Red Tide Rising: Harmful Phytoplankton Blooms

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Thousands of red abalone washed up on the Sonoma coast after a phytoplankton bloom turned the waters red—and toxic. Photo: loarie.

About a month ago, thousands of abalone and other invertebrates washed up along the Sonoma coast, killed by what people thought was probably a red tide, a.k.a. a harmful algal bloom. Phytoplankton—photosynthetic organisms like algae and bacteria—can multiply in number, turning the water red with their bright-colored cells and wreaking havoc on marine organisms. An interdisciplinary team of researchers banded together to find out what was going on along the Sonoma coast. Within a few weeks, they’d figured it out: sure enough, it was a red tide.

This was the first time a red tide had widespread impact on wildlife along the coast of California. From Bodega Bay to Salt Point, 50 miles north, invertebrates like abalone, urchins, and gumboot chitons were affected by the red tide. I talked with Laura Rogers-Bennett, Senior Biologist Specialist with the California Department of Fish and Game and the UC Davis Wildlife Health Center, who did surveys to quantify the extent of the damage. She and her colleagues surveyed several sites and found a lot of dead abalone. At Fort Ross, 30% of the abalone had died. At Salt Point, abalone mortality was 12%, and at Timber Cove, mortality was 25%. They took tissue samples from abalone and other invertebrates, and they conducted underwater surveys to look at the geographic distribution of affected organisms.

Red tides can kill marine life in two ways. The first way is by depleting the oxygen in the water, during the algal bloom and during the subsequent die-off and decomposition of the phytoplankton. With low levels of oxygen in the water, marine life suffocates. Red tide events that kill through oxygen depletion have particular characteristics. They are usually small and localized—the size of a large living room, says Rogers-Bennett. They often occur in areas with very little water movement, like the back of coves, and they affect all the organisms in the area, including the fish, which either die or swim away. Recent underwater surveys indicated that this red tide was not killing via oxygen depletion. For one, the area affected was far larger than a living room. And the red tide affected organisms in multiple patches, some of which were in very exposed areas, not just the still backwaters of coves. Plus, the fish did not seem to be affected. Because of this evidence, scientists suspected the phytoplankton bloom was releasing a biotoxin.

To identify the biotoxin, the researchers sent water and tissue samples to Rita Horner at the University of Washington and David Crane at Fish and Game’s Office of Spill Prevention and Response. They determined that the most abundant phytoplankton species was a dinoflagellate called Gonyaulax spinifera, which releases a toxin called a Yessotoxin. However, the dinoflagellate and the toxin are poorly understood, says Rogers-Bennett. Yessotoxin was present in very low amounts—about one part per billion—and we don’t know how much toxin must be present for an organism to suffer ill effects. So we don’t know for sure whether Gonyaulax spinifera and its Yessotoxin are responsible for the death of the invertebrates, or whether the true guilty party is some other, unidentified toxin. Rogers-Bennett and her colleagues are seeking funding to do additional tests of the Yessotoxin, its toxicity, and its effects on the invertebrates.


Studying this red tide is a team effort, involving researchers from UC Davis, UC Santa Cruz, Sonoma State, the California Department of Public Health, and NOAA. Researchers have done bird surveys, which indicate that the toxin is not moving throughout the food web. And they’ve tested for toxins that affect humans, but thankfully none present. Next, researchers plan to look at archived satellite images of the red tide to see how it moved throughout the area, and they want to do more extensive subtidal surveys, to learn more about the geographic pattern of its effects.

Because of the recent mass mortality of abalone, the Fish and Game closed the Sonoma abalone fishery in September. Normally, recreational abalone collectors can take abalone from the coast through November, but because the population took a big hit, it needs some time to recover. Rogers-Bennett hopes that the fishery and its managers will keep this in mind as the next abalone season approaches in April. The abalone fishery is not a commercial fishery; it is illegal to buy and sell abalone.

There is now a new red tide off the Sonoma coast. This one, called Ceratium, is not toxic, but it is lending the water a brownish red hue. Recent conditions have been perfect for a red tide: calm water and abundant sunshine, thanks to fog-free days. The conditions preceding last month’s deadly red tide were similar. Globally, red tides are on the rise, because of warmer sea surface temperatures and an increase nutrient in coastal waters because of human activity. However, neither of these red tides is tied to warm surface waters or higher-than-usual nutrient input. The causes of these recent red tides remain a mystery.