An empirical model for estimating phytoplankton productivity in estuaries

We have previously shown that primary productivity in San Francisco Bay, USA, is highly correlated with phytoplankton biomass B (chlorophyll a concentration) and an index of light availabil - ity in the photic zone, Z,I, (photic depth times surface irradiance). To test the generality of this relation, we compiled data from San Francisco Bay and 5 other USA estuarine systems (Neuse and South Rivers, Puget Sound, Delaware Bay and Hudson hver Plume), and regressed daily productivity (P (mg C m-2 d-') against the composite parameter B Z, I,. Regressions for each estuary were signifi- cant and typically over 80 % of the variation in 5 P was correlated with variations in B Z, I,. Moreover, the pooled data (n - 211) from 4 estuaries where methodologies were comparable fell along one regression line (r 2 = 0.82), indicating that primary productivity can be estimated in a diversity of estuarine waters from simple measures of phytoplankton biomass and light availability. This implies that physiological variability (e, g. responses to variations in nutrient availability, temperature, salinity, photoperiod) is a secondary control on phytoplankton production in nutrient-rich estuaries, and that one empirical function can be used to estimate seasonal variations in productivity or to map productivity along estuarine gradients of phytoplankton biomass and turbidity.