Significance of biomass and light availability to phytoplankton productivity in San Francisco Bay

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Abstract
Primary productivity was measured monthly at 6 sites within San Francisco Bay, 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 95-150 g C m-2, and was highest in regions of lowest turbidity. Daily photic zone net productivity PNpd ranged from 0.05-2.2 g C m-2 d-1, and was significantly correlated with the composite parameter BIo/.epsilon. (where B = phytoplankton biomass; Io = daily surface insolation; .epsilon. = attenuation coefficient). Linear regression of PNpd against B Io/.epsilon. 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. Annual water-column net productivity PNwy was calculated as a fraction of annual gross productivity PGy. The ratio PNwy:PGy was inversely related to the ratio of water depth H to annual mean photic depth .hivin.Zp. This linear relation indicates that the water column of San Francisco Bay is a net photosynthetic source of organic C only when the ratio H:.hivin.Zp < 6. In deep turbid habitats, where H: .hivin.Zp > 6, respiratory loss exceeds productivity. Two empirical formulations allow productivity to be estimated over the photic zoine and water column from simple properties that are easily measured.