California Sea Grant College Program

We measured suspended sediment and chlorophyll-a fluxes between San Francisco Bay and the coastal ocean for two days in March 2002, October/November 2002, and June 2003, one day during neap tide and one during spring tide. We applied a harmonic analysis to velocity and chlorophyll-a data to model scalar and velocity fields during a spring-neap cycle. We then integrated these modeled data over the fortnightly period to calculate net dispersive fluxes. The net flux consisted of an advective and a dispersive component. Dispersive flux was decomposed into physical mechanisms such as tidal pumping, steady circulation and unsteady circulation.

Net flux of both sediment and chlorophyll changed seasonally. The net chlorophyll flux was out of the bay during spring and fall, but in during summer. The net flux of suspended sediment was large and in during the fall and out during spring and summer. The direction of advective flux was always out of the estuary and the magnitude depended on advective speed and mean scalar concentration. Dispersive flux was of similar magnitude as advective flux each season and changed direction seasonally. The dispersive flux was larger than the advective flux, contributing over 63% to the net flux of both scalars across season. Tidal pumping was the dominant dispersive process year round.

The dominance of tidal pumping implies that seasonal variability of ocean-estuary exchange is set almost entirely by variation in the east-west gradient of scalar concentrations between the ocean and the estuary. If concentrations are higher during flood tide compared to ebb tide, the tidal pumping flux will be into the estuary, whereas the converse is true if the concentration is higher on ebb tide.

The average chlorophyll and sediment concentrations are governed by different processes. During the summer while coastal upwelling occurs, chlorophyll concentration is higher in the ocean than in the estuary creating a gradient driven dispersive flux of coastal phytoplankton into the estuary. The opposite is true during spring when estuarine concentration is higher and the dispersive flux is driven out of the estuary. During fall there were relatively low gradients and the net dispersive chlorophyll flux was relatively small. The seasonal direction of chlorophyll fluxes measured in this study are consistent with physical and biological processes of a typical year, though, the magnitude and timing of these fluxes may change annually or inter-annually depending on the specific physical and biological conditions.

While the spatial and temporal distribution of favorable growth conditions determines the direction of the chlorophyll flux, growth within Central Bay plays a relatively small role in the local chlorophyll balance. Blooms or the accumulation of phytoplankton within Central Bay are limited by the large dispersive transport rates.