The Tisbury Great Pond has had its nitrogen health checkup and is in good shape, especially compared with other Island ponds.
This was the conclusion of the final Massachusetts Estuaries Study report for the pond, presented this week to Chilmark and West Tisbury selectmen, town planners and residents.
“The pond is just over its [nitrogen] limit . . . but it’s not bad,” said Brian Howes, a marine scientist and technical director of the estuaries project, speaking at a meeting on the Vineyard Tuesday night. “It’s just over the limit by a little bit so you only have to bring it back by a little bit. You have a lot to work with.”
“This is a good system for Massachusetts,” he added. “But it is a little impaired.”
The Tisbury Great Pond is the fourth Island pond to be studied by the University of Massachusetts at Dartmouth in an ongoing examination of coastal ponds and embayments throughout southeastern Massachusetts. Completed studies for Lagoon Pond in Vineyard Haven, Edgartown Great Pond and Sengekontacket in Oak Bluffs and Edgartown found high levels of nitrogen, much of it coming from residential septic systems. A study was also done for Farm Pond in Oak Bluffs. Chilmark Pond is next on the list for study.
The Tisbury Great Pond embayment system includes 743 acres on Tisbury Great Pond and 64 acres on Black Point Pond, which is connected to the larger pond through a tributary. Black Point Pond has a better nitrogen tolerance because of a saltmarsh wetland around the pond, Mr. Howes said. No management plan is recommended at this time for Black Point Pond.
The entire pond system has a total maximum daily load of nitrogen of 62 kilograms per day, which needs to be reduced by 19 per cent, the study said.
To lower nitrogen levels, the study recommends continued opening of the pond on a regular basis, with at least one opening in high summer when water quality is poorest.
Nitrogen levels are higher in the finger coves of the pond that see less flushing, the report found. In Town Cove, for example, nearly 90 per cent of the animal population is limited to one species, Mr. Howes said.
“It’s a very productive pond in terms of cranking out bodies, but it’s almost one or two species,” he said.
The study found that there are 20 species within the main basin of the pond; the target number of species is around 25.
“This system is really in pretty good shape and has the ability to support eelgrass with openings,” Mr. Howes said.
According to the report, the primary habitat issue in Tisbury Great Pond is a general loss of eelgrass beds that support shellfish. The study estimates around 50 acres of eelgrass has been lost due to nitrogen impairment. Too much nitrogen spurs the growth of algae which blocks sunlight from the pond bottom, inhibiting eelgrass growth.
Eelgrass is a key species targeted for renewal in this region, Mr. Howes said.
In Black Point Pond, animal diversity is good, Mr. Howes said, and its salt marsh helps keep its ecosystem in balance.
“Black Point Pond is below its limit for nitrogen, and can take more without damaging it,” he said.
The report found the main source of nitrogen in the Tisbury Great Pond system is agriculture (44 per cent), followed by septic systems (40 per cent). “We were very surprised at the number of animals that live in this watershed,” Mr. Howes said.
But an analysis of future buildout in the pond watershed also came back with good news. The potential future buildout under current zoning laws would result in an increased nitrogen load of 13 per cent. For the sake of comparison, in the Lagoon Pond the nitrogen load could increase 50 per cent with future buildout around the pond.
“You’re pretty well set,” Mr. Howes said about the Tisbury Great Pond.
Rick Karney, director of the Martha’s Vineyard Shellfish Group, said the buildout number was the most promising aspect of the report.
“That’s a really nice thing to know,” he said Thursday, following the presentation. “The other ponds don’t have as much preserved land around them. You want to solve the problem that exists, but look at the larger problem coming down the pike . . . when you see something like that, it’s reassuring.” Bill Wilcox, former longtime water resource planner for the Martha’s Vineyard , echoed Mr. Karney’s remarks. He said large swaths of conservation land and forestland have played a key role in protecting the Tisbury Great Pond watershed.
“There’s a great savings in nitrogen loading that occurs by having conservation land in the watershed; it’s a big plus,” Mr. Wilcox said. “There’s really no people-related nitrogen. It just falls on the ground [in the form of rainfall] and gets processed through the soil and the vegetation, and that takes it from those areas in the system.”
Meanwhile, Mr. Karney had just come off the Tisbury Great Pond where he had surveyed a healthy new set of young oysters. The longtime shellfish biologist said he would like to see future studies of the pond include the contribution of shellfish to nitrogen reduction.
“We’ve had this incredible increase in the number of oysters in the pond and I believe that probably most of the sampling [for the study] was done prior to that,” he said.
Mr. Karney said he’s seen a dramatic increase in oysters over the past 40 years, including large beds where they’ve never existed before. The pond saw a burst of oyster growth in 2010.
“It’s a pretty dramatic situation right now,” he said. “To see this new setting on top of [the older set] seems like a really healthy situation, at least from the shellfish standpoint. I like to think they’re definitely having an impact and are going to help to keep the pond healthy.”