Sinking microplastics in the water column: simulations in the Mediterranean Sea

Abstract

Abstract. We\xa0study the vertical dispersion and distribution of negatively\nbuoyant rigid microplastics within a realistic circulation\nmodel of the Mediterranean sea. We first propose an equation\ndescribing their idealized dynamics. In that framework, we\nevaluate the importance of some relevant physical effects (inertia, Coriolis force, small-scale turbulence and\nvariable seawater density), and we bound the relative error of\nsimplifying the dynamics to a constant sinking velocity added\nto a large-scale velocity field. We then calculate the amount\nand vertical distribution of microplastic particles on the\nwater column of the open ocean if their release from the sea\nsurface is continuous at rates compatible with observations in\nthe Mediterranean. The vertical distribution is found to be\nalmost uniform with depth for the majority of our\nparameter range. Transient distributions from flash releases\nreveal a non-Gaussian character of the dispersion and various\ndiffusion laws, both normal and anomalous. The origin of these\nbehaviors is explored in terms of horizontal and vertical flow\norganization.