A large number of beaches in West Tisbury and Oak Bluffs were closed to swimming this week due to high levels of enterococcus bacteria found during routine water testing. Health agents in those two towns put out notices about the closures.

Then some, but not all, of the beaches were opened again.

At press time Thursday the following beaches remained closed: Lambert’s Cove and the ocean side of Long Point in West Tisbury; and Inkwell Beach, Pay Beach, Madeiros Cove (near the drawbridge on the Lagoon Pond) and Eastville Beach in Oak Bluffs.

Other beaches, including Seth’s Pond and Salt Works Beach at Seven Gates Farm, both in West Tisbury, had reopened after being closed.

There were no closures in Tisbury, Aquinnah or Edgartown.

By week’s end there was widespread public confusion and many questions but few answers about the mysterious high bacteria readings at Vineyard beaches, where the water is so clear on most days you can see your toes as you walk in the water.

“Enterococcus is an indicator bacteria; it doesn’t necessarily mean something bad but it indicates something is there, so the state mandates a closure. It could be a completely harmless bacteria,” explained Kendra Newick, the person who tests the water samples at the Wampanoag Tribe testing lab in Aquinnah.

Five of the six Vineyard towns send their water samples to the tribe lab for testing. Tisbury conducts its own tests at its sewage treatment facility.

The enterococcus bacteria can cause sore throat, cold-like symptoms, or gastrointestinal illness including diarrhea. Usually the elderly, people with compromised immune systems and young children are more at risk of illness from disease causing bacteria in the water.

“What is so frustrating is that it is happening right now in the middle of summer, on hot summer days. There were so many beaches closed for swimming around the Island. It is frustrating,” said Chris Kennedy, the Vineyard superintendent for The Trustees of Reservations.

Beaches at Long Point Reservation were closed to swimming on Tuesday. Yesterday the beach at Long Cove was reopened but the ocean beach remained closed.

Squibnocket Beach in Chilmark had one high reading but a second test came back clean, so there was no swimming closure, said health agent Marina Lent on Wednesday.

More testing will be done over the weekend at all beaches.

Testing will be done in Edgartown on Monday, health agent Matt Poole said.

The Gazette will continue to post information about beach closures and openings on its Web site (mvgazette.com) as details become available.