Modeling residence time with a three-dimensional hydrodynamic model: Linkage with chlorophyll a in a subtropical estuary

Residence time calculated with a three-dimensional hydrodynamic model was analyzed together with field measurements of chlorophyll a collected through long-term monitoring programs and synoptic-scale water quality mapping in the Caloosahatchee Estuary located in Florida, USA. Freshwater inflows to the estuary are highly managed at the head water control structure. A total of 14 freshwater inflow rates ranging from 0 to 283 m3 s−1 were simulated to represent the whole spectrum of hydrologic and water management conditions. Residence time, reported as the e-folding time, ranged from a few days to more than 60 days, depending on the magnitude of freshwater discharge and location in the estuary. The spatial heterogeneity of residence time indicated that there existed a zone in the estuary where water parcels reside for a longer period of time than in other areas of the estuary. The location of this “residence time maximum” zone progressed further toward the mouth of the estuary with increasing freshwater discharge. The location of peak chlorophyll a concentration along the longitudinal axis of the estuary as measured through long-term monitoring and synoptic water quality mapping, also fluctuated with freshwater discharge and coincided with the zone of maximum residence time. The results confirmed the fundamental role of freshwater inflow in the control of residence time and accumulation of phytoplankton biomass. The study helps elucidate how and where phytoplankton respond to freshwater inflows at the head of the estuary.