The town of Oak Bluffs closed two Lagoon Pond beaches Wednesday morning to swimming. Samples of saltwater taken on Monday tested positive for high counts of a bacteria known as enterococci, a bacteria that can indicate the presence of fecal contamination.

The town closed Medeiros Cove off the Sailing Camp Park and the drawbridge beach at Eastville. A harborside Eastville beach remains open.

Swimming in water with high levels of enterococci can pose a safety risk to those with sensitive immune systems, such as the very young and the elderly.

A sample of water from Medeiros Cove showed bacteria at 1,050 colony forming units per 100 milimeters, which surpasses the state acceptable standard of 104 CFU/100 ml for salt water. At Eastville town beach on the drawbridge side, bacteria was found in the amount of 331 CFU per 100 milliliters of salt water.

Bacterial contamination of bodies of water is often associated with high rainfall. Though the Island experienced heavy rainfall this past weekend, the town board of health would not confirm this explanation.

Follow-up samples will be tested Thursday. The beaches will be reopened Friday night at the earliest, said Shirley Fauteux, health agent for the town.

Shellfishing is still in operation at the Lagoon Pond hatchery, said Rick Karney, director of the Martha’s Vineyard Shellfish Group. Mr. Karney is also a member of the Lagoon Pond Association, and says pet owners who walk near the shore of the pond should be encouraged to pick up after their dogs, as their excrement can leak into the water and contribute to water contamination. The association has provided disposal sites along the shore for this purpose.

The pond at Lucy Vincent Beach in Chilmark has remained closed for several weeks after repeated tests of that water showed high levels of enterococci. The pond is shallow, stagnant and subject to contamination from goose excrement, said Marina Lent, town health inspector.