Investigation of dynamic change in microplastics vertical distribution patterns: The seasonal effect on vertical distribution

This paper analyzes the variability of microplastics vertical distributions in the oceanic water column. Data were obtained from targeted sampling in the Bay of Marseille (France) and from a numerical simulation forced by realistic physical forcings. By fitting model and in-situ data in a simplified vertical dimension, three microplastics classes may be deduced: settling, buoyant and winter neutrally-buoyant microplastics. Buoyant microplastics are mainly concentrated at the surface but they can be mixed throughout the whole water column during episodes with strong winds and no water stratification, inducing an implicit underestimation of buoyant microplastics in surface sampling. Almost symmetrical to the distribution of buoyant microplastics, settling microplastics are mainly found at the bottom but they can sometimes reach the surface