A river flowing from the sea: the effects of atmospheric rivers on U.S. West Coast oyster populations
Dr. Danielle Zacherl, CSU Fullerton
Dr. Matt Ferner, San Francisco State University

September 18, 2018

With predicted climate change, extreme weather events will increase in severity and frequency, including a doubling of intense drought and flooding in California (CA) by 2100. One important source of flooding events in CA is atmospheric rivers (hereafter AR). An AR is a narrow ribbon of concentrated moisture in the atmosphere, up to a few hundred km wide, that travels across the ocean and releases excessive precipitation upon landfall. The 2017 AR events brought an abrupt end to the extended drought in CA over much of the state and contributed to significant coastal flooding. With AR events come significant shifts in the salinity of CA estuaries with potentially catastrophic effects on local marine flora and fauna. A recent study attributed a significant mass-mortality event among Olympia oysters, Ostrea lurida, in San Francisco Bay to an extreme series of AR events in 2011 that resulted in an intolerably long period of sustained low salinities (< 6.3 practical salinity units, or psu).

Olympia oysters

​Olympia oysters (Ostrea lurida) nestled in the mud with
mussels and sponges. 

Olympia oysters

Pacific oysters (Crassostrea gigas)  Photo by Stefan Nehring, 
via http://www.NOBANIS.org, date of access 9/17/2018. 

The potential for Olympia oysters, the only oyster species native to the U.S. West Coast, to be profoundly and negatively impacted by AR events is problematic. Olympia oysters may act as a powerful shoreline stabilizer and habitat provider and are increasingly viewed as part of a CA state-wide solution to tackling vexing predicted climate-change impacts to coastal communities, including storm surges and sea-level rise. Within CA, human-introduced structures (e.g., seawalls and riprap) are being replaced by “Living Shorelines” initiatives (e.g., oyster reefs and eelgrass beds) that accumulate sediment, stabilize shorelines, and provide essential fish and invertebrate habitat. Such initiatives are underway in San Francisco and Newport Bays and are planned for San Diego Bay.

Concurrent with this building appreciation for the Olympia oyster is a growing concern about a renowned global invader, the Pacific oyster, Crassostrea gigas, which has established populations that regularly recruit new young oysters in southern CA estuaries including Newport Bay and San Diego Bay. If allowed to proliferate and establish high densities, these invasive oysters could reduce the growth and survival of our native oysters. The relative abilities of the two oyster species to tolerate locally significant departures from average salinity and thermal conditions are not well-documented. Information that can fill the knowledge gap about these species’ responses to major climatic events could contribute to management decisions for controlling the spread of non-native Pacific oysters and influencing restoration success for native Olympia oysters.

The convergence of the 2017 AR events across a broad swath of California with the need for information about oyster success under extreme climate scenarios created a strategic opportunity for us to explore the extent to which AR events can broadly impact Olympia oysters. We hypothesized that AR events in 2017 would have a measurable effects on the mortality and reproductive success of Olympia oysters, and that those effects would vary among estuaries. The overarching objective of the project was to link changes in local salinity driven by AR events to oyster mortality and subsequent oyster recruitment from San Diego Bay to San Francisco Bay, as well as to deploy key instrumentation in preparation for detecting population responses to future extreme climate events.

Map of atmospheric river events

Atmospheric river events impacting the U.S. west coast on February 16, 2017. Southern California was forecasted to receive 1-4.5 inches of precipitation over the 72 hours following the AR event. Both Newport Bay and San Diego Bay were impacted by these AR events, receiving 1.49 inches and 1.43 inches of precipitation respectively.

Several AR events occurred from December 2016 to March 2017, leading to precipitation that fell within the San Diego Bay (3 ARs), Newport Bay (4 ARs), and San Francisco Bay (>4 ARs) watersheds. Following each AR event was a surge in rainfall and runoff, which drove potentially lethal drops in estuarine salinity (below 6.3 psu) that varied in duration. Graduate and undergraduate students from CSU Fullerton surveyed densities of oysters at two sites each in Newport Bay and San Diego Bay in May 2017, and a post-graduate intern surveyed densities in San Francisco Bay. We had pre-AR density data from December 2016 for each site within each bay that we were able to use to determine the impact of the ARs on oyster populations.

Olympia oysters Olympia oysters

(Left) Following four Atmospheric river events impacting the bay from January to May 2017, CSU
Fullerton undergraduate research scholars quantify oyster densities on a cobble field in Newport Bay, 
CA during May, 2017. (Right) CSU Fullerton graduate student and undergraduate students quantify 

oyster density on a seawall in Newport Bay, CA in May, 2017.

In southern CA (Newport Bay and San Diego Bay), no major die-off and no detectable change in density of Olympia oysters occurred between December 2016 and May 2017 despite ARs significantly affecting salinity. Salinity dropped below established tolerance thresholds on multiple occasions, but none of these drops in salinity was long enough to be lethal to Olympia oysters. However, Pacific oyster density declined at both sites in San Diego Bay and at one site in Newport Bay at some tidal elevations. The decrease may have been due to ARs or due to other sources of background mortality. 

In contrast, there was substantial Olympia oyster mortality in San Francisco Bay during 2017-18. Salinity in 2017 dropped below established tolerance thresholds on multiple occasions and for extended periods of time, including for several consecutive weeks in February. This sustained low salinity likely had lethal consequences for many if not most Olympia oysters in the estuary. Pacific oysters are uncommon in San Francisco Bay and were not detected in density surveys before or after the AR events. Settlement and recruitment of both oyster species following the AR events ranged from average to significantly higher than average in both southern CA bays. Settlement of Olympia oysters was low and delayed in San Francisco Bay and no Pacific oysters were detected there.

Olympia oysters in San Francisco Bay may be more susceptible to AR impacts because the bay experiences more extreme ARs and has a larger watershed; both can lead to more extreme fluctuations in estuarine water quality such as lower salinity for longer durations. Because southern CA populations of Olympia oysters may be less at risk to extreme weather events such as ARs, they may provide a buffer against large-scale regional mortality and recruitment failure following such extreme weather events. By finding relationships among AR events, water quality, and oyster mortalities, restoration efforts can be directed towards areas within estuaries better suited for long-term maintenance of oyster reefs.

Dr. Danielle Zacherl is a Professor of Biological Science at CSU Fullerton. Dr. Matt Ferner is Research Director of SF Bay NERR at San Francisco State University's Estuary & Ocean Science Center. COAST provided funding for this project: Rapid Response Funding Program Award# COAST-RR-2016-003, April 2017.