Apalachicola Bay a shallow, microtidal, multiple inlet estuary along Florida’s coastline in the northeastern Gulf of Mexico, is an economically and ecologically important estuary.  Historically, the bay has provided 90% of the oyster harvest for the state of Florida and 10% of the oyster harvest for the United States, and also serves as nursery habitat for commercially and recreationally important finfish.  A series of droughts in recent decades, combined with overharvest of oysters, has led to a serious decline in the oyster population with potential ramifications to the ecosystem health and regional fisheries.  To support ongoing and future activities aimed at restoring oyster populations within the bay, a high-resolution hydrodynamic model of the bay and surrounding northeastern Gulf of Mexico coastal waters has been developed using the Finite Volume Coastal Ocean Model (FVCOM).  The model is run for historical and altered climate and water management scenarios, with particular focus on low-flow periods.  Comparisons of the simulated salinity to observations show that, although the model simulates well the salinity variability during years of normal rainfall, the model tends to have a low bias in its simulated salinity during drought years.  Analysis of model experiments reveals that reducing the freshwater input from the Apalachicola River based on its measured discharge at the observing station closest to the bay alone is not sufficient to raise the bay’s salinity to the observed range.  As the hydrodynamic model output is used to drive individual-based oyster larvae simulations, the low salinity bias results in erroneous estimates of larval survival with the model not reproducing the larval mortality that likely leads to the decline in oyster populations.  Thus, additional physical mechanisms are found to be of increased importance for accurate simulation of the bay’s salinity during drought regimes compared to times of normal hydrological conditions.  Model experiments reveal three such factors whose importance is enhanced during low-flow conditions are: high-frequency (diurnal) wind variability, local evaporation and precipitation deficit, and newly understood and measured diversions of the Apalachicola River flow downstream of its closest measurement gauge.  Including these factors results in a dramatic improvement of the hydrodynamic simulation and corresponding larval simulation.

Primary Presenter: Steven Morey, Florida A & M University (steven.morey@famu.edu)

Authors:

Xu Chen, Florida State University  (xchen@coaps.fsu.edu)