Factors Controlling Circulation at the Terminus of a Shallow Seagrass-Dominated Estuary, Fort DeSoto Bay, West-Central Florida

Fort DeSoto Bay is a shallow seagrass-dominated estuary located landward of Mullet Key that is home to the most extensive seagrass beds in all of Tampa Bay. During a period of development in the 1950s and early 1960s, the bay underwent extensive physical modifications, including the construction of earthen causeways and dredged channels, which altered the bathymetry of the seafloor and changed natural circulation patterns. The stagnation of water in the southern bay and increased estuarine residence time led to elevated summer temperatures, reduced dissolved oxygen levels, and die-offs of seagrasses. Two 40-foot circulation bridges were installed on each causeway in 2004 and 2016 to restore hydrological connectivity between the estuarine cells, improve water quality, and improve ecosystem health. The purpose of this study is to evaluate the influence of dredged channels, causeways, circulation bridges, and seagrass beds on circulation patterns at the Fort DeSoto Bay using a calibrated and verified numerical model, the Coastal Modeling System (CMS). The model was constructed and calibrated using in-situ velocity, tide fluctuation, and bathymetric data collected by the USF Coastal Research Laboratory. A series of simulated scenarios were created within the numerical model, and compared with a baseline scenario (i.e. the existing circulation regime) to determine the influence of natural and anthropogenic factors on circulation patterns. The scenarios were designed to simulate predevelopment, post-development, and post-restoration conditions to understand the evolution of flow regimes in a heavily modified estuary. Modeling results show that the natural circulation patterns follow a north-south trajectory as water enters the bay from Bunces Pass, and an east-west trajectory along the southern coast of the estuary where tidal flow encounters the southern boundary of Mullet Key. Overall, water flows in a counterclockwise direction during the flood tide, and a clockwise direction during the ebb tide. Causeway construction in the late 1950s disrupted the natural east-west flow and reduced current velocity within the seagrass beds in the southern estuary by up to 76%. Following construction of the circulation bridges, velocity in the previously stagnant seagrass beds increased by up to 226%. Tidal prism analysis revealed that up to 26% of the tidal prism in the lower half of the bay passes through the bridges during a spring flood-tidal cycle. Thus, the bridges significantly improved tidal flushing between the estuarine cells; especially the eastern bridge, which was found to have a larger area of influence than the western bridge. The differential frictional and geometric characteristics of the dredged channels and seagrass beds were also accounted for in the numerical model. The linear mostly north-south trending and unvegetated dredged channels serve as conduits that facilitate penetration of tidal currents into the southern reaches of the bay, leading to significantly higher current velocity in the dredged channels and corresponding reduced velocity over the adjacent seagrass beds. The channels allow for more efficient flushing during the ebb tide