Riverine runoff has been recognized as one of the most important routes for transporting plastic debris from terrestrial environments to the global ocean.  We developed an integrated modeling framework using Human Development Index (HDI) as the main predictor.  We further estimated the country-specific riverine plastic outflows from 161 countries.  Although India, China, and Indonesia are the largest contributors to riverine plastic outflows in the world, their per capita contributions rank only 23rd, 79th, and 13th, respectively.  Once entering the oceans, plastic debris may undergo a variety of processes and eventually deposit in deep seas, which unfortunately have remained poorly understood.  Laboratory simulations were also proved fruitless due to the complex conditions in reality.  Previous modeling efforts suggested that plastic debris only oscillate within certain depths of the water column, which was contradicted by recent field measurements on the vertical profiles of plastics in deep seas.  Our new model, which considers previously overlooked calcite precipitation accompanied with biofouling on plastic surfaces and a new motion equation for irregular particles under fluid drag, predicts that plastic debris can distribute at all depths along the water column, but ultimately deposit at the seabed and twilight zones.  Under similar environmental conditions, the simulated vertical distribution patterns of plastic debris are consistent with recent field measurements in the Pacific Ocean and the Indian Ocean.  These results highlight the importance of the seafloor as the final fate of plastic debris.

microplastics,transport,behavior