Sea-level rise impacts on longitudinal salinity for a low-gradient estuarine system

Salinity response to sea-level rise is evaluated for a low-gradient, tidally active estuary, the lower St. Johns River, Florida. A high-resolution numerical model is forced by continuous data of water levels and freshwater inflows for the offshore and upstream boundaries, respectively. The modeling approach is configured for salinity simulation over a 10-year record, 1997–2007, and validated at four salinity-gauging stations inside the river. The initial condition of salinity field was found to be a critical factor in the numerical simulation. Adjustments in the initial salinity condition of ± 10% required 6–9 months for the model salinity solution to dynamically equilibrate with the applied boundary conditions. Model predictions of salinity response to sea-level rise of 0.05, 0.15, and 0.30 m were diagnosed in terms of salinity change. Salinity was found to increase over the entire river, regardless of the magnitude of sea-level rise. Linear rates of salinity increase were predicted as high as 6 ppt m−1 inside the river. The change in salinity was nonuniform throughout the system and exhibited a moderate-to-strong nonlinear component. The results uncover a hotspot in the river where salinity was predicted to increase as much as ~ 2.3 ppt due to the nonlinear system response to sea-level rise.