Toxic algae found at high levels in SF Bay mussels

In this 2014 photo provided by the University of California, Irvine, a blue mussel clings to a rock at Mount Desert Island, on the Maine coast. A 2016 scientific study said the mussels, which are beloved by seafood fans, have declined dramatically in the Gulf of Maine. (Kylla Benes/University of California, Irvine via AP) less In this 2014 photo provided by the University of California, Irvine, a blue mussel clings to a rock at Mount Desert Island, on the Maine coast. A 2016 scientific study said the mussels, which are beloved by ... more Photo: Kylla Benes, Associated Press Photo: Kylla Benes, Associated Press Image 1 of / 1 Caption Close Toxic algae found at high levels in SF Bay mussels 1 / 1 Back to Gallery

High levels of a toxic freshwater algae known to be harmful to humans have been detected in mussels at several locations in San Francisco Bay, scientists said Monday.

Researchers found 10 to 20 times the regulatory limit of the algae, called microcystin, in mussels collected in Berkeley and Alameda during tests in 2015, said Raphael Kudela, a professor of ocean sciences at UC Santa Cruz.

The toxin, also known as blue-green algae, apparently formed in reservoirs, lakes and stagnant freshwater ponds and then flowed into the bay in rivers and creeks, according to a report written by Kudela and published in the journal Harmful Algae.

It is the same naturally occurring toxin blamed for killing dozens of California sea otters in Monterey Bay area in 2010.

The discovery is alarming because there is no regular year-round testing for microcystin, which researchers have found remains toxic for two weeks in the ocean and becomes concentrated in shellfish.

Kudela’s research team collected mussels between April and September 2015 from the Berkeley Marina, Alameda, Tiburon, and Point Isabel and Point Potrero in Richmond. Concentrations were highest in September, Kudela said.

The recommended limit for microcystin in food is 10 nanograms per gram. Concentrations between 100 and 200 nanograms per gram were found in mussels in Berkeley and Alameda in September. The mussels in Richmond had 20 nanograms per gram, and Tiburon mussels were at the limit.

The levels found in Berkeley and Alameda “won’t kill you,” Kudela said in an interview, “but they would certainly make you sick.”

Bay mussels are sometimes eaten by local shellfish gatherers, but they are not sold commercially.

“At this point, we are not as concerned about humans, but it raises concerns about animals that eat shellfish,” Kudela said.

In addition to sea otters, birds, crabs and some seals and sea lions eat mussels from the bay.

The researchers also detected low levels of the toxin in commercial oysters from Tomales Bay, but they did not find any oysters above the guidelines for microcystin in fish.

Kudela’s findings are unrelated to the quarantine that the state Department of Public Health puts in place every May through October on eating locally gathered mussels. That quarantine is designed to protect people from domoic acid poisoning, which is common in mussels between spring and fall.

Kudela began investigating the algae phenomenon six years ago after investigators determined that sea otters had been dying of liver damage from ingesting microcystin. The bloated, bright yellow corpses of the otters had been turning up near Monterey Bay since 2007.

Microcystin forms bright green blooms. It is produced by a type of cyanobacteria that thrive in warm, nutrient-rich water conditions and has been found in many lakes and rivers in California, including the Sacramento and San Joaquin rivers, which flow into the bay. In the Bay Area, it has forced closures of public swimming areas at Lake Anza in Berkeley and Lake Temescal in Oakland.

The bacteria have been linked to the death of cattle and even dogs that drank the water or swam through the green slime and then licked their fur. It does not hurt shellfish, Kudela said, but the bivalves appear to relish the blooms, accumulating large quantities of the toxins in their tissues.

Although the study samples were from last year, Kudela said microcystin is still being found in the bay, with as many as three-quarters of mussels testing positive in some locations. He said the toxin appears to be multiplying in the environment to the point that it is becoming a global health concern.

The warmer the temperatures, the more bacteria there are, he said. Global warming is likely to increase the frequency of deadly green “super-blooms.”

“The takeaway is that the things we are doing on the land side do have an impact on the marine environment,” Kudela said. “It’s a wake-up call that we really need to be thinking about our coastal system more holistically.”

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite