A LAGRANGIAN VIEW OF THE JUAN DE FUCA EDDY: MACRONUTRIENTS AND CIRCULATION

Barbara M. Hickey¹, William P. Cochlan², Vera L. Trainer³, Evelyn Lessard¹, and Amy MacFadyen¹

¹School of Oceanography, University of Washington, Seattle, WA 98195, USA
²Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA 94920, USA
³NOAA Fisheries, Northwest Fisheries Science Center, Seattle, WA 98112, USA

A number of surface drifters were deployed in the Juan de Fuca eddy during ECOHAB PNW cruises to study transport pathways and to allow shipboard measurements following a patch of water. In September 2004, three drifters were followed for periods of more than a week, with shipboard profiles of biological/chemical/physical water properties measured as frequently as 3 hr intervals. During this cruise, the bloom of Pseudo-nitzschia in the Juan de Fuca eddy was highly toxic, containing up to 15 million cells per liter of P. cuspidata. One of the drifters was followed as it entered, transited and exited the eddy region, and the water mass that it tracked was sampled for a period of ten days; another drifter remained in the eddy for 21 days, clearly illustrating the retentive nature of the eddy. Particulate domoic acid and chlorophyll increased and then decreased following the drifter pathways.

Comparisons between nitrate and temperature or salinity data distinguish the biological versus physical control of the nutrient supply. For example, differences in trends in temperature and nitrate in shallow layers versus deeper layers provide a clear illustration of biological utilization in surface layers versus re-supply at deeper layers during the first few days of the drift. The data demonstrate that the Juan de Fuca eddy has several physical mechanisms for providing nitrate to sustain offshore blooms-these processes distinguish the eddy region from the coast and help to sustain the toxic blooms observed in the eddy. Internal wave activity is accentuated in this region by the complex topography and the pycnocline is shoaled over the eddy throughout the summer season. Accordingly, vertical motion due to internal wave activity in the upper 50 m causes large variability in all data series at a fixed depth, with colder, more saline water associated with higher nitrate, chlorophyll-depleted water. The upward movement of the high nitrate waters, combined with wind mixing could provide a mechanism to re-supply nutrients to near surface layers in the eddy region. The time series also elucidated another important mechanism for re-supplying nutrients to the upper water column-cooling at the surface followed by vertical mixing, a process not usually considered important in coastal upwelling systems. This cooling occurs only during periods of downwelling favorable winds, generally associated with colder air temperatures in this region in summer.