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

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Abstract
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 < 5 to > 40 mg m-3. The bloom persists for 2-4 wk, and then dissipates. Phytoplankton biomass remains low (chlorophyll a < 5 mg m-3) from May through Dec., although light and nutrient availability are sufficient to sustain growth rates of 1-1.5 divisions d-1 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.