Fifty-six years ago almost to the day, a strange and horrifying event in a seaside town in California inspired Alfred Hitchcock’s thriller “The Birds.” Scores of sooty shearwaters flew into the sides of buildings and at residents who ventured outside. The seabirds vomited anchovies and died en masse in the streets.
What caused those birds to go crazy in Capitola? Probably toxic algae from what’s commonly called a “red tide.”
A red tide is not necessarily red, and it has nothing to do with tides. Rather, it’s an overgrowth of algae, aka an algal bloom, and in the case of the shearwaters, a harmful algal bloom (HAB).
Algal blooms occur all over the world. In some places, they do turn the water blood red, which certain religious sects see as a sign of the end times. But there is an explanation grounded in science. An overgrowth of algae can occur for a variety of reasons, including agricultural runoff, leaks of human sewage and weather patterns. When the concentration of algae becomes high enough, the water color changes to red, pink, brown or green.
The algae involved go by tongue-twisting names: Pseudo-nitzschia. Akashiwo sanguineum. Cochlodinium polykrikoides. Karenia brevis. All are types of plankton. These single-celled, plant-like organisms are an indirect food source for small fish, mollusks and crustaceans, which are in turn eaten by larger fish, birds, marine mammals and humans.
Unfortunately, some algae produce toxins that adversely affect consumers higher up the food chain.
Pseudo-nitzschia creates domoic acid, which causes a neurological disorder featuring disorientation, vomiting, memory loss, seizures and even death. The shearwaters that attacked Capitola were poisoned after eating fish contaminated by an HAB likely caused by leaking septic tanks. This summer, the Marine Mammal Center in California is dealing with a wave of seal and sea lion strandings from domoic acid poisoning. In 1987, three people in Canada died after eating contaminated mussels.
On the other side of the country, especially in Florida, Karenia brevis is more prevalent. It produces toxins that have adverse respiratory effects (coughing, wheezing, runny nose) when particles aerosolized by surf and winds are inhaled, and unpleasant digestive effects (diarrhea, nausea) and neurological impacts (numbness, partial paralysis) when contaminated shellfish are consumed.
Manabu Tokunaga is a frequent surfer in Half Moon Bay, California, who says he’s allergic to red tide. It subjects him to long bouts of sneezing and coughing, “like living in a chamber full of pollen.” Tokunaga doesn’t dare surf when an algal bloom arrives. “For me, if I go in the water, it would be a week of misery,” he says
If you don’t eat potentially contaminated seafood, or suffer allergic symptoms in the presence of an HAB, is it safe to surf or swim in a red tide?
In general, there is no need to fear immersion in the oddly colored water. Niel Dilworth, an ocean swimmer on California’s central coast, runs the Blue Water Task Force program for the San Luis Obispo chapter of Surfrider. “We test for enterococcus bacteria, which is an indicator of the presence of human waste,” he says. “The presence of enterococcus is not related to the algae blooms that cause red tides.”
Clarissa Anderson, executive director of the Southern California Coastal Ocean Observing System, is more cautious. “A red tide will eventually decay and that will be due to bacteria,” she says, “so even if there isn’t a storm, it’s hard to say that there’s zero risk to someone swimming in a patch of red tide.”
Anderson notes that “there’s some evidence of respiratory distress caused by a dinoflagellate called Akashiwo sanguineum that blooms occasionally in California.” But no other adverse effects on human health from water contact are documented in relation to the common bloom formers, she says.
Unfortunately for surfers like Tokunaga and affected marine wildlife, there is evidence that red tides are getting worse. The likely cause is climate change — specifically, rising ocean temperatures. Anderson says a “blob” of warmer water appeared in the Pacific in 2013 and has persisted, causing longer and more toxic algal blooms on the west coast of North America.
On the positive side, red tides can display a lovely, sparkly bioluminescence.
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