Physical Oceanography and HABs in the California Current System

Barbara M. Hickey

School of Oceanography, University of Washington, Box 355351, Seattle, WA 98195, USA
EMAIL: bhickey@u.washington.edu

The California Current System is an Eastern Boundary System of currents, including the equatorward flowing California Current and the poleward flowing California Undercurrent. The Undercurrent, which hugs the upper continental slope, is the primary source of the nutrient rich water upwelling to the coastal marine ecosystem. In this system in the summer and early fall growing season physical processes are large scale; upwelling events occur over distances exceeding several hundred kilometers along the coast. Nevertheless, mesoscale features are important to the plankton and to the occurrence of HABs along the US west coast. Within the California Current System, at least three types of physical environments can be identified; we speculate that these environments have a first order role in the occurrence or non-occurrence of HABs. In the first environment, upwelling occurs on a nearly straight coastline; plankton are swept primarily downstream and slightly offshore as they grow, returning to the coast during periods of downwelling or relaxation. In the second environment, where promontories and headlands occur, upwelling jets break free of the shelf, delivering plankton to regions up to a few hundred kilometers offshore, rarely returning plankton to the coast. In the third environment, local topographic irregularities such as banks or canyons alter the flow in such a way as to retain plankton longer in the coastal environment than in either of the other environments, often providing as well a higher or more continuous source of nutrients. In ECOHAB PNW, two of these environments are being contrasted; namely straight coast upwelling and a mesoscale bank. Results to date suggest that macronutrients are indeed provided more continuously to the bank, by virtue of estuarine-driven upwelling into the nearby Strait of Juan de Fuca. Moreover, our results suggest that a nearby river plume (in this case the Columbia) may provide mitigating effects on HABs events, effectively preventing them from impacting the beach at some locations and times. Plumes from rivers may play a role in HABs incidence by this physical mechanism as well as by controlling the supply of micronutrients such as iron to both the sediment and the water column.