Influence of Near-Surface Currents on the Global Dispersal of Marine Microplastic

Buoyant microplastic in the ocean can be submerged to deeper layers through biofouling and the consequent loss of buoyancy or by wind-induced turbulent mixing at the ocean surface. Yet the fact that particles in deeper layers are transported by currents that are different from those at the surface has not been explored so far. We compute 10-year trajectories of 1 million virtual particles with the Parcels framework for different particle advection scenarios to investigate the effect of near-surface currents on global particle dispersal. We simulate the global-scale transport of passive microplastic for (i) particles constrained to different depths from the surface to 120-m depth, (ii) particles that are randomly displaced in the vertical with uniform distribution, (iii) particles subject to surface mixing, and (iv) for a 3-D passive advection model. Our results show that the so called garbage patches become more leaky in deeper layers and completely disappear at about 60-m depth. At the same time, subsurface currents can transport significant amounts of microplastic from subtropical and subpolar regions to polar regions, providing a possible mechanism to explain why plastic is found in these remote areas. Finally, we show that the final distribution in the surface turbulent mixing scenario with particle rise speed w(r) = 0.003 m/s is very similar to the distribution of plastic at the surface. This demonstrates that it is not necessary to incorporate surface mixing for global long-term simulations, although this might change on more local scales and for particles with lower rise speeds.