Farm Pond, the 42-acre great salt pond that hugs the edge of the Harthaven section of Oak Bluffs along Beach Road, is ailing and at risk due to too much nitrogen, a draft study for the ongoing Massachusetts Estuaries Project concludes.
The draft report, which is now circulating among town leaders and water quality planners, also finds that rehabilitation of the pond is easily within reach and could be largely accomplished by significantly widening the culvert that allows tidal exchange between the pond and Nantucket Sound.
In the 1970s blue crabs, softshell clams and oysters were all being harvested in Farm Pond, one of the Island’s unsung Great Ponds that has the public advantage of being in a highly visible and accessible location. This has also been its downfall, due to development around the shoreline in recent decades.
Excess nitrogen is now a well-known cause of degradation in pristine saltwater estuary habitats. In the same way that it stimulates growth in grassy lawns and garden plants, nitrogen stimulates the growth of algae in ponds which uses up available oxygen in the water column, a process called eutrophication which eventually causes naturally occurring eelgrass and the marine life it supports, including shellfish, to die.
The primary source of nitrogen in Farm Pond is septic system discharge from residential development, the draft study concludes.
The study also examined other sources of nitrogen, including fertilizers from lawns and the Farm Neck Golf Club, whose emerald greens and groomed fairways lie partly in the watershed for the pond, and the town sewage treatment system, which discharges effluent beneath nearby Ocean Park, also in the watershed for the pond.
All are contributing varying amounts of nitrogen to Farm Pond, the scientific study concludes, including the sewage treatment plant, which ironically was built to help solve the problem of too much nitrogen leaching into the groundwater from residential septic systems. But the benefits from sewering within the Farm Pond watershed far offset any return contribution of nitrogen to the watershed from treated wastewater, the report also finds.
A collaborative scientific, academic and government project, the estuaries project has been in progress for more than a decade and seeks to establish highly accurate profiles of the saltwater estuaries and embayments throughout Southeastern Massachusetts, using sophisticated marine science computer modeling methods. Marine scientists from the University of Massachusetts at Dartmouth have been at the forefront of the study, but the towns have played a key role in gathering baseline information through intensive data collection such as water sampling. On the Vineyard in general and in Oak Bluffs especially, the collaborative effort has been strong, involving town shellfish departments, the Martha’s Vineyard Commission and active citizen friends groups. The Friends of Farm Pond is one such group.
A draft study for the Edgartown Great Pond was completed two years ago. The draft report for Farm Pond and Sengekontacket are next up in the order; studies are also in progress at the Tisbury Great Pond, Chilmark Pond and James Pond.
The work has been slowed in recent years by governmental tangles and occasional squabbles between the academic marine science institutions and state fisheries agencies involved in the project.
But in the end reports profiling the health of the Island ponds and estuaries are expected to be valuable tools for planning and policy-making.
Using narrative and an array of technical data and illustrations, the 125-page Farm Pond draft report sketches a profile of a pond that has been shaped by both man and nature through the years.
The pond is 42 acres at high water with about eight acres of salt marsh. The name is derived from the old deBettencourt Farm which once occupied its shores. The pond has a single main basin with a small upper basin and a small, undeveloped island named Woody Island. It is a shallow pond — only about six feet deep at its deepest point. Like the entire Vineyard, the pond is a remnant of a retreating glacier, although its formation is relatively recent, perhaps a few thousand years old.
It is called a simple estuary or great salt pond, and exchanges its water with Nantucket Sound through a single culvert that runs beneath the road. Occasional storm surges also flood the pond with fresh saltwater. A second culvert near the old Harthaven inlet is no longer functioning. Unlike the Edgartown and Tisbury Great Ponds, Farm Pond is not mechanically opened to the sea several times a year due to the fact that its barrier beach is also a road.
And as the tidal exchange between the pond and the sound has slowed in recent years, eutrophication has set in at Farm Pond.
“The primary ecological threat to the Farm Pond embayment system, as is the case for virtually all other estuaries in Southeastern Massachusetts, is degradation resulting from nutrient enrichment,” the report says.
“Any management plan for Farm Pond must take into account the significant issue of sustaining tidal exchanges in addition to managing nitrogen loadings,” it concludes.
The draft report finds that increased development still poses a threat around the pond, where up to 37 more houses could be built under existing zoning rules.
But extensive pond surveys conducted for the report also show that eelgrass beds, while diminished, have not disappeared. “At present, eelgrass exists across a relatively large portion of the [pond],” the draft report says. Historical data compiled for the report shows that in 1997 there was just over 19 acres of eelgrass is Farm Pond; in 2006 the number dropped to 16 acres. The report documents a 12 per cent decline in eelgrass beds between 1995 and 2001.
“Restoration of the eelgrass . . . is the primary target for overall restoration of . . . Farm Pond,” the report says.
The draft study also concludes that reducing nitrogen in Farm Pond can be a relatively easy fix: widen the culvert, which is currently approximately three feet wide, to 16 feet, it suggests, and the pond will flush better and more frequently.
“Inlet improvements offer a very cost-effective alternative to sewering,” the report concludes.