A laboratory experiment on the effect of waves on the transport and dispersion of macro, meso, and microplastics in the surf zone

Plastic debris is currently a significant threat to marine and coastal ecosystems. Most previous research focused on the behavior of drifting macro and mesoplastics on global and regional ocean scales. Furthermore, a few more recent studies provide some first insights into the microplastic dispersion in coastal areas. These studies found that waves and wind, as well as the density, size, and shape of microplastics, drive their transport and dispersion in coastal areas; however, they point to the need for a more extensive characterization. This laboratory study assesses the effect of waves and wave-induced currents on the input rate from land to sea and on the cross-shore transport and dispersion of different types of plastic debris, including the macro and mesosizes, in addition to microplastics. A total of 15 types of plastic debris characterized by different sizes, shapes, and densities, including face masks, were analyzed under regular and irregular wave conditions. The results show that the input rates and transport of plastic debris in the marine environment depend on the position they acquire in the water column, which is related to the terminal velocities and the wave steepness. A higher input rate from the beach was found for plastic materials moving closer to the sea bottom and under less steep wave conditions, as these conditions allow items to escape from coastal entrapment. Furthermore, greater onshore transport was observed for plastic debris that showed greater buoyancy under steeper wave conditions. Regarding the cross-shore distribution, the heaviest plastic debris that managed to be transported accumulated in the breaking zone, while the buoyant elements showed a predominant accumulation closer to the shoreline.