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 availability 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 h v e r Plume), and regressed daily productivity ( P (mg C m-2 d-') against the composite parameter B Z, I,. Regressions for each estuary were significant and typically over 80 % of the variation in 5 P was correlated with variations in B Z, I,. Moreover, the pooled data (n211) from 4 estuaries where methodologies were comparable fell along one regression line (r = 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.