Aspects of the physical control of phytoplankton dynamics over the Southern California Bight continental shelf

Evidence gathered three decades ago showed persistently elevated total and new primary production over the continental shelf of the Southern California Bight. This dissertation examines the mechanisms that drive the flux of nitrate necessary to support the phytoplankton productivity over the shelf. Using results gathered from an intensive field experiment, I show that strong, persistent cross-shelf gradients in phytoplankton standing stock, primary productivity and community composition result from nitrate flux due to internal waves of tidal and higher frequency. I report the first estimates of the horizontal flux of nitrate due to the internal tide, and demonstrate that it is quantitatively similar to the nitrate demand of the inner shelf phytoplankton population. The local winds, contrary to expectations based on previous research, support a biologically- relevant tilt of the nitracline. Variability in the magnitude of the cross-shore shoaling of the nitracline in turn impacts the magnitude of the internal tide mediated nitrate flux. Remotely-forced large scale variability in the offshore depth of the nitracline controls the continental shelf ǹitrate climate.' During episodes of anomalously warm ocean temperatures, a correspondingly deep nitracline can shut off nitrate supply to the continental shelf, resulting in low phytoplankton biomass. During more typical conditions, the phytoplankton community over the inner shelf is dominated by taxa capable of rapid nitrate assimilation, while offshore waters have a proportionally greater amount of oligotrophic species. The results of this dissertation indicate that relatively small changes in the offshore depth of the nitracline could account for the long-term increase primary productivity and phytoplankton biomass noted by recent studies, even in the absence of increased local wind-forced upwelling. Finally, I examine the baroclinic semidiurnal variability in currents and water column structure in the shallow water of the inner shelf. The results of this analysis demonstrate that the behavior of the semidiurnal variability is not consistent with a propagating or standing low mode internal wave. Analyses of linearized momentum balances suggest that the semidiurnal variability may result from the interaction of barotropic tidal flow, friction in the bottom boundary layer, and rotation