Modeling Water Residence Time and Connectivity in the Northern Indian River Lagoon

The northern Indian River Lagoon (IRL) has been experiencing severe water quality degradation and phytoplankton blooms in the last decade or so. Two important factors for maintaining high water quality in estuaries are the water residence time and connectivity. Here we used a newly developed high-resolution hydrodynamic model to investigate both factors in this area. Two types of numerical experiments have been carried out: (1) tracer dilution experiments in which an inert tracer was released from selected sub-basins and tracked for 3 months, and (2) particle experiments in which a number of neutrally buoyant particles were released from selected areas and tracked for 1 month. The results indicate that surface winds may drive complex yet coherent residual circulation, including semi-enclosed re-circulation cells, within each subbasin, with the patterns mainly depending on the directions of prevailing winds. These patterns may promote retention of waters and transport barriers within sub-basins, leading to long water residence time with limited connectivity between sub-basins, and spatial heterogeneity of water properties such as salinity, nutrients, organic matter, and dissolved oxygen. Despite their transient nature (weekly to monthly), these dynamic features and associated processes may also significantly affect the development of phytoplankton blooms and marine habitats in this region. On the other hand, water throughout the lagoon was strongly influenced by the water level in the coastal oceans, especially the frontal position of the Gulf Stream. This suggests that water exchanges through the inlets, albeit limited, are important to the water quality and phytoplankton blooms in the lagoon.