Bacteria Ride the Tide: Moon’s phases predict water quality at beaches

At many ocean beaches, full and new moons coincide with the greatest concentrations of bacteria in the water, researchers in California have determined. The new finding suggests that extreme tides, which occur fortnightly in synchrony with lunar phases, generate water conditions that could make swimmers sick.

To prevent waterborne microbes from causing diarrhea and other illnesses, authorities at U.S. beaches periodically test concentrations of bacteria such as enterococci and temporarily close sites where samples exceed regulatory limits. While most enterococci aren’t pathogenic, studies link their prevalence to the risk of infections from other waterborne microbes. Last year, closures affected 1,000 ocean and freshwater beaches out of the 3,400 that are monitored nationwide.

However, microbial concentrations fluctuate rapidly, and it takes a day or more to cultivate and count bacteria from a given sample. Often as a result, says Alexandria B. Boehm of Stanford University, a “pollution event is gone by the time the sign goes up” warning beachgoers to stay out of the water.

To assess whether information about tides could be useful in predicting water quality, Boehm and Stephen B. Weisberg of the Southern California Coastal Water Research Project in Westminster assembled data for 60 beach sites along 120 kilometers of the southern California coast. For each site, enterococci had been measured in samples taken daily or weekly for several years. The researchers recorded the phase of the moon, tidal conditions, and other characteristics associated with each sample.

At most sites, so-called spring tides—those associated with full and new moons—significantly elevated average enterococci concentrations and more than doubled the likelihood that a sample would exceed regulatory standards. The negative effect on water quality was greatest when a spring tide was going out, or ebbing, Boehm and Weisberg report in an upcoming Environmental Science & Technology.

It’s not surprising that the highest bacterial concentration “shows up at low tide during an exaggerated tidal cycle, when the sea water is at its lowest ebb,” says coastal oceanographer Willard S. Moore of the University of South Carolina in Columbia. During ebb tides, the open ocean receives subterranean waters, which can be rich in microbes and their nutrients, he says.

Richard L. Whitman, an ecologist at the U.S. Geological Survey in Porter, Ind., offers an alternative explanation. The wave action enhanced by the tide stirs up bacteria in the sand, he says.

“If you’re risk averse,” Boehm says, “avoid going to the beach during spring tides, and particularly during spring ebb tides.”

That’s good advice, says Whitman. But he cautions that nobody directly monitors disease-causing bacteria, so the link between tides and risk isn’t confirmed.

The Environmental Protection Agency already advises people not to swim at beaches after heavy rains, which dump bacteria into the water.

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