Effect of wind and wave-driven mixing on subsurface plastic marine debris concentration

Buoyant micro-plastic marine debris (MPMD) is a known widespread ocean pollutant. Every year, surface tows are conducted to collect MPMD and to estimate MPMD concentrations. However, it has been observed that such surface concentrations are lower when wind speeds are elevated, suggesting that wind and wave-driven mixing is submerging MPMD to greater depth. Motivated by MPMD observations, this study applies a turbulence resolving large eddy simulation (LES) and a parametric one-dimensional column turbulence model to examine the vertical distributions of MPMD. The parametric model, which is tuned to accurately capture LES results, is computationally efficient, so that it can be applied to long time series of MPMD observations. Both models capture wind-driven turbulence, Langmuir turbulence (LT), and enhanced turbulent kinetic energy input due to breaking waves (BW). Therefore, these models depend explicitly on wind and wave properties, which are estimated from archived wave simulation data for the analysis period. Model results are only consistent with the observed MPMD concentration profiles if LT effects are included; neither BW nor shear-driven turbulence is capable of deeply submerging MPMD, suggesting that observed MPMD concentration profiles are a characteristic signature of wave-driven Langmuir turbulence. The parametric model is applied to several years of observations in the Pacific and Atlantic Oceans to show that total MPMD concentrations are significantly underestimated by surface measurements.