Technical note: On the importance of a three-dimensional approach for modelling the transport of neustic microplastics

Abstract

Understanding and estimating the distribution and transport of microplastics in marine environments has been recognized as a major global research issue. Most of the existing research on transport modelling has focused on lowdensity particles floating in surface waters, using a 2-D Lagrangian approach and ignoring the vertical displacement of particles. In this work, we evaluate to what extent the vertical movement of particles within surface waters by mixing processes may affect the horizontal transport and fate of microplastics. The aim is to determine whether a 2-D approach is sufficient for the accurate modelling of neusticmicroplastics transport or whether a 3-D approach is necessary. For this purpose, we compare visually and statistically the microplastics transport patterns of three simulations in a coastal system: one using a 2-D approach; and two using a 3-D approach with weak and strong vertical turbulence, respectively. The 2-D simulation roughly reproduced the transport and accumulation patterns, but accurate results required a 3-D approach. This was particularly important for strong vertical turbulence and regions characterized by strong vertical current shear. Moreover, a 2-D approach can lead to errors in the results even with negligible turbulence due to simplifications in the velocity field. A 3-D modelling approach is therefore key to an accurate estimation and prediction of microplastics distribution in coastal systems and consequently for planning mitigation and cleaning programmes.Published by Copernicus Publications on behalf of the European Geosciences Union.

Resolution dependency of sinking Lagrangian particles in ocean general circulation models

“…They serve as a primary mode of carbon export out of the exogenic carbon pool and deliver sediment to the world ocean floor: An important archive for understanding the climate system. The lateral advection of the sinking particles by ocean currents complicates the estimation of downward particle fluxes captured by sediment traps [ 1 ], the paleoceanographic reconstructions based on sedimentary microplankton distributions [ 2 – 5 ], and the estimation of micro-plastic distributions in the ocean [ 6 ]. Initially buoyant micro-plastic in the ocean sinks when it gets biofouled and its density increases [ 7 ], meaning that a large fraction of the plastic in the ocean has already sunk to the ocean floor [ 8 ].…”

Microplastics in a salt-wedge estuary: Vertical structure and tidal dynamics

“…For the water compartment, experimental and numerical approaches are generally limited to floating microplastics, such as surface sampling and 2D Lagrangian particle-tracking models coupled with ocean circulation models (Isobe et al, 2009;Kako et al, 2010;Lebreton et al, 2012;Murray et al, 2018;Neumann et al, 2014;Sherman and Van Sebille, 2016), assuming that most of the microplastics load is floating (Mani et al, 2015;McCormick et al, 2016;Yonkos et al, 2014), and focusing on the longitudinal spread of the plastics load from cities and sewage plants (Dris et al, the open ocean where most heavy particles would have sunk well beyond the resuspension (closure) depth. However, the vertical structure of the plastics load can certainly not be ignored in coastal and estuarine environments where the hydrodynamics is generally able to maintain in suspension sediments which are heavier than typical polymers (Forsberg et al, 2020;Jalón-Rojas et al, 2019;Kukulka et al, 2012). To numerically study the dispersion of microplastics in areas of intense turbulence or wave mixing, it was shown that vertical turbulence model and particle inertia are key parameters (Jalón-Rojas et al, 2019;Stocchino et al, 2019;DiBenedetto et al, 2018).…”

Sea Waves Transport of Inertial Micro-Plastics: Mathematical Model and Applications

“…Assuming a perfect match between sea water and plastic debris density inhibits any physical mechanism of sedimentation, which is indeed neglected when assuming neutrally buoyant particles. On the contrary, other approaches allowed to consider a settling velocity disregarding any drag-induced effect, both in a lagrangian [14,19] and in an eulerian [20] approach; the settling velocity of the particles was also taken into account in Liubartseva et al [17], who simulated the sedimentation of particles through a statistical approach based on Montecarlo simulations. Nevertheless, it has to be pointed out that the drag may be reasonably neglected only for some regimes of the controlling parameters, i.e., Froude and Stokes number.…”