The Nature Conservancy called it a globally rare community type.

The Massachusetts Natural Heritage and Endangered Species Program said it is one of the most critical habitats to protect in all of New England.

Many Island residents know it best as a secret late-night skinny-dipping spot.

Hidden in the woods off Lambert's Cove Road in West Tisbury, Old House Pond is a coastal plain kettle pond with a northern shore that boasts one of the most diverse and fragile rare species habitats in the region. It is also one of the Vineyard's most private and peaceful places to swim.

Hoping to share that experience with the public, the Martha's Vineyard Land Bank purchased 11 acres on the pond in 2003 and intended to open the property earlier this summer. But state environmental officials, citing concerns about potential impacts to water quality and protected species, denied the land bank's proposed management plan in June and required another year of scientific study before it decides whether it is appropriate to open the pond to the public.

On a recent visit to Old House Pond, also known as Ice House Pond, Judith Lane and Mark Mattson - the married couple that owned one of the two properties the land bank purchased on the pond - spoke with the Gazette about what they are still fighting to protect. Ms. Lane and Mr. Mattson no longer own property on the pond - they rent a home in Aquinnah during the summer - but as career freshwater scientists they aggressively lobbied the state this spring and were instrumental in its decision to require additional studies.

"Most people don't understand what it is we're trying to get out of this. Frankly, we want the land bank to do what's right," said Ms. Lane, who has been visiting Old House Pond since she was a child and said it first inspired her to become a freshwater scientist.

"We're both ecologists. We've dedicated our lives to protecting places like this," added Mr. Mattson, who works for the Massachusetts Department of Environmental Protection in the division of watershed management. "We just want to see it taken care of."

Wearing hats for sun protection, Ms. Lane and Mr. Mattson paddled an old green canoe around the pond late last month. The 11-acre freshwater pond is surrounded by native high bush blueberry, some scrub oak and a tall grove of pitch pines. Almost all of the homes on the pond have been set back from the pond, with the natural buffer preserved.

Even on a hot day in the peak of summer - when much of the Island is covered in crowds - Old House Pond remained relatively silent. A few young swimmers dove off a floating dock in the southern corner of the pond. The only other boat on the water belonged to Martha's Vineyard Commission water resource planner William Wilcox, who was doing some testing.

Mr. Wilcox began testing at Old House Pond last summer and will provide his data to the land bank for its ongoing studies. He said the quality of the pond appears to have dropped slightly from the previous summer, but that it is still much cleaner than other ponds he tests on the Island.

Kettle ponds are unique to areas of glacial outwash, where more than 10,000 years ago retreating glaciers left enormous chunks of ice buried in the ground that later melted, leaving behind large craters in the landscape. The craters that intersected with the water table filled with fresh water to form self-contained ponds.

Coastal plain kettle ponds are rare and found mostly on Long Island, Rhode Island and the Cape and Islands. The Vineyard has about a dozen of significant size - including Fresh Pond in Oak Bluffs and Black Pond in Aquinnah, among others.

The water quality of an untouched kettle pond is remarkably clear and extremely vulnerable. The pond is fed only by groundwater, rain and snow and loses water solely through seepage and evaporation.

"A lot of the pollutants get filtered through the soil and are kept out of the pond - which is what keeps these ponds much cleaner than those with streams coming in," Mr. Mattson explained. "But because there are no streams going out, any pollutants that get in - either from septics or swimmers - are going to stay for quite some time."

Mr. Mattson and Ms. Lane pointed to Uncle Seth's Pond nearby on Lambert's Cove Road as an example of a kettle pond that has been degraded from overuse and pollution. They expressed concern that Old House Pond, which has seen decline in its quality over the last few years, might be headed for a similar fate.

"Around 2000 or 1999 the pond was the most incredible blue-green," Ms. Lane said. "Since then milfoil started to appear, and the animal populations have started to change. Everybody on the pond starting noticing it and getting concerned."

