Seasonal Differences in Sources of Toxic Pseudo-nitzschia

Seasonal Differences in Sources of Toxic Pseudo-nitzschia Cells on Washington's Razor Clam
Barbara M. Hickey1, Nicolaus Adams2, and Vera L. Trainer2
University of Washington, School of Oceanography, Seattle, WA 98195
Northwest Fisheries Science Center, 2725 Montlake Blvd. East, Seattle, WA 98112
Time series of domoic acid (DA) in razor clams on Washington's coastal beaches over
the last 15 years show along-coast patterns that are different in spring compared to late
summer to fall. During fall events, toxin in razor clams is highest at northern beaches.
Research from the ECOHAB PNW program has demonstrated that the primary northern
source region of DA to Washington outer coast beaches is the Juan de Fuca eddy, a cold,
50-km wide, seasonal eddy located offshore of the Strait of Juan de Fuca. Toxic cells are
ejected from the eddy during periods of strong upwelling-favorable winds when the eddy
elongates and becomes less retentive. Cells are transported southward toward the latitude
of clamming beaches in the ambient flow that occurs over the shelf and upper slope in the
upwelling season. In spring events, on the other hand, toxin in razor clams often is
generally higher at southern beaches. During spring 2005, data collected by MERHAB
programs, namely Oregon's MOCHA and Washington's ORHAB, suggest that the source
of the toxic cells is likely Heceta Bank, a semi-retentive topographic feature offshore off
the central Oregon coast. Transport from a southern source is consistent with the
northward modeled and observed currents during that period. In situ species and toxin
data from an offshore water property section at the time of the observed toxicity in the
razor clams confirms that toxic species were present offshore of coastal beaches at that
time, consistent with a southern source.