Scientists at the University of California, Santa Cruz have detected high levels of microcystin, a toxin, in mussels drawn from San Francisco Bay.

According to the researchers, the toxin, which can cause liver damage, is produced by a type of blue-green algae that thrives in warm, nutrient-rich freshwater 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.

University of California, Santa Cruz researcher Kendra Negrey.University of California, Santa Cruz researcher Kendra Negrey."There is potential for this toxin to affect humans, but most of our samples are still below the recommended limits for human consumption, so people shouldn't panic and think they can't eat shellfish," says Raphael Kudela, professor of ocean health.

The greater concern, he says, is the potential impact on marine mammals such as sea otters, which eat large amounts of shellfish. In 2010, Kudela and scientists with the California Department of Fish and Wildlife reported that microcystin poisoning had killed several sea otters in Monterey Bay.

Blooms of the algae that produce microcystin are a growing problem throughout the U.S. and around the world due to warming temperatures and nutrient pollution, the researchers say. Microcystin from algal blooms in Lake Erie, for example, contaminated the water supply for Toledo, Ohio, in 2014. In the Bay area, it has forced closures of public swimming spots at Lake Anza, in Berkeley, and Lake Temescal, in Oakland.

Kudela says he suspects the toxins detected in San Francisco Bay are coming from multiple sources, including the rivers that flow into the Delta as well as local sources. The recent drought appears to have exacerbated the problem throughout California, he says.

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