SEASONAL AND INTERANNUAL VARIABILITY OF PSEUDO-NITZSCHIA AND DOMOIC ACID IN THE JUAN DE FUCA EDDY REGION AND ITS ADJACENT SHELVES

Evelyn J. Lessard¹ , Barbara M. Hickey¹, William P. Cochlan², Charles G. Trick³, Mark L. Wells⁴, and Vera L. Trainer⁵

¹School of Oceanography, University of Washington, Seattle, WA 98195 USA
²Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA 94920
³Department of Biology, University of Western Ontario, London, ONT N6A 5B7, Canada
⁴School of Marine Sciences, University of Maine, Orono, ME 04469 USA
⁵NOAA, Northwest Fisheries Science Center, Seattle, WA, 98112 USA

The Ecology and Oceanography of Harmful Algal Blooms of the Pacific Northwest (ECOHAB PNW) program was motivated by a hypothesized physical pathway between the Juan de Fuca Eddy, an apparent initiation site for toxigenic Pseudo-nitschia blooms, and coastal clamming beaches on the Washington coast. Data from 6 cruises over 3 summers have shed light on the processes controlling the presence of domoic acid (DA)-producing Pseudo-nitzschia in the eddy region, those species responsible for toxic events, and the environmental parameters controlling bloom development. The field program used a combination of synoptic surveys and lagrangian drifter studies. The synoptic surveys provided information on the scales of variability and insights into the factors governing Pseudo-nitzschia and toxin occurrence. We identified more than 10 species of Pseudo-nitzschia off the WA coast, but those responsible for the most toxic blooms were P. cuspidata and P. australis. However, the presence of a particular species cannot be used as an absolute indicator of toxicity due to the high level of variability in DA production by field assemblages of Pseudo-nitzschia. Although sometimes achieving high abundances (>10⁶/L), Pseudo-nitzschia spp. were always a small component of the total phytoplankton biomass, therefore satellite imagery is not useful for bloom prediction in this region. Throughout the study area, Pseudo-nitzschia usually occurred as a minor member of diatom-dominated assemblages; notably, it was often the major diatom taxa present in euglenoid and dinoflagellate blooms that occurred in the eddy region. Over the entire data set, no simple predictive relationship was found between environmental parameters (nitrate, phosphate, silicate, chlorophyll, temperature or salinity) and either Pseudo-nitzschia abundance, species or DA. Both Pseudo-nitzschia abundance and DA were highly variable in time and space. On a 21 day time scale, measurable toxin was always observed in the eddy region, and the highest level of toxin (>90 nM) and Pseudo-nitzschia cell numbers (>11-13x10⁶/L) were observed in an eddy bloom in September 2004. The field as well as modeling studies clearly demonstrated that toxic blooms can escape the eddy and move southward along the WA coast. On two occasions, toxin found in the coastal region was associated with the presence of Columbia River plume water. Our observations confirm the idea that the eddy region and not recently upwelled coastal water is the primary initiation site for most toxic blooms on the WA coast.