Experimental studies of phytoplankton growth and grazing processes were conducted in the California Current Ecosystem off Point Conception, California to test the hypothesis that growth and grazing losses determine, to first order, the local dynamics of phytoplankton in the upwelling circulation.  Eight experiments of 3-5 days each were conducted over the course of two cruises in May-June 2006 and April 2007 following the trajectories of satellite-tracked drifters. We compared directly the net rates of change observed for the ambient phytoplankton community to the net growth rates predicted from experimental determinations of process rates.  The resulting relationship accounted for >90% of the variability observed, providing strong support for the growth-grazing hypothesis.  Grazing by mesozooplankton was unexpectedly high and variable, driving a substantial positive to negative shift in phytoplankton net rate of change between years despite comparable environmental conditions and similar high growth rates and suggesting strong top-down control potential.  The demonstrated agreement between net ambient and experimental community changes is an important point of validation for using field data to parameterize models.  Data sets of this type provide an important source of new information and rate constraints for developing better coupled biological-physical models of upwelling system dynamics.