A drifter study of a toxic Pseudo-nitzschia bloom from the Juan de Fuca Eddy in the Pacific Northwest

C.G. Trick¹, E.J. Lessard², W.P. Cochlan³, B.M. Hickey², V.L. Trainer⁴, and M.L. Wells<sup>5

1</sup> Schulich School of Medicine, University of Western Ontario, London, Ontario N6A 5B7, Canada
² School of Oceanography, University of Washington, Seattle, WA 98195, USA
³ Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA 94920-1205, USA
⁴ NOAA Fisheries, Northwest Fisheries Science Center, Seattle, WA 98112, USA
⁵ School of Marine Sciences, University of Maine, Orono, ME 04469, USA

As part of the ECOHAB-PNW program to study the community formation, domoic acid (DA) toxicity and transport of Pseudo-nitzschia from the coastal waters of Washington State and British Columbia to the shoreline, we employed drifters to follow blooms emitted from their putative origin, the Juan de Fuca Eddy. In September 2004, we followed the dynamics of a bloom of a highly toxic Pseudo-nitzschia cuspidata and a small, vertically migrating flagellate. Over ten days, particulate DA reached greater than 40 nM and dissolved DA reached up to 15 nM. Our analysis of temporal changes allowed us to evaluate nutrient drawdown rates and corresponding biomass increase, and to test hypotheses regarding bloom success: 1) Is the bloom community composition a direct result of physical processes of eddy mixing of nutrients? 2) Is community composition the result of grazing removal of competing species? 3) Is bloom formation a result of increased well-being of the successful phytoplankton species (measures of photosynthetic efficiency and potential)? We also tested the effect of both cellular and extracellular DA production on the competitive success of Pseudo-nitzschia (grazing reduction, improved cell physiology). This is one of the most comprehensive studies of an in situ toxigenic diatom bloom to date.