This week Island swimmers were greeted by the three worst words a vacationer can read: CLOSED NO SWIMMING. While sharks have garnered the regional headlines this has been a summer increasingly interrupted for beachgoers by much smaller animals: bacteria.

A number of beaches were closed in Oak Bluffs and West Tisbury on Wednesday due to bacterial contamination.

Inkwell Beach, Pay Beach, Eastville Beach and Madeiras Cove in Oak Bluffs were all closed on Wednesday. Shirley Fauteux of the Oak Bluffs board of health said elevated bacterial counts were found in water samples taken on Monday.

By Thursday morning though, Dave Caron of the Oak Bluffs board of health confirmed that the beaches were once again safe for swimming. He said that at this point it is unclear what caused the high bacteria reading, but results from samples taken Wednesday came back negative.

“We all want the President to swim at Inkwell,” Mr. Caron said.

Meanwhile in West Tisbury, beach closings have been similarly intermittent. On Wednesday Sepiessa Point on the Tisbury Great Pond was closed to swimming, as was all of the Long Point Wildlife Refuge including Long Cove, its Tisbury Great Pond access and even the Atlantic Ocean, due to erratic bacteria readings. Everything has since reopened at Long Point save the Tisbury Great Pond beachfront, where Long Point superintendent Chris Egan says readings are still high, as they have been on and off for weeks.

“We pride ourselves on our beaches and keeping them clean but we are concerned,” he said. “The results have been bouncing all over the place this summer, and we’re not exactty sure why that is.”

West Tisbury health agent John Powers who has been working with The Trustees of Reservations this summer to monitor the situation, said he too is perplexed by the erratic sample readings.

“Over last eight years when we’ve tested all these places they normally don’t come back high,” Mr. Powers said, noting that earlier in the season there were unusually high bacteria numbers registered at Makonikey Head and Seven Gates Farm. “I’ve been asking people much more knowledgable than me, ‘Are tides different? Does it have to do with the opening of Great Pond? Is there an influence from somewhere else? What could it be?’ ”

A reading of 819 parts per milliliter of enterococci was recorded in the Atlantic Ocean at Long Point on Tuesday. The upper boundary for acceptable enterococci is 104 ppm, and in order to reopen the beach an average reading over several days must register at under the 104 threshold to avoid what is called a geo-mean failure. But Tuesday’s unusually high results were registered the day after heavy rainfall. As a result they were thrown out, as rainfall can introduce nutrients and runoff that can artificially spike samples, and lower readings this week allowed beaches to be reopened.

Before the sample readings were disqualified, though, The Trustees of Reservations regional director Chris Kennedy registered his frustration with the situation.

“This is traditionally the busiest week of the year for us,” he said on Thursday morning. “The president is coming. The situation could not be worse.”

The Long Point staff had been informing incoming beachgoers about the test results, but Mr. Kennedy said it was ultimately up to them to decide whether they should swim.

He said many shrugged off the unusual results and went swimming in the ocean anyway.

At Lucy Vincent Beach in Chilmark the ocean is open for swimming, although the pond has been closed for weeks due to high coliform bacteria readings.

“We’re all scratching heads around the Island,” said Chilmark health inspector Marina Lent. “The only thing that jumps out at us as a possible explanation is that the summer has been so hot, but other than that we have no idea.”

Meanwhile, on Thursday morning before the beaches were officially reopened in Oak Bluffs, beachgoers at Inkwell generally took the closings in stride.

“We won’t go in the water until there’s more information,” said Cathy LaGuardia. “I hope it gets better before the end of our vacation, though.” A pair of hopeful kadima players shrugged at the warning sign and got back in their car.

On Pay Beach, Maria Keogh of Manchester, N.H., was unaware of the swimming ban and took objection to the lack of signage. “It’s just lousy communication. There’s one little sign over there,” she said. “Nobody knows they can’t swim here today.”