The entire Oak Bluffs side of Lagoon Pond was opened to shellfishing this week, in response to the forced closure of Sengekontacket Pond late last week due to high levels of bacteria.

Shellfish constable Dave Grunden said he had adjusted the management regime for Lagoon Pond to open early one area which had been closed until August, as well as another area which had been closed for reseeding.

Sengekontacket was closed last Friday after water quality testing showed fecal coliform bacteria concentrations well above the safe level for shellfish consumption at eight of nine testing stations on the pond. The contamination level at the ninth testing station was at the upper limit of safety.

The samples were taken on June 5 and the pond was closed immediately after the results came back. It is likely to remain closed for another couple of weeks at least while further tests are made and analyzed.

The maximum allowed level of fecal coliform is 28 bacterial colonies per 100 milliliters of water. The readings at eight stations were greater than 51, which is the highest concentration the testing system registers.

The ninth station registered 28.

The closure does not affect swimmers and boaters.

At a meeting of the Oak Bluffs selectmen this week, the testing regime came under some criticism from shellfishermen, who claimed the results could have been skewed by the fact that samplings were taken close to shore and several small islands where large numbers of cormorants and other birds congregate.

If the samples had been taken from a boat in deeper water, critics suggested, the concentrations would have been lower.

But Mr. Grunden said the sampling was done by the book under national shellfish guidelines. And he said while samples were not taken by boat, that did not necessarily change the results.

"The most likely cause of the high counts on June 5 is that on June 4 we had an abnormally high tide and it would have flooded the three small islands in the marshes. Then when it ebbed, it took all the fecal matter from where the birds have been nesting," he said.

Both Mr. Grunden and Edgartown shellfish constable Paul Bagnall remained cautiously hopeful that contaminants will have been flushed from Sengekontacket by the time it is retested.

Meanwhile there are shellfishing alternatives for residents of both towns.

"We just changed our management a bit so people would have places to go shellfishing," Mr. Grunden said.

"The biggest problem with Lagoon Pond is access, but the quahaugs are there. Two of my selectmen were out on Sunday and each got close to their limit of half a bushel," he added.

But even if the bacteria levels fall and Sengekontacket is promptly reopened, concerns remain about the long-term health of the vast saltwater pond that spans Edgartown and Oak Bluffs, divided from Nantucket Sound by the winding ribbon of Beach Road. On the Edgartown end Sengekontacket is historically known as Anthier's Pond. The health of the pond is expected to be a key topic for discussion this week when the Massachusetts Shellfish Officers association meets on the Island next Thursday.

Rick Karney, shellfish biologist and director of the Martha's Vineyard Shellfish Group, said the major concern centers on the loss of eelgrass in the pond - also believed to be related to pollution.

"From our standpoint, in terms of shellfish seeding, our biggest problem has been a loss of eelgrass," Mr. Karney said.

Eelgrass is vital habitat for shellfish, particularly bay scallops.

"A lot of that decline in eelgrass is due to nitrogen pollution," Mr. Karney said. "Nitrogen-rich water encourages algal blooms, which will either shade out the light that's needed by the eelgrass, and or cause the growth of organisms on the eelgrass which kills it." He added:

"A lot of work has been done to try to seed the pond to bring the grass back, but they have not been very successful in getting it to take."

One of the presentations to this week's meeting of the shellfish officers will be by Alison Leschen of the Division of Marine Fisheries, who has developed a new eelgrass regeneration technique which promises better results.

"We're probably going to try her technique out this summer. It sounds quite simple and she's had great success with it in Boston harbor," said William Wilcox, water resource planner for the Martha's Vineyard Commission.

Mr. Wilcox noted that the same forces which caused the bacterial contamination of the pond also would have introduced more nitrogen into the water.

"You've had that manure piling up, and it's on a very shallow, low-angle tidal area, so when you get an extreme tide it can flood quite a substantial area, and release not only bacteria, but whatever nitrogen is in there," he said, adding:

"Birds are a source. They are not adding nitrogen from outside the watershed, but they are making it much more available in a readily usable form by phytoplankton and slime algae that impact the eelgrass."

The major source of nitrogen contamination, however, is human activity. Fertilizers and septic systems are the main contributors.