A mystery came to Monterey Bay in 2007: Hundreds of seabirds washed ashore looking and even smelling as though they'd run into an oil spill. The slimy substance that covered the struggling and dead birds smelled "like linseed oil," says Raphael Kudela, an associate professor of ocean sciences at the University of California, Santa Cruz.

But after water testing, the researchers found no indication of excess oil, pesticides, acids or commercial products that might have caused the foam and gooey coating on the birds. All told, 207 birds were found dead and 550 were stranded.

Then the slime disappeared.

Now, the "mystery spill" has been solved. It wasn't the Cosco Busan oil spill in nearby San Francisco Bay or a controversial aerial pesticide spraying along the Central Coast. It was harmless-looking foam from an ordinarily nontoxic algal bloom churned up by November waves, according to a study coming out in PLoS ONE on Monday.

This unassuming foam looks like "if you beat egg whites into a meringue and sprinkle a little dirt in with it," study co-author Kudela tells ScientificAmerican.com. Although the protein-rich froth didn't contain toxins, it acted as a surfactant, which lowers the surface tension of water—especially where it comes into contact with oil. This disabled the natural water-repellent coating on the feathers of floating loons, grebes, northern fulmars and other birds in the bay, soaking them through and rendering them susceptible to the chilly autumn Pacific water.

The first clue actually appeared before any signs of trouble: A satellite had picked up images of a bloom, or "red tide" of the nontoxic algae Akashiwo sanguinea in the bay. Algal blooms can be triggered by a variety of causes—many of which are not easy to predict—including an excess of nutrients and other just-right conditions for the phytoplankton populations to explode.

Armed with that knowledge and water samples containing the species, researchers tried to reproduce the event in the lab by agitating—to mimic the waves—a broth of healthy versions of the suspected culprit. But no foam appeared.

Then they tried the same experiment on dead and dying cultures of the same algae—which is what would be present in the latter stages of an algal bloom. Voila. Foam. They then dipped bird feathers into the froth. "It's really quite dramatic," says Kudela. "As soon as this feather touches it, [the feather] melts right down and [would] touch the bird's skin." The bird would then get wet and their body temperatures would drop, he says.

To add insult to injury, many of the affected birds had just returned from their fall migration and were weak, making them more susceptible to the stressful event.

Carmelo Thomas, an associate professor of biology at the University of North Carolina in Wilmington and an expert in harmful algal bloom species, was surprised to hear that a nontoxic algae species was responsible for the bird deaths. He has cultures of Akashiwo sanguinea in his lab and says he's always just thought of it as a nuisance and not one that causes real harm. Any algal bloom, says Thomas, who wasn’t involved in the study, can generate foam (given enough wind and waves), but he hadn't counted on anything less than toxic foam producing a threat to wildlife.

Although algal blooms often cause large foam events, says Kudela, there are no other documented cases of them causing a large wildlife die-off. But Kudela notes a similar bird stranding in the late 1990s in which "they never found oil or anything" and a curious coral bleaching event that was linked to this same sort of algae. And Kudela says that there's reason to suspect we'll see more of these sorts of incidents. In the past five years, algae blooms have been increasing along that region of California's coast, and the Scripps Institution of Oceanography has found that winter storms have been gathering strength over the past 50 years (so we'll be seeing more turbulence on the surface), which could churn up more foamy danger for birds.

For humans in the area, the foam didn't appear to have any documented health effects. Kudela says that surfers complained of some respiratory problems and skin rashes. But for the most part the biggest impact for locals, says Kudela was, "It stunk a lot."

Image (an unharmed California seagull walks through the foam) courtesy of Raphael Kudela/UC Santa Cruz