Invisible Rivers in Our Atmosphere Killed Oysters in the San Francisco Bay

Atmospheric rivers are corridors of water vapor predicted to increase under climate change.

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Dec 14 2016, 3:00pm

Oysters cling to the shoreline at China Camp State Park in San Francisco Bay before a 2011 mass die-off. Image: Brian Cheng/UC Davis

Churning and rolling above us through the Earth's atmosphere are invisible "rivers" that can transport seven to fifteen times the amount of water that flows out of the Mississippi River. These narrow corridors of concentrated water vapor and winds are called Atmospheric Rivers (ARs) and are a critical component of the global water cycle.

The rivers contribute about 30 to 50 percent of the annual precipitation for the West Coast of the United States, and are known to occasionally reach extreme levels. But no direct biological impact of ARs had ever been documented. Researchers at the University of California, Davis, however, have now concluded that atmospheric rivers influenced a mass die-off of wild oysters in San Francisco bay in 2011. This is the first recorded example of the biological impact of ARs on an ecosystem, which are due to have even more repercussions as the climate changes. Their study was published in the science journal Proceedings of the Royal Society B.

Olympia oysters are filter feeders that can clean up to 30 gallons of water a day, removing excess nitrogen and other pollutants that can be harmful to the environment. The beds, or reefs, that they create provide habitat for fish, crabs and numerous other sea creatures.

Overharvesting and habitat destruction have caused massive declines in numbers of the once abundant mollusk, but up until 2011 in North San Francisco Bay, there were still about 3,000 oysters per square meter. Then, they were suddenly close to obsolete.

Ecologist and lead author of the study Brian Cheng was studying that exact population of oysters when the die off occurred, and immediately went to work investigating what happened. Cheng and his colleagues had figured out that oysters have a specific threshold for low salinity, or salt content in the water, that they need in order to survive. It turns out that the salinity in the bay had plummeted past that threshold due to a surge of freshwater runoff into the bay. The runoff was the result of three atmospheric rivers that had made landfall and dumped rain all over Northern California that month.

Atmospheric rivers, like drought, heatwaves and other extreme weather events, are predicted to intensify due to climate change and become much more common. "This shows us one way in which extreme events might affect coastal ecosystems," said Cheng. "Oysters can help buffer shorelines and enhance biodiversity" so blows to them represent blows to the larger ecosystems they serve as well.

ARs don't, and won't, affect just oysters however. Though this is the first documented instance of a biological impact from the "rivers in the sky," the strongest ARs have also been
attributed to severe rainfall and flooding that pose serious risks to human health and property.

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