Does the Benthos Control Phytoplankton Biomass in South San Francisco Bay?

South San Francisco Bay, USA, is a shallow coastal embayment that receives large inputs of nutrients (N. P, Si) and small local inputs of freshwater. Phytoplankton dynamics are typically characterized by a spring bloom when surface chlorophyll a increases from 40 mg m-3. The bloom persists for 2 to 4 wk, and then dissipates. Phytoplankton biomass remains low (chlorophyll a < 5 mg m-3) from May through December, although light and nutrient availability are sufficient to sustain growth rates of 1 to 1.5 divisions d-' in the expansive shallows. Transport processes apparently exert a small influence on phytoplankton biomass, and calculated zooplankton grazing accounts for only a small reduction in net rate of phytoplankton population growth in the shallows. However, suspension-feeding bivalves are sufficiently abundant to filter a volume equivalent to the volume of South Bay at least once daily. These observations suggest that grazing by benthos is the primary mechanism controlling phytoplankton biomass during summer and fall.