Modeling phytoplankton productivity in a shallow, microtidal, subtropical estuary

Urban and agricultural development around the Caloosahatchee Estuary on the southwest coast of Florida (USA) has altered the flow and quality of water in the system since the late 1800s. Increasing algal blooms have highlighted these changes and their influence on phytoplankton production. Our primary objectives were to (1) measure phytoplankton production to test a previously developed empirical model based on simple measures of phytoplankton biomass and light availability, (2) assess changes in water quality and phytoplankton community composition for direct and indirect effect on the model, and (3) apply the model to estimate annual primary production in the estuary. This study is the first field test of the Cole & Cloern (1987; Mar Ecol Prog Ser 36:299–305) model in a low latitude subtropical system. Experiments were conducted using integrated water samples collected monthly between February 2009 and February 2010 at 4 sites. Primary production rates, in terms of oxygen evolution, were measured using simulated in situ light:dark bottle incubations in a flow-through raceway. Daily gross primary productivity (GPPd) ranged from 90 to 3121 mg C m−2 d−1 with the annual mean estimated at 346 g C m−2 yr−1. Pooling estimates from all 4 sites revealed a strong linear relationship between GPPd and the BZ p I₀ model predictor (r² = 0.84, p < 0.001). The model tended to overestimate productivity during the dry period when dinoflagellates were dominant and underestimate productivity during the wet period when diatoms dominated. These model deviations indicated the secondary influence of nutrients, salinity, water residence times, and species composition on phytoplankton productivity in the system.