Nearshore circulation during upwelling inferred from the distribution of dissolved cadmium off the Oregon coast

The effectiveness of dissolved cadmium (Cd) as a tracer of coastal upwelling is demonstrated by comparing its water column distribution to that of traditional upwelling indicators, in particular phosphate (P) and salinity, as well as equatorward wind forcing at the Oregon coast. Cd and P concentrations in samples collected on board ship on 17–27 August 1995 between 3 and 100 km from the coast offshore of Cape Blanco, Oregon, ranged between 0.2 and 0.9 nmol kg21 and ,0.1 and 2.5 mmol kg21, respectively. Both tracers behaved conservatively during onshore transport at depth; linear Cd-salinity and P-salinity relations in offshore source waters were preserved in bottom waters over the shelf. Following upwelling to the surface, however, Cd and P distributions diverged, with Cd remaining essentially constant while P was taken up by phytoplankton, predominantly diatoms. The Cd content of nearshore water collected from the beach inshore of the cruise area was very similar to that of upwelling source waters sampled from the ship, suggesting efficient exchange of waters between the shelf bottom layer and the very nearshore region, including the surfzone. A simple box model, in which onshore Ekman transport occurs through a well-mixed bottom layer extending from the edge of the continental shelf to the very nearshore region, illustrates a close link between upwelling-favorable wind forcing and very nearshore Cd concentrations. The close agreement between Cd time series generated by the model and surfzone Cd sampled every 2–4 weeks during 1994–1996 suggests that the effectiveness of Cd as a tracer of wind-driven coastal upwelling off the Oregon coast is due in large part to conveyor-like circulation and the remarkably efficient mixing of newly upwelled water into the very nearshore region. Eastern boundary currents such as the California Current system are among the most productive waters in the world ocean (Ryther and Dunstan 1969; Chavez and Toggweiler 1995). While a complex set of factors regulate productivity in the California Current (Abbott and Barksdale 1991; Chavez et al. 1991; Mann 1993), the supply of nutrient-rich water by coastal upwelling undoubtedly plays an important role. Coastal upwelling is particularly intense at midlatitudes along the northwestern U.S. coast during spring and summer because the gradient between warm air temperatures (low atmospheric pressure) over the central valley of California and comparatively cool temperatures (high atmospheric pressure) over the coastal ocean drives strong equatorward upwelling-favorable winds (Huyer 1983; Beardsley et al. 1987; Bakun and Nelson 1991). By using the trace element cadmium as a tracer of nearshore circulation, the present study provides a new perspective on the relation between