Bacteria Ride the Tide: Moon's Phases Predict Water Quality at Beaches
Harder, B., Science News
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. …