Using lagrangian coherent structures to study coastal water quality

The accumulation of pollutants near the shoreline can result in low quality coastal water with negative effects on human health. To understand the role of mixing by tidal flows in the coastal water quality of four beaches in Biscayne Bay, Miami, Florida, we reveal Lagrangian Coherent Structures (LCSs), i.e., distinguished material curves which shape global mixing patterns and thus act as skeletons of the Lagrangian circulation. This is done using the recently developed geodesic theory of transport barriers. Hobie Beach and Crandon Park Beach are recreational, subtropical marine beaches, characterized by high microbial levels. Possible sources of pollution include human bather shedding, runoff, sand efflux at high tides, and dog fecal matter in the case of Hobie Beach. Virginia Key Beach and Bill Baggs State Park are two other popular beaches in the area. While the four beaches are within 9 km of each other, the latter two have no problems with water quality. Consistent with the patterns formed by satellite-tracked drifter trajectories, designed for this purpose, the LCSs extracted from simulated currents reveal a Lagrangian circulation favoring either the retention or release of pollutants released along the shoreline.