Oyster deaths linked to ‘atmospheric rivers’

Narrow channels of moisture snaking through the atmosphere can bring storms that wreck beachfront bungalows — and leave oyster beds bare. Several of these channels, called atmospheric rivers (SN: 2/26/11, p. 20), dumped particularly heavy storms on California in early 2011. The resulting freshwater influx probably left part of the San Francisco Bay without enough salt for oysters to survive, researchers report online December 14 in Proceedings of the Royal Society B.

Oysters worldwide have been struggling in recent years because of climate change, ocean acidification and overharvesting. Their disappearance hurts the coastal ecosystems they inhabit.

“Oysters build habitat on the coast for other species. They’re kind of like a coral reef in that regard,” says study coauthor Brian Cheng, an ecologist now at the Smithsonian Environmental Research Center in Edgewater, Md.

That’s why scientists were regularly monitoring oyster populations in China Camp State Park, in the northern San Francisco Bay. When the tides went out and exposed the oyster beds along the coast, Cheng and his colleagues could rush in to count the shellfish. In March 2011, almost all the oysters at the park suddenly died.

“You could smell the rotting, decaying oysters well before reaching the site,” says Cheng, who did the work while at the University of California, Davis. “But we’re biologists, so we forged ahead.”

Cheng’s team charted the oyster population’s sudden plummet and then gradual rebound over the next few years. Then the researchers examined how water and atmospheric conditions fluctuated over the same time period.

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The bay hadn’t become especially acidic or changed dramatically in temperature. But around the same time as the die-off, atmospheric rivers had carried several big storms to the California coast, and the bay’s salinity had markedly decreased. Oysters need a certain salinity level to survive, and the salt content of the China Camp area was now too low.

It’s tricky to pin down a direct causal link between the atmospheric rivers and the oyster deaths. But given the evidence, Cheng thinks it’s likely that the extreme storms triggered a huge release of freshwater into the bay, diluting the salt water and ultimately killing the oysters. (Other species were probably affected, too — they just weren’t being monitored.)

China Camp’s oysters have since recovered somewhat. And because the oyster records at this site go back only to 2009, it’s hard to say whether this is a one-off event or whether similar atmospheric phenomena have hurt coastal ecosystems in the past, says Michael Dettinger, a hydrologist with the United States Geological Survey in Carson City, Nev., who wasn’t part of the study. But atmospheric rivers and the extreme weather they bring are predicted to become more frequent in the coming years.