Vertical transport of buoyant microplastic particles in the ocean: The role of turbulence and biofouling

This study investigates the interactions between turbulence and biofouling and their influence on the vertical transport of buoyant microplastic particles in a marine environment. We explore the sinking characteristics for a range of particle densities and sizes, focusing on comparing laminar and turbulent flows with diffusivity profiles typical of the North Pacific Ocean. The results show the existence of three flow regimes based on the relative importance between turbulent fluctuations and biofilm growth. The biofouling process determines the vertical motion of microplastic particles of sizes in the millimeter range. In contrast, particles in the micrometer range are found to follow flow trajectories without any significant influence from biofouling. We observe that turbulence, on average, promotes the beginning of the vertical particle settling; for example, a high-density polyethylene particle of 1 mm in size has an average settling onset of 10 days in the presence of turbulence, while in its absence, this occurs in 19 days. We also show that turbulence causes buoyant microplastic particles smaller than 0.1 mm to spend their entire lifespan underwater. Finally, the probability distributions for particle size after 100 days in the ocean reveal that particle density strongly influences the biofilm thickness for particles larger than 10μm. We will discuss the implications of these results for tracking the motion of microplastic particles in large-scale regional or global numerical models.