Microplastics segregation by rise velocity at the ocean surface

Predicting the vertical distribution of microplastics in the ocean surface mixed layer is necessary for extrapolating surface measurements and comparing observations across conditions. The competing mechanisms that control the vertical distribution are particle buoyancy, which is primarily a function of particle properties and drives microplastics to accumulate at the ocean surface, and turbulent mixing, which disperses microplastics throughout the mixed layer and depends on local hydrodynamics. In this study, we focused on the physical properties of microplastics collected within one vertical profile in the North Pacific. We measured the size, shape, and rise velocity of all microplastics collected, finding that average size and rise velocity decay with depth. In addition, we demonstrate how the vertical distribution of the microplastics depends on the rise velocity of the microplastics by segregating the data into three regimes based on a ratio of microplastic rise velocity and a characteristic turbulence velocity scale. Using an individual model for each regime, we can extrapolate the vertical distribution of microplastics to the bottom of the mixed layer and find the total concentration of microplastics. The total extrapolated concentration using the combined model results in 10× the concentration of the surface net alone and 47% more than a model which does not consider the different microplastic regimes. Finally, we discuss how the vertical distribution also depends on microplastic form, finding that lines are approximately well-mixed whereas the concentration of fragments decays with depth. These observations indicate the importance of considering the appropriate rise velocity regime when predicting the vertical distribution of microplastics.