Intra-seasonal and Interannual Variability in the Circulation and Biochemical Water Properties in the Juan de Fuca Eddy Region

A. MacFadyen¹, B.M. Hickey¹, V. Trainer², W. Cochlan³

¹University of Washington, School of Oceanography, Box 355351, Seattle, WA 98195 United States
²NOAA Fisheries, Marine Biotoxins Program, Northwest Fisheries Science Center, 2725 Montlake Boulevard East, Seattle, WA 98112 United States
³Romberg Tiburon Center for Environmental Studes, San Francisco State University, 3152 Paradise Drive, Tiburon, CA 94920 United States

Recent studies suggest that the Juan de Fuca Eddy, a seasonal nutrient-rich retentive feature off the Washington and British Columbia coasts, may be an initiation site for toxic Pseudo-nitzschia blooms that impact the Washington state razor clam fishery. The ECOHAB PNW project, which aims to understand the physiology, toxicology, ecology and oceanography of toxic Pseudo-nitzschia species off the Pacific Northwest coast, has conducted five 3-week long cruises in the summers of 2003 through 2005. One of these surveys (September 2004) coincided with the highest levels of Pseudo-nitzschia cells and domoic acid ever measured in this region. Through an examination of patterns and differences in biochemical water properties and circulation among the five surveys to date we illustrate factors that may be important to the development and sustenance of large, toxic blooms within the eddy. Surveys were conducted in June/July and September of 2003 and 2005 when local winds were predominantly upwelling favorable and in September 2004 when local winds were from the north or downwelling favorable. Macronutrient supply to the eddy is shown to be much more persistent than to the inner shelf--it is governed by estuarine forcing rather than by wind-driven upwelling. To examine variability in circulation in the eddy region, a simple diagnostic model has been utilized for the hindcasting of flow fields. Results show a more developed Juan de Fuca eddy later in the season contributing to a broader region of high nutrients off the mouth of the Juan de Fuca strait. During periods of upwelling winds, surface waters leave the eddy at the southern edge and join the Washington shelf upwelling jet. Under intermittent upwelling/downwelling conditions (September 2004), a much tighter recirculation is observed in the eddy and residence time increases dramatically.