How does buoyancy behavior impact microplastic transport in an estuarine environment?

Much is still unknown about the transport behavior of microplastic pollutants within the marine environment, particularly smaller scale coastal systems such as estuaries. Through the use of a Lagrangian particle-tracking model coupled with a validated 3D hydrodynamic model, we examined the transport, pathway and ultimate fate of microplastic particles, both in an idealized estuary and Galveston Bay, Texas, USA. Emphasis was placed on differences based on settling behavior (neutrally versus negatively buoyant), use of random walk for diffusion processes, and release location. For Galveston Bay, settling behavior had a noteworthy impact on both the transport pathway of microplastic particles, as well as overall time spent within the bay. Particles with negative buoyancy were retained approximately seven times longer than those with neutral buoyancy. Negatively buoyant particles also showed a tendency to be dispersed eastward to Trinity Bay through the bottom baroclinic flow, while neutrally buoyant particles took a more direct route along the ship channel to the mouth of the bay. Idealized model simulations suggest impact of settling depends on the vertical mixing strength. For a system with stronger tidal mixing, negatively buoyant particles with small settling velocities may still behave similarly to neutrally buoyant particles, and differences only become apparent for particles that sink rather quickly (> 10 m d^-1). Future sea-level rise or channel deepening tends to flush out neutrally buoyant particles more quickly, while increasing the retention time for negatively buoyant particles. Our results suggest that plastics within estuaries could show substantially different behavior depending on their buoyancy characteristics, highlighting a need to quantify specific settling velocities of plastic pollutants entering the coastal estuarine system.