Identifying plastic accumulation zones in coastal seas: The Roatan Island case study

The issue of plastic in the marine environment is a growing concern. Images of floating plastic island are often found in mainstream press publications. In this study, we focus on such an episode that occurred in October 2017 near the island of Roatan in the Gulf of Honduras. Our objective is to find out if Lyapunov Exponents, a type of Lagrangian Coherent Structure diagnostic can characterize such plastic accumulations. We use routinely available weather and ocean model data as inputs to a Lagrangian drift model. With the drift simulation outputs we compute the Lyapunov exponents. We perform sensitivity studies by varying the separation ratio value, the wind coefficient, the start date and the simulation duration to investigate the ability of the Lyapunov exponents to provide useful information. We find that with the right settings, Lyapunov Exponents can indeed highlight areas of plastic accumulation and their transient nature in coastal seas.