The asymmetric distribution of chlorophyll associated with a coastal upwelling center

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.