Heavy rains in recent days and weeks have washed polluted runoff water into streams, rivers, and bays, leading to high bacteria levels that on Wednesday forced New Jersey officials to close 15 beaches, including ones in Atlantic City and Ventnor.

No need to panic yet. The vast majority of ocean beaches along the coast are fine. The other closed beaches are either river or bay beaches, including one in Somers Point, according to the state's Department of Environmental Protection.

What is the cause? Probably animal waste, said Larry Hajna, a DEP spokesman. Geese, gulls, and other popular shore birds are eating, resting, and leaving their messes. It's hard to predict where the flocks will land or the amount of rainfall that could raise bacterial levels.

For those looking for a more scientific explanation, the DEP's website states: "The New Jersey State Sanitary Code requires that the concentration of bacteria not exceed 104 colonies of Enterococci bacteria per 100 milliliters of sample. Enterococci is a type of bacteria that is an indicator of possible contamination within bathing waters."

Vacationers expressed concern about Wednesday's closures that prohibit swimming, but allow people to use the beach.

"It's a bummer," said Susan Tavella, whose granddaughters, Bianca, 10, and Giovanna, 5, could not use their new boogie boards near Dorset Avenue in Ventnor. But after learning why the beach closed, she said, "We don't want to  swim in that."

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Nearby, Michelle Asatone was making the best of the situation while her sons, Tom, 7, and Nicky, 9, were engrossed in their books (Diary of a Wimpy Kid and The Enemy Above).

The day was cloudy anyway, she said. "We're making good use of the time. It's still cool today, but kind of gross."

Ventnor Mayor Beth Holtzman said elevated levels were found in beaches from Jackson to Wissahickon Avenues, but she made the decision to close all the beaches Wednesday to "err on the side of safety."

"Hopefully the levels will go back to normal," she said, "the sooner, the better."

There was no word on when the beaches would reopen, but testing is done daily. Hajna was hopeful that the waters would clear up soon, possibly Thursday or Friday.

In some areas, the levels spiked so quickly that officials issued immediate closures.

Click here for the latest beach-closure information.

"River beaches went straight to closure, which is unusual," Hajna said. Typically, the beaches would have advisory warnings first, he said.

On Wednesday, only four beaches had advisories, down from 31 the day before.

"We rarely see an ocean beach close," Hajna said. "If we get more rain, it could be a problem."

Mother Nature may not cooperate as the National Weather Service is predicting heavy rain Thursday and Friday in Atlantic City and nearby locations.

The beaches closed in Atlantic City are Bartram, Georgia, Annapolis Avenue, Missouri, North Carolina, Pennsylvania, and Montgomery Avenue.

All beaches were closed in Ventnor after 1 p.m. Wednesday, after Austin Avenue, Dorset, New Haven and Washington were ordered closed initially. Washington was  closed as a precaution due to the the closures of surrounding beaches.

The Atlantic City and Ventnor beaches had been under advisories on Tuesday, but were closed after testing Wednesday.

Staff writer Amy S. Rosenberg contributed to this article.