Modelling the sedimentation of macro-, micro- and nanoplastics in the ocean from surface to sediment

The ocean is considered a sink for plastic waste, with buoyant plastics remaining at the surface for up to 50 years. Removal of plastic waste from the ocean by manpower is not feasible, but natural-based removal strategies are promising. Among these, the biological pump stands out as a crucial process responsible for transferring materials and nutrients from the surface to the deep ocean. In this context, plastic particles can be incorporated into and transported with marine snow aggregates, providing a potential mechanism for removing plastic waste from the upper ocean (human food chain) to the deep ocean. However, the longevity of plastic debris at the ocean surface remains poorly understood. To address this gap, we developed a degradation-aggregation model to predict the longevity of different types, sizes and shapes of buoyant plastics at the ocean surface. Our results show that the longevity of plastic debris is primarily determined by the time it takes for plastic debris to degrade sufficiently and become small enough (< 100 µm) to be incorporated into marine snow aggregates. Larger plastics take decades to degrade into small microplastics that can be trapped in marine snow, prolonging their presence in surface waters. Conversely, once microplastics are small enough to be incorporated into marine snow, they can reach the seafloor sediment within two years. Interestingly, our model highlights that vertical settling of microplastics occurs through multiple incorporation and settling via vector transport of marine snow, which differs from previous studies reporting oscillations of microplastics in the ocean subsurface. This study explains the mechanisms of plastic debris removal by the biological pump and the longevity of plastic debris at the ocean surface. In addition, this theoretical model can be extended to different aquatic ecosystems to predict the fate and longevity of plastic debris in different environments.