A well-developed upwelling plume was observed off the California coast near Point Conception and Point Arguello (∼34°20′N latitude, 120°35′W longitude) during April 1983. The core of the upwelling plume was cool, nutrient-rich, and low in phytoplankton biomass. The distributions of temperature and nutrients were relatively symmetric about the offshore axis of the cool core of the upwelling center. However, the distribution of chlorophyll was asymmetric about the center with the highest abundance observed on the southern (equatorward) edge of the feature. Comparison of the near-surface currents measured by Doppler acoustic profiling, with the distribution of temperature, nutrients, and chlorophyll, suggests that the phytoplankton asymmetry is determined by the horizontal velocity field. High offshore velocities on the poleward side of the center advect the upwelled water offshore before accumulation of biomass can occur. Lower velocities, more variable current direction, and possible shoreward flow on the equatorward side of the upwelling center permit a longer residence time for the upwelled water near the upwelling center allowing accumulation of phytoplankton biomass to occur This study was supported by National Science Foundation grants OCE 8215228 and OCE 8507438 (BI-IJ), OCE 8506468 (KHB), OCE 8511526 (LA), and OCE 8511520 (DB). Contribution no. 422 of the Allan Hancock Foundation, University of Southern California Peer reviewed