Toxins secreted by freshwater algae blooms have found their way into shellfish populations in San Francisco Bay.

Scientists regularly monitor shellfish for toxins produced by marine aglae, but don't often measure concentrations of microcystin, a toxin produced by blue-green algae, a type of freshwater cyanobacteria.

The toxin has previously been measured in water samples collected from the Sacramento and San Joaquin rivers, both of which empty into the San Francisco Bay.

Researchers collected and tested mussels from five sites in the bay. Several of the samples featured dangerous levels of the toxin. Exposure to microcystin can cause liver damage.

Scientists also measured how fast mussels and oysters in lab tanks aborbed the toxin from tainted water. They published their findings this week in the journal Harmful Algae.

"We 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," Raphael Kudela, a professor of ocean health at the University of California, Santa Cruz, said in a news release.

Researchers are currently working with the California Department of Fish and Wildlife and NOAA to monitor and address the problem.

"There is monitoring of shellfish for marine-derived toxins, but because this is a freshwater toxin no one has been looking for it," said study author Corinne Gibble, a former UCSC grad student and now a environmental scientist with CDFW. "Now it seems microcystin is something we should be monitoring as well."

Cyanobacteria blooms prefer warm, nutrient-rich water. Hot summers and fertilizer-rich runoff from farms can trigger blooms in lakes and rivers. The problem is likely being exacerbated by ongoing drought conditions in California.

"The rains help by flushing things out. Warm, dry conditions favor these blooms, so we've been seeing more of them lately than we would without the drought," Kudela said.

As of now, researchers don't believe the toxin is an immediate threat to seafood consumers. Scientists found low levels of the toxins in mussels from a shellfish farm in Tomales Bay, but the concentrations were lower than safety thresholds.

"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," Kudela said.