Wild oysters in San Francisco Bay are threatened by climate change

Climate change could supercharge the powerful storms often hailed for bringing drought-busting rains to California.

The storms, called atmospheric rivers, are long stretches of water vapor that “flow” from the tropics up to the West Coast. In California, they can deliver up to half of the state’s annual precipitation in just 10 to 15 days, and cause floods and landslides. Sometimes, the deluges from atmospheric rivers can do ecological damage, too.

A new study describes how a series of atmospheric rivers in the winter of 2010-2011 contributed to a die-off in the San Francisco Bay’s struggling oyster population. The findings, published in the journal Proceedings of the Royal Society B, are the first to demonstrate the ecological impacts of the storms — an example of the extreme climatic events expected to increase in size and frequency with climate change.

In California, climate models project that both intense droughts and floods, linked to strengthening El Nino and La Nina events, will increase by 50 percent by the end of the century. The reasoning is warmer air has an increased capacity to store moisture and energy, which could be unleashed in the form of bigger storms.

While some ecological effects of climate change have been well-documented — such as higher temperatures driving range shifts in plants and animal species — the impacts of extreme weather have proved difficult to study because of their unpredictable nature. However, scientists may have seen a preview of what’s to come over five years ago.

On March 16, 2011, torrents of freshwater discharged into the San Francisco Bay from the Sacramento-San Joaquin River Delta. At its peak, 220,000 cubic feet of water rushed into the bay per second. The influx coincided with a drop in the Bay’s salinity, which proved catastrophic for the little Olympia oyster.

A previous study tested the range of environmental conditions — such as air and water temperature, acidity, dissolved oxygen and salinity — in which oysters can survive.

Oysters match their internal body salinity with their external environment — when the surrounding water gets less salty, they seal themselves up. They can stay this way for about eight days, until they asphyxiate or build up too much waste in their bodies.

Before the deluge, China Camp State Park was home to one of the most abundant populations of the mollusks in the northern San Francisco Bay, with about 180 oysters per square foot stuck to the shoreline and below the tide. (That’s 30 times more oysters than in the second-most populated site.)

The numerous China Camp oysters likely provided other populations around the Bay with “recruits” — larval oysters that float through the water until they find a hard surface to latch onto and grow.

In the 1850s, the booming population along the West Coast — especially in San Francisco — could not get enough of the little oysters, which have a coppery taste compared with the shellfish often served in restaurants today.

Around the late 1800s or early 1900s, the fishery collapsed, and people began farming Pacific oysters introduced from Asia. The West Coast’s only native oyster species, the once-abundant bivalve has struggled to recover since.

For many years, the San Francisco Bay has been the site of ambitious oyster restoration projects. Government agencies and environmental groups hope the filter-feeding, reef-building bivalves could protect the shoreline from erosion and sea level rise, as well as reinvigorate the underwater ecosystem.

But the study suggests that more trouble could be in store for the diminutive Olympia oyster as climate change threatens to bring supercharged precipitation events to Northern California.

The March 2011 event wasn’t the biggest freshwater discharge into the bay — storms in January 1997 and January 2006 dumped 16,000 and 10,000 cubic meters per second into the bay, respectively. Although oyster data from that time is sparse, surveys found a relatively abundant population at China Camp in 2003. A November 2006 count found no living oysters, but many shells belonging to the dead mollusks.

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