems that we really can’t control.”

According to U.S. census figures, the Vineyard year-round population grew by 240 per cent between 1970 and 2020, MVC cartographer Chris Seidel said, displaying an animated series of Island maps showing increased building density over time.

According to the maps, development in the pond areas began to accelerate in the second half of the 20th century and is projected to continue through the 21st century, based on current zoning laws.

“Where things are positioned on the land certainly has a key impact on the water quality,” Ms. Seidel said.

Thursday’s presentation and panel was the first of three online workshops on Island ponds sponsored by the nonprofit foundation, which is actively involved in a variety of initiatives, including a project launched last summer to track harmful algal blooms in the ponds.

The series continues Dec. 9 with Pond Systems Out of Whack and concludes Jan. 13 with Data in a Changing Climate, both at 1 p.m.

To attend, email [email protected] for a link.