Saxitoxin concentrations in coastal Oregon shellfish: The influence of El Niño and the Pacific Decadal Oscillation. Jacqui Tweddle, Boston University (previously at Oregon State University), Pete Strutton, Oregon State University and Michelle Wood, University of Oregon Figure 1: Time series of the NOAA/OAR/ESRL/PSD Multivariate El Niño Southern Oscillation Index (MEI), the JISAO Pacific Decadal Oscillation (PDO) Index, NOAA ERD Upwelling Index at 45 ºN and saxitoxin concentrations in Oregon Department of Agriculture coastal shellfish samples (showing only those positive for saxitoxin.) Darker lines on MEI and PDO plots show July-June mean, as used by Newman et al. (2003) and Moore et al. (2008). Analysis of shellfish saxitoxin concentrations included only samples in which saxitoxin was higher than the previous sample at that site. This was to ensure samples were associated with active Alexandrium blooms (shellfish actively exposed to, and taking up, toxin) rather than periods of depuration of previous toxic bloom events. Saxitoxin concentrations were considered over a calendar year. The comparisons below all use July-June means of MEI or PDO, and calendar year analyses of the Upwelling Index. Figure 2: Effect of ENSO on shellfish saxitoxin concentrations Positive MEI values (El Niño) were significantly related to the mean annual upwelling index (r 2 = 0.12, p<0.05), with positive MEI leading to weaker upwelling and warmer SST. A significantly higher percentage of sites along the coast were affected by saxitoxin in positive MEI years, and the whole coast experienced significantly increased proportions of samples testing positive for the toxin (p<0.05). The highest annual mean and maximum saxitoxin concentrations occurred during positive MEI years. Figure 3: Effect of the PDO on shellfish saxitoxin concentrations A higher proportion of shellfish samples and sites were positive for saxitoxin during periods of positive PDO. But only 6 years were associated with negative PDO, so statistics comparing saxitoxin concentrations from positive and negative PDO were not robust. No significant relationships were found between the PDO index and upwelling, however mean July-June PDO values were significantly correlated to maximum sea surface temperature (r 2 = 0.41, p<0.05, AVHRR SST extracted for ºN ºW). Positive PDO was significantly related to an increase in the number of days over 13˚C (r 2 = 0.54, p<0.05). The MEI had no significant effect on SST. Effect of Upwelling on shellfish saxitoxin concentrations There was a significant inverse relationship between maximum upwelling strength and SST (r 2 = 0.51, p<0.05). Neither the length of the summer upwelling period, nor the start or end dates, were significantly related to saxitoxin concentration parameters. However, years of weaker upwelling, and warmer waters, were significantly associated with lower mean shellfish saxitoxin concentrations the next year (r 2 =0.21, p<0.05). HABs occur mostly during periods of upwelling (or transitions) along the OR coast. PDO and ENSO affect upwelling (refs and data) and other climatic conditions, and therefore may influence HABs. E.g. Chhak & Di Lorenzo (2007) found cool PDO was associated with deeper upwelling cells. What relationships are apparent between MEI, PDO, upwelling and toxic events? PDO increases max SST and number of days when SST is >13˚C. Positive PDO also seems to show increased number of samples are event related, and more sites are affected. Shellfish on the Oregon coast have been tested for saxitoxin by the Oregon Department of Agriculture since This saxitoxin time series was analyzed for relationships to the Pacific Decadal Oscillation (PDO), the El Niño-Southern Oscillation (ENSO), an upwelling index, and AVHRR sea surface temperature (SST). A possible link is observed between years with warmer surface ocean conditions and an increased proportion of positive saxitoxin samples. This is hypothesized to be due to an increase in the number of days conducive to Alexandrium growth (number of days SST > 13ºC, Nishitani & Chew 1984, Aquaculture) during positive phases of ENSO and the PDO. Defining relationships such as these enhance our understanding of what causes toxic blooms and aids prediction, therefore helping to protect public health. Shellfish toxin data were provided by the Oregon Department of Agriculture. Funding was provided by NOAA MERHAB and Center for Sponsored Coastal Ocean Research.