Regional Oceanography leading to Toxic Pseudo-nitzschia Events on Beaches in the Northern California Current

Hickey, B.M.¹, A. MacFadyen¹, V.L. Trainer², E.J. Lessard¹, W.P. Cochlan³, C.G. Trick⁴ and M.L. Wells⁵

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

Recent interdisciplinary studies of Pseudo-nitzschia off the Washington/Oregon coasts have improved our understanding of the physical/chemical/biological oceanographic elements leading to significant toxic conditions in razor clams on coastal beaches. First, phytoplankton must become concentrated in a bloom source region such as the Juan de Fuca eddy. ECOHAB PNW studies suggest this requires a period of downwelling-favorable or lightly fluctuating winds. 2) Environmental conditions leading to elevated domoic acid (DA) levels must be present. In this region, cellular DA levels appear to be influenced by iron and copper availability and not by macronutrient levels. 3) The Juan de Fuca eddy region, which has been shown to be a regional source of domoic acid, is located offshore and well to the north of coastal beaches that have experienced toxic outbreaks. Patches of toxic phytoplankton must first escape this eddy-like offshore source region--escape is favored during upwelling-favorable wind conditions, which allow the geostrophic constraint of the eddy circulation pattern to be broken. 4) The patch must then move alongshore to sites with shellfish populations and 5) the cells must retain their cellular toxin content during the time period of transport. For a toxic source in the Juan de Fuca eddy this requires southward advection along the shelf, as occurs during periods of upwelling-favorable winds in summer and early fall. ECOHAB PNW studies show that toxin can be maintained in the 7-14 days required for transport. For an Oregon source of toxin such as Heceta bank to impact the Washington shelf, this requires northward advection along the shelf, as occurs during periods of downwelling-favorable winds in spring. Last, 6) the patch of toxic phytoplankton must move onshore to coastal beaches and/or estuaries and 7) must remain there for a period sufficient for significant ingestion by shellfish. In the northern California Current this requires a period of significantly strong downwelling-favorable winds, as occurs during a storm. The requirement to fulfill each of these several steps illustrates why toxic events occur sporadically on coastal beaches in spite of the frequent observations of toxicity in offshore locations.