Vertical distribution of microplastics in upper-ocean turbulence: Laboratory modelling

, Poulain-Zarcos Marie, Mercier Matthieu, Ter Halle Alexandra.

Global estimation for plastic waste in the ocean is about 200,000 pieces/km2, which represents only 1% of annual plastic inputs (Jambeck et al., 2015). Amongst the several explanations that could justify this discrepancy, the precision of current modelling tools could be one, with improvements required to better reproduce plastic concentrations at sea surface (Law et al., 2014; Brach et al., 2018) or in depth. Models for the vertical transport are based on a balance between the upward buoyant flux and the downward turbulent one due to waves and wind (Kukulka et al., 2012). Our aim is to improve these models taking into account the particles shape and size (Poulain et al., 2019), the surface turbulence properties and the fluid-particle coupling. We present here an experimental approach focused on the dynamics of nearly-buoyant particles, of different sizes (from 0.5mm to 6mm), dispersed in a turbulent flow induced by a vertically-oscillating grid (whom intensity decreases with depth). First, we characterize the turbulence, using PIV measurements, to estimate the depth profile of turbulent quantities, to ultimately get the eddy viscosity profile. Second, we study the dynamics and the transport of nearly buoyant particles in such flow. We then compare our results with previous studies on tracers (Kukulka et al., 2012) and heavy particles (Michallet & Mory, 2004); and discuss how to extend our results to global plastic pollution.

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