Liver-damaging toxin from freshwater algae is found in San Francisco Bay shellfish

San Francisco Bay

Recently, scientists have detected high levels of a toxin produced by freshwater algae in mussels from San Francisco Bay.

Although shellfish harvested from California’s coastal waters are monitored for toxins produced by marine algae, they are not routinely tested for this freshwater toxin, called microcystin.

The toxin, which causes liver damage, is produced by a type of blue-green algae that thrives in warm, nutrient-rich water conditions.

It has been found in many lakes and rivers in California, including the Sacramento and San Joaquin Rivers, which flow into the San Francisco Bay Delta, and in several Bay Area lakes.

Researchers from UC Santa Cruz investigated the potential for the toxin to contaminate shellfish after detecting low levels of the toxin in water samples from San Francisco Bay.

The researchers tested mussels collected from five sites in the Bay.

They also did experiments with both mussels and oysters in tanks to determine how quickly the shellfish take up the toxin and how long it takes to clear it from their tissues.

The results are published in the journal Harmful Algae.

Researchers found that this freshwater toxin accumulates in shellfish, both mussels and oysters, and that in San Francisco Bay, the toxin levels in some mussels exceed the recommended guidelines for consumption by quite a bit.

The researchers also detected low levels of the toxin in commercial oysters from Tomales Bay, although the levels were well below the guidelines for toxin levels in fish.

These findings suggest that tests for the toxin should be added to existing shellfish monitoring programs.

Researchers also suggest that the toxins detected in San Francisco Bay may come from multiple sources, including the rivers that flow into the Delta as well as local sources.

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Citation: Gibble CM, et al. (2016). Evidence of freshwater algal toxins in marine shellfish: Implications for human and aquatic health. Harmful Algae, 59: 59-66. DOI:
Figure legend: This image is for illustrative purposes only.