State officials this summer requested additional baseline data about the pond's current quality, so future monitoring - if the pond is opened to the public - will indicate whether swimmers have a significant impact. Mrs. Lane, Mr. Mattson and Mr. Wilcox all said they would also like to pinpoint the cause of the recent decline in water quality before opening the pond to further risk of pollution, and agreed that the only way to do so would be through multiple years of study.

Ms. Lane said little information is available about kettle ponds because they are so rare. They are also particularly difficult to study because one of their primary characteristics is a dramatic fluctuation in size and depth from year to year and even from season to season, she said.

Ms. Lane said Old House Pond can be almost unrecognizable some years, when a four or five-foot shoreline is exposed all the way around its edge. Heavy snow and rainfall this winter and spring led to a relatively high water level earlier this summer, with little or no shoreline exposed.

"One more year of study isn't enough to really understand this pond," Mr. Mattson said. "Unfortunately, the land bank wants to do the studies right now, but a lot of the rare plants are under water."

Natural fluctuation in the water level is in fact largely responsible for the abundance of rare species in the shoreline communities.

The Natural Heritage and Endangered Species Program identified more than 40 rare plant species found in coastal plain pondshores in New England, including 10 that are endangered in Massachusetts. The habitat is also home to more than 45 species of dragonflies and damselflies. The state has asked the land bank to try and inventory dragonflies and damselflies at Old House Pond.

The plant species for the most part have adapted to find a niche in the natural fluctuations. Many can withstand being submerged for substantial parts of their lifespans, while typical upland and aquatic plants are killed off during particularly wet or dry spells.

But because many of the pondshore plant species are so specialized, they can exist only in these restricted habitats. And if the water quality of the pond is degraded - and flooded with nutrients, either from groundwater seepage or swimmers - other less fragile plants will crowd out the rare native species.

The pondshore plants and seedbeds are also extremely sensitive to trampling and other forms of human disturbance, which The Nature Conservancy described as the primary threat to the habitat.

Ms. Lane and Mr. Mattson are also concerned. The prevailing wind patterns on the pond blow seeds to the windward north end of the pond, where they owned the property that the land bank now holds. This means the most fragile pondshore habitat on Old House Pond is right where the land bank wants to allow swimmers.

Paddling around the pond last month, the northern shoreline was noticeably different. Tall plants grew out of the water, with blue and red dragonflies hovering above. A narrow, sandy, lifeless path was visible on the bottom where the natural habitat had been disturbed by trampling.

"Admittedly, we probably caused some of that ourselves," Ms. Lane said, pointing out the disturbance somewhat sheepishly. "But at least we used the same path."

The two differ slightly when it comes to their opinions about access. Island residents have traditionally enjoyed casual use of the pond, but the land bank management plan would for the first time create formal public access.

Mr. Mattson said he believes the proposed nearby swimming perch is a good compromise.

"I'm a little bit more in favor of public access as appropriate. I thought the swimming perch was an excellent idea," he said. "If they actually could keep people out of the pondshore area, it would be okay."

But Ms. Lane believes the property should remain a sanctuary with no access to the water, like the Sheriff's Meadow Foundation property at Cedar Tree Neck.

"I don't want any swimming in the pond," she said. "I don't think you can control it."

Deed restrictions grant Ms. Lane and Mr. Mattson access to the pond, but they both said they would waive their rights if the land bank agreed to disallow public use of the water.

"We would have given up that deeded access in two seconds if they were willing to make it a preserve," Ms. Lane said.

Both also said they would like to see the land bank make science a higher priority in the management of its properties. They said the land bank should subcontract experts in different fields to help author management plans for different properties.

Mr. Mattson and Ms. Lane said they are glad that the state secretary of environmental affairs required at least an additional year of study, and are hopeful that the revised management plan will serve to protect the pond.

"I don't want to sound like there's rare species here and people should be upset by that," Mr. Mattson said. "In fact, they should be thrilled the land bank purchased a rare ecosystem - that's part of the land bank's purpose. This could be a great thing for the Island if they do it right."