Primary productivity was measured monthly at 6 sites within San Francisco Bay, USA, throughout 1980. The 6 sites were chosen to represent a range of estuarine environments with respect to salinity, phytoplankton community composition, turbidity, and water depth. Annual net production over the photic zone ranged from 95 to 150 g C m-2, and was highest in regions of lowest turbidity. Daily photic zone net productivity PN,, ranged from 0.05 to 2.2 g C m-2 d-', and was significantly correlated with the composite parameter B I,/& (where B = phytoplankton biomass; I, = daily surface insolation; E = attenuation coefficient). Lnear regression of PN,, against B Io/€ indicated that most (82 %) of the spatio-temporal variability in primary productivity within this estuary is explained by variations in light availability and phytoplankton biomass. We also calculated annual water-column net productivity PN, as a fraction of annual gross productivity PGx The ratio PN,, : PG, was inversely related to the ratio of water depth H to annual mean photic depth Z,. This linear relation indicates that the watercolumn of San Francisco Bay is a net photosynthetic source of organic carbon only when the ratio H : Z, < 6. In deep turbid habitats, where H : Tp > 6, respiratory loss exceeds productivity. Thus, 2 empirical formulations allow us to estimate productivity over the photic zone and water column from
simple properties that are easily measured.