Development of water quality model in the Satilla River Estuary, Georgia

A coupled three-dimensional physical and water quality model was developed for the Satilla River Estuary, Georgia. The physical model is a modified ECOM-si version with inclusion of flooding/draining process over the intertidal salt marsh. The water quality model is a modified WASP5 with inclusion of nutrient fluxes from the bottom sediment layer. The coupled model was driven by tidal forcings at the open boundary in the inner shelf of the South Atlantic Bight (SAB) and real-time river discharge at the upstream of the estuary. The initial condition for salinity was specified using the field measurement data taken along the estuary. The water quality components were assumed as constant values everywhere at the initial, with assumption that the spatial and temporal variations of these variables were caused by physical–biological–chemical interactions under strong tidal mixing environment. The model-predicted concentrations of inorganic nutrients (ammonium, nitrate plus nitrite, and ortho-phosphorus), chlorophyll-a, and dissolved oxygen (DO) in an along-estuary transect were in reasonable agreement with observational data. Process studies suggest that the intertidal salt marsh acts as a main sink for particulate materials and a major consumer of DO. The low DO concentration in the Satilla River Estuary was mainly due to high sediment oxygen demand (SOD) over the intertidal salt marsh. This feature is the nature of the estuarine-salt marsh ecosystem with nothing related to anthropogenic activities. Tidal mixing-induced bottom resuspension process played a critical role in supplying the nutrients in the water column in addition to the nitrification process.