Effect of freshwater on the long-term salt balance in the Northern and Central Indian River Lagoon (Florida)

Effect of freshwater discharge to the long-term salt balance in the Northern and Central Indian River Lagoon (IRL) is successfully simulated by a newly-developed analytical solution to a water balance-based one-dimensional salt conservation equation. The water balance calculation shows that fresh groundwater discharge is the primary factor, with surface runoff from gauged and ungaged areas as the second freshwater contributor. Precipitation and evaporation are almost in balance for the entire IRL. Due to high freshwater discharge from groundwater, the annual net flow is outward from the IRL to the continental shelf of the Atlantic Ocean, resulting in a relatively short flushing time, denoted as T_0.5 (50% flushing time) and T_0.99 (99% flushing time). T_0.5 and T_0.99 without a tidal effect in the Northern IRL are 17 and 114 days, respectively, during the dry season. During the wet season, they are 10 and 65 days, respectively. Tidal flushing effects are considered in central IRL due to the proximity to Sebastian Inlet.; Longitudinal salinity features distinctively occur within each four-segmented zones in the study area. The urbanized Northern Central IRL releases freshwater through surface runoff even after wet season (June to October). A salinity minimum occurs in October, and continues until next April. This continuous surface runoff causes dry season salinity to be lower than the wet season salinity by about 2 psu. The annual net flow of the less urbanized Northern IRL is outward through the Haulover Canal into the Mosquito Lagoon, which in turn provides salt to the Northern IRL by inward saline water movement during April to September. This saline water is gradually diluted by freshwater from groundwater and surface runoff. Sebastian Inlet provides salts to the Northern and Southern Central IRL. The upper region of the highly urbanized Northern Central zone and lower region of the less urbanized Northern zone show a minimal salinity level of about 18 psu due to their urbanization and remoteness from both Haulover Canal and Sebastian Inlet.; Whereas the Northern Central zone is affected by strong freshwater, resulting in small salt diffusivity of order of 0.29 × 106 cm2/sec for the dry season and of 0.65 × 106 cm2/ sec for the wet season, the Southern Central zone is controlled by high diffusivity of order of 3 × 106 cm 2/sec for the dry season and of 0.7 × 10 6 cm2/sec for the wet season. However, salinity drops rapidly down to about 22 psu in the region of the Southern Central IRL farthest from Sebastian Inlet. The high freshwater discharge during wet season decreases the diffusion coefficients for the dry season except for the Northern Central zone.; Sensitivity tests show that the salinity levels drop abruptly even during the dry season due to the high surface runoff discharge caused by tropical storms, depressions, and passages of cold front. (Abstract shortened by UMI.)...