Oystermen in San Francisco Bay haul in their catch in the late 1800s. University of Washington

From the shores of Manhattan to the craggy coastline of Puget Sound, the country’s seaboards were once flush with oysters. Unfortunately, overharvesting and habitat destruction over the past century have taken a toll on our bivalves. Officially, stocks are thought to be about 12 percent of what they were in the early 1900s. So when nearly every single oyster in the northern stretches of San Francisco Bay kicked the bucket in March of 2011, there was good reason for alarm.

No one can definitively say who or what the culprit was, but the mass die-off might have had something to do with a series of storms that hit the area around the same time. Almost 70 percent of the region’s annual rainfall poured out of the sky over the course of just a few weeks. A recent study takes a closer look at the water conditions that may have led to the oysters’ demise—and the researchers think it is related to massive, invisible “rivers in the sky.”

Rivers in the Sky?

I know, it sounds like I’ve been smoking the Pineapple Express, but that movie actually gets its name from an atmospheric river that picks up moisture in the tropics and passes over Hawaii before crashing into America’s west coast. In the winter of 2014, the Pineapple Express rocked California from end to end, fueling blizzardlike conditions all over the state and perhaps even contributing to a tornado in Los Angeles.

“Technically speaking, atmospheric rivers are long and narrow plumes of intense water vapor that travel in the lower atmosphere,” says lead author Brian Cheng, a postdoctoral fellow at the Smithsonian Institution.

via GIPHY

While you can’t see these rivers with the naked eye, scientists map the moisture corridors via microwave sensors aboard Department of Defense satellites. We now know that atmospheric rivers occur over every continent and are responsible for a heckuva lot of extreme precipitation events. For example, California’s Russian River flooded seven times between 1996 and 2007, and scientists have linked atmospheric rivers to every instance. Across the pond, the landfalls of atmospheric rivers have synced up with every one of the United Kingdom’s 10 largest floods since the 1970s.

This atmospheric river (shown in red) hit California at the end of 2010. U.S. Naval Research Laboratory

Hell in a Half Shell

So let’s go back to San Francisco in March of 2011. Oysters thrive in salty and brackish waters, and because the mollusks cement themselves to the seafloor or river bottom, they have to make do with whatever the environment throws at them. And in the case of atmospheric river formation, that means a deluge of freshwater.

Close to 40 percent of the rain that falls in California drains into the San Francisco Bay, and that March, three atmospheric rivers made landfall in the state. (That whole winter had been especially wet, with 20 total atmospheric rivers noted in a season that averages nine.) With biblical amounts of freshwater pouring into one place, the bay’s salinity plummeted.

Oysters may be stuck in place, but they aren’t exactly sitting ducks when water quality goes sour. The shellfish have been around for 200 million years or more, and in all that time their genes have evolved to cope with changes in temperature, water quality, and salinity. When conditions are really bad, the mollusks can go into a hibernation-like state called mantle cavity hermetization. The oysters clamp up their valves and sort of hold their breath until things get better, Cheng says.

But turning yourself into a waterproof panic room has its limits. According to lab experiments conducted by Cheng in an earlier study, published in Global Change Biology in 2015, the main species inhabiting San Francisco Bay can withstand up to eight days of low salinity. And wouldn’t you know it—eight days is precisely the amount of time the salt levels dipped in the bay that fateful spring.

Cheng admits that these findings are a correlation, not necessarily a causation, but he’s quick to add that researchers ruled out every conceivable factor besides salinity. Temperature? Oxygen? pH levels? Each one measured within the oysters’ survivable range.

“Salinity was the only variable that reached lethal levels, and it matched our predictions from oyster laboratory experiments almost perfectly,” Cheng says. Not only do the findings essentially close the case file for the Bay Area’s Great Oyster Massacre, but they mark the first time biological impacts have ever been pinned to atmospheric rivers.

via GIPHY

What Now?

The study of these ghostly rainmakers is pretty new, and the sheer vastness of atmospheric rivers makes it impossible to create similar conditions in the lab. That’s why it’s pretty darn lucky that Cheng and his team were able to examine a whole bunch of data that weren’t collected after an oyster die-off or a sudden bout of extreme weather, but were the result of routine monitoring by satellites.

“This is a part of the very important work that’s being done by NOAA and NASA,” Cheng says. “We need to maintain funding for these agencies to study Earth’s climate.”

We know that global warming will increase the frequency and intensity of extreme weather events, but what role atmospheric rivers will play in this saga remains unclear.

As for San Francisco’s oysters, there’s good and bad news. While the mollusks’ numbers seem to have bounced back in the past five years, the oysters are much smaller than usual. That could mean they aren’t reproducing at full capacity and that the population will therefore be less likely to bounce back the next time the Pineapple Express and its wrath roll into town.

Obviously we can’t push a button and make atmospheric rivers go away—nor is it likely we would want to. But in addition to fighting climate change by reducing carbon pollution, Cheng says we should be looking at studies like his for ways we might be able to adapt our communities, whether human or mollusk, to stronger storms.

“Let’s think carefully about infrastructure and building near the coast,” he says. “Let’s make sure that coastal ecosystems are resilient to these changes.”

In other words, it’s time to mount up.

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