Overview
Dissolved oxygen (DO) levels in estuarine bottom waters are a critical indicator of ecosystem health, with low-oxygen conditions posing significant risks to aquatic life. The Caloosahatchee River Estuary in southwestern Florida is subject to a complex interplay of river inflow, tidal exchange, and wind forcing — all of which influence the distribution of DO, salinity, and temperature throughout the system.
This study applied the Environmental Fluid Dynamics Code (EFDC) to simulate the hydrodynamics and eutrophication dynamics of the estuary, with the goal of quantifying the relative influence of each physical forcing mechanism on bottom-water DO.
Model Setup
The EFDC model was configured to simulate the full extent of the Caloosahatchee River Estuary, incorporating three-dimensional hydrodynamics coupled with a eutrophication module. Boundary conditions included freshwater inflows from the S-79 structure, tidal forcing from Charlotte Harbor, and spatially varying wind fields. Calibration and validation were performed against observed DO, salinity, and water temperature data from the Florida Department of Environmental Protection and the South Florida Water Management District.
Key Findings
Modeled results showed good agreement with field observations across the estuary. Sensitivity analyses revealed distinct roles for each physical forcing mechanism:
- Wind forcing increased vertical mixing throughout the water column, elevating bottom-layer DO concentrations.
- River discharge enhanced stratification in the deeper sections of the upper estuary, reducing vertical mixing and bottom DO — while simultaneously driving vertical mixing in the shallower upstream reaches.
- Tidal forcing exerted a strong influence on bottom-layer DO concentrations throughout the entire estuary, underscoring the role of tidal exchange in regulating estuarine water quality.
These findings provide a mechanistic basis for evaluating management strategies that target freshwater delivery and flow regulation in the Caloosahatchee system.