Kamikaze gulls that inspired Hitchcock's The Birds may have been doomed by leaky septic tanks.
Urea pollution can trigger ocean algae to produce a deadly toxin called domoic acid, scientists have discovered.
The research may help explain several mass animal deaths, including a historic bird stranding event thought to have inspired Alfred Hitchcock's horror film The Birds.
Raphael Kudela, an ocean scientist at the University of California, Santa Cruz, and his team made the discovery after studying a form of sea algae called Pseudo-nitzschia australis. Although the algae's blooms are normally benign, they have long been known to sometimes begin making domoic acid.
Much like the kamikaze gulls portrayed in the 1963 horror film, animals poisoned by domoic acid have erratic behaviour patterns. On 18 August, 1961, residents in the town of Capitola, California, awoke to find sooty shearwaters slamming into their rooftops, and their streets covered with dead birds. According to the local paper, Alfred Hitchcock — who lived a few miles away — requested news copy to use as "research material for his latest thriller", which was based on Daphne du Maurier's 1952 short story, The Birds.
Although researchers can only speculate that domoic acid caused this historic event, modern toxicologists have conclusively linked the toxin to more recent cases. In 1987 contaminated shellfish poisoned 100 people on Prince Edward Island in Canada, killing three and causing many cases of amnesia. In 1998, 400 disoriented sea lions died along California's central coast — domoic acid was traced back to contaminated fish that swam through a toxic bloom before being eaten by the sea lions. "Every few years there is a big outbreak that causes otters, pelicans or sea-lion deaths," says Kudela.
"The acid binds very tightly to surface receptors on excitatory neurons, which makes it hard for the cells to switch off," says Melissa Miller, a veterinarian at the California Department of Fish and Game in Santa Cruz. While the toxin doesn't make animals homicidal, the brain damage it causes accounts for the strange behavior patterns that precede death, says Miller.
Human pollution was thought to play a role, but researchers were never able to identify which contaminant causes P. australis to start producing domoic acid. "It's definitely a combination of factors, which makes it hard to show a cause and effect relationship," says Kudela.
So he and his colleagues tested a range of chemicals found in fertilizers, including nitrate, ammonium and urea, to determine their effects on the algae. Urea was the only chemical that increased domoic-acid production. In cases where the plankton mysteriously began making low levels of the toxin in clean water, adding urea nearly doubled production.
After taking water samples off the coast of California, they also found that urea concentrations in Monterey Bay and San Francisco Bay were high enough to account for some recent harmful algal bloom events.
Urea isn't commonly found in agricultural fertilizers, but it is present in many garden products. Sewage-treatment plants tested as part of Kudela's study did not discharge much urea, but leaky septic tanks have been known to dump urea into the Chesapeake Bay and the Gulf of Mexico. "Marine animals release small amounts of urea, but the pollution problem is almost entirely human caused," says Kudela. The findings will appear in the November issue of Harmful Algae.1,2
"This work directly links human activities to the hundreds of marine mammal mortalities resulting from exposure to domoic acid," says Frances Gulland, director of veterinary science at the Marine Mammal Center in Sausalito, California.
Although there is no way to know for sure if urea caused the famous incident that inspired The Birds, Kudela says the pollutant was probably leaching into the ocean at the time. "There was a lot of new development back then, with many unregulated septic tanks going in," he says.
Kudela now plans to look for other chemical factors that may cause the P. australis tides to turn toxic.
Cochlan, W. P., Herndon, J. & Kudela, R. M., Harmful Algae doi:10.1016/j.hal.2008.08.008 (2008).
Kudela, R. M., Lane, J. Q. & Cochlan, W. P., Harmful Algae doi:10.1016/j.hal.2008.08.019 (2008).
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Nature Geoscience (2012)