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...