MEPS 337:51-61 (2007)  -  doi:10.3354/meps337051

ABSTRACT: We measured chlorophyll a (chl a) fluxes between San Francisco Bay and the coastal ocean for 2 d in March 2002, October and November 2002, and June 2003; 1 d during neap tide and 1 d during spring tide. We applied harmonic analysis to velocity and chl a data to model scalar and velocity fields during a spring–neap cycle. We then integrated these data over the fortnightly period to calculate net dispersive fluxes. The net flux consisted of an advective and dispersive component. Dispersive flux was decomposed into physical mechanisms such as tidal pumping, steady circulation and unsteady circulation. Net flux was large and directed out of San Francisco Bay during spring, large and into the estuary during summer, and effectively zero during fall surveys. The direction of advective flux was always out of the estuary and the magnitude depended on advective speed and mean chl a concentration. Dispersive flux was of a similar magnitude to advective flux each season and changed direction seasonally. Based on historical records and simultaneous observations, we conclude the reversal of the dispersive flux is most likely due to difference in phytoplankton growth conditions (or difference in timing of blooms) in the coastal ocean and estuary. During the spring, phytoplankton bloom in the estuary, creating a net seaward flux. In summer, during upwelling, phytoplankton bloom in the coastal ocean, driving a net flux into the estuary. Tidal pumping accounted for 79% of spring, 63% of fall and 93% of summer dispersive flux. Steady fluxes were about 1 order of magnitude smaller than tidal pumping, and unsteady fluxes yet another 1 order of magnitude smaller. The dominance of tidal pumping implies that seasonal variability of ocean– estuary exchange is set almost entirely by variation in the gradient of chl a concentrations between the ocean and the estuary such that the variability of ocean-estuary exchange is set by variation in the occurrence of estuarine and oceanic blooms.

KEY WORDS: Chlorophyll a physical transport · Phytoplankton ecology · San Francisco Bay · Statistical analysis