A toxic algae that forms in reservoirs, lakes and stagnant freshwater ponds was responsible for the deaths of at least 21 threatened California sea otters in the Monterey Bay area, a scientific study revealed Friday.

The discovery, reported in the scientific journal PLoS ONE, is alarming because the toxin, called microcystin, had never been linked to the death of a marine mammal and was not believed to be capable of surviving for long periods in saltwater.

"Based on what we know, this is the first documentation of a freshwater algal bloom being transmitted to upper-level marine mammals, specifically a federally listed species," said Melissa Miller, a senior wildlife veterinarian and pathologist for the California Department of Fish & Game and the study's lead author.

The three-year average population of California otters, also known as southern sea otters, declined 3.6 percent this year, the second consecutive drop after a decade of increases. The reason for the decline is a mystery, but scientists believe a variety of causes are at play, including toxic runoff.

Microcystin, which is commonly referred to as blue-green algae, can cause liver damage when ingested. All 21 sea otters that tested positive for the bacteria died from liver failure, according to the study, which was completed with the help of experts from UC Santa Cruz and a variety of state and federal agencies.

It is believed the toxins flowed to the ocean off the coast of Monterey in rivers and creeks. Sea urchins and shellfish near the outflow filtered the water and the poison accumulated in their bodies, which were, in turn, eaten by otters.

Microcystis is a naturally occurring algae. Its bright green blooms have long been found behind dams, particularly along the Klamath River, and in stagnant pools.

But researchers say the microcystin toxin appears to be multiplying in the environment to the point that it is becoming a global health concern. That's because cyanobacteria, as scientists call bacterial blooms, thrive in warm water that is rich in nutrients from lawn, city and agricultural runoff. The warmer the temperatures, the more bacteria there are

Global warming, scientists say, has increased the frequency of deadly green "super-blooms."

Microcystins have been detected in brackish water before, including in the Sacramento-San Joaquin River Delta and San Francisco Bay, scientists say. The bacteria has 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 is also potentially dangerous to humans.

"What's really astonishing to us is that it had not been on anyone's radar as a problem in the marine environments until now," said co-author Tim Tinker, a research biologist with the U.S. Geological Survey and an adjunct professor of ecology and evolutionary biology at UC Santa Cruz.

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