Is San Francisco Bay resistant to Pseudo-nitzschia and domoic acid?

San Francisco Bay (SFB), California, USA is the largest estuary in the western United States and is home to more than 7 million people in nine counties and 101 cities. It is highly nutrient enriched and is directly connected to the Gulf of the Farallones and coastal Pacific ocean through the Golden Gate strait. The Gulf of the Farallones is one of several "hotspots" for the neurotoxin domoic acid, produced by members of the genus Pseudo-nitzschia. Despite the close proximity, SFB has few reports of harmful algal blooms and low concentrations of domoic acid, suggesting that SFB is somehow resistant to toxic blooms. Here we evaluate the potential growth and toxicity of the dominant toxigenic species in California coastal waters, P. australis and P. multiseries, to directly test the hypothesis that SFB waters confer resistance to blooms. We specifically evaluate the effect of varying temperature, salinity, and to a lesser extent, nutrients on growth and toxin production. Results show equivalent growth in SFB water (maximum growth rates of 0.71 and 1.35 d-1 for P. multiseries and P. australis) compared to open-coast water, and comparable or greater toxicity (0 to >100 pg DA cell-1). The historical resistance to blooms in SFB is hypothesized to be caused by a combination of insufficient acclimation time for advected Pseudo-nitzschia populations to become established and suppression of toxin production in warm waters.