Reason cited as massive freshwater discharge due to intense rainfall
In 2011, there was a mass die-off of wild Olympia oysters in north San Francisco Bay, the cause of which remained a mystery for a long time.
But not anymore.
Researchers from University of California, Davis have discovered the reason for the mass die-off: atmospheric rivers.
In March 2011, a series of atmospheric rivers found their way to California, bringing with them intense rainfall. This led to a massive discharge of freshwater into the San Francisco Bay, which experienced a drastic drop in salinity within a few days to a level that it was too low for the oysters to survive. These atmospheric rivers comprise narrow stretches of concentrated moisture – several thousand kilometres long – in the lower atmosphere. They have been the cause of heavy rain in the region.
The research team, led by Brian Cheng, reconstructed the 2011 incident, and published their findings in the recent issue of Proceedings of the Royal Society B.
Atmospheric rivers transport huge amounts of moisture around the world. If they are at land, severe precipitation such as rain or snow can result, leading to floods. According to the researchers, the ten most severe floods in Great Britain since 1970 have been associated with atmospheric rivers.
The research team had been studying a population of Olympia oysters (Ostrea lurida) in north San Francisco Bay from October 2009 to July 2011. Every three months, they would take note of the population size.
Then came March 2011, which brought along the rain caused by atmospheric rivers. The salt content in the bay dropped to a great extent and, for eight consecutive days, remained below the level which the researchers had concluded was the threshold for low salinity for the oysters. By July 2011, almost all the oysters had died.
By November 2013, the population of oysters in the region had recovered. However, according to Cheng, “These new oysters are smaller and less fertile, and that may have consequences for restoring oysters in San Francisco Bay.”
More information: www.ucdavis.edu
Link to the study: rspb.royalsocietypublishing.org/content/283/1844/20161462