A largely unmonitored but worrisome toxin produced by freshwater algae has shown up in shellfish in San Francisco and Tomales Bays, though its presence in West Marin was low, according to a study by University of California researchers published last month.
The toxin, microcystis aeriginosa, is produced by freshwater algae that blooms with the right mixture of water temperature, sunlight and nutrients. If consumed in large quantities, it can sicken both humans and marine mammals by causing liver damage.
The California Department of Public Health requires routine monitoring of commercial shellfish operations for domoic acid and paralytic shellfish poisoning toxins, but does not require microcystin testing. Yet that could change.
A lead author of the study, Corinne Gibble, who now works for the state health department as an environmental scientist, said in an announcement of the study, “There is monitoring of shellfish for marine-derived toxins, but because this is a freshwater toxin no one has been looking for it. Now it seems microcystin is something we should be monitoring as well.”
Terry Sawyer, who co-owns Hog Island Oyster Company, said more testing would be “an acceptable burden. It has to be…We support research. We’re in the business of providing food, not of making people sick.” Another of the study’s authors, Raphael Kudela, said in the announcement that “people shouldn’t panic and think they can’t eat shellfish,” since levels of microcystin have been mostly low.
But they should be concerned about the effects on marine mammals, which eat lots of shellfish: research Mr. Kudela conducted in 2010 found that some sea otters in Monterey Bay died from microcystin poisoning.
The toxin has been documented in rivers that feed San Francisco Bay and some Bay Area lakes. Given evidence in recent years that it can show up in brackish bays and harm marine mammals, researchers from the University of California at Santa Cruz tested mussels at five locations in San Francisco Bay over a period of six months. They found the toxin at all locations, and in every month except August at one site.
The prevalence surprised researchers, since samples were collected throughout the dry season, when freshwater loads from rivers are typically minimal. Samples from mussels in San Francisco Bay contained a wide range of toxin concentrations, with some samples far exceeding human consumption recommendations.
Mr. Kudela said that though most mussels were collected during the annual recreational mussel quarantine, researchers found the toxin in mussels both before and after the quarantine.
In Tomales Bay, researchers opted to test commercial oysters, given the bay’s numerous aquaculture businesses and three creeks—Lagunitas, Olema and Walker—that could be potential toxin sources. They found “low but detectable” levels of the toxin, though none of the oyster samples exceeded levels for human consumption.
Mussels also fared worse than oysters when exposed to the toxin. In experiments conducted in controlled tanks, mussels became more concentrated with the toxin and cleared it from their system more slowly than did oysters, which nevertheless contained low levels for up to two months after exposure.
The reason that oysters cleared microcystin more quickly is unknown. “We’re not sure why, but it’s pretty common that different organisms have different responses,” Mr. Kudela said in an email.
The lower level of toxins in Tomales Bay oysters could also be tied to a less prominent freshwater influence, compared to San Francisco Bay.
“Tomales Bay has much less exposure to rivers and creeks, which is where the toxin is probably originating. S.F. Bay is downstream from the San Joaquin and Sacramento Rivers, where we know there are large blooms,” he said.
Mr. Sawyer said he supports more research to better understand toxins such as microcystin and what influences their growth, particularly given the current and future impacts of climate change.
“Here’s the message: we have an incredible body of water preserved by a lot of hard work. To continue to produce food, we need to preserve that. We need to research to understand what’s going on.”