Courtesy: Doug Bush

Bioluminescence, caused by algae, along the Sonoma coast before a red struck last month

It began with a glowing blue sea.

It ended with unprecedented carnage, when a thick bloom of blood-red algae swarmed down the Sonoma coast, decimating shoreline sea life and killing animals kept in research tanks that were connected to the open ocean at the Bodega Marine Laboratory.

Weeks later, scientists are still at a loss to explain why the mass of deadly plankton, known colloquially as a red tide, blossomed in the Pacific Ocean off Sonoma and Mendocino counties.

Nor are they sure how it killed crabs, urchins, starfish, mollusks and up to 70 percent of the coveted abalone in some spots. At Fort Ross, about 30 percent of abalone died.

Scientists warn that it could strike again.

In fact, waters along the Sonoma coast have again grown discolored this week, although it could take days for scientists to know whether the red tide has returned.

“We’ve never had an event like this,” said Gary Cherr, a University of California, Davis, professor who directs the Bodega Marine Laboratory. “When something like this happens, you suddenly realize that you’re vulnerable.”

The waters of Bodega Bay and the Sonoma coast are occasionally home to remarkable blue glows caused by bioluminescent algae. Researchers and locals noticed an especially bright glow before last month's red tide washed in. Cherr said next time it is witnessed, the glow should be considered a potential harbinger of pending devastation.

“It was one of these things that was nice to look at,” Cherr said. “Now we know better.”

Courtesy: Jerry Dodrill

A red tide that struck the Sonoma coast, pictured Aug. 25 near Fort Ross

In the wake of the devastation, the California Department of Fish and Game closed the Sonoma County abalone fishing season. It's unclear whether it will reopen next spring.

Biological oceanographer Raphael Kudela, of the University of California, Santa Cruz, is an expert in the type of algae that causes red tides and has been studying samples of the killer plankton that were captured last month by researchers working along the Sonoma coast on an unrelated project.

“The conditions are really right for red tides,” Kudela said Friday. “They really do well with warm days, calm conditions and really nice, warm surface temperatures. And that’s exactly what you guys are getting up there right now.”

The red tide was dominated by two types of algae called dinoflagellates, including one species that can glow bright blue and another that is capable of producing a mild poison called yessotoxin, according to Kudela.

Dinoflagelletes are single-celled members of the algae family. They can build microscopic shelters and hide inside them for extended periods when water conditions do not suit them, grouping together in what's called a seed bed.

“When these organisms come in, it could be something we don’t ordinarily see, but if it establishes itself, it could be hanging around for several years,” Kudela said. 

Once conditions shift, a dinoflagellate can burst out of its hard shell, using one whip-like tail called a flagella to propel itself and another to steer. It grows by feeding on other tiny organisms or using energy from the sun, sometimes co-opting its prey's cells for photosynthesis. It reproduces either by cloning itself or by mating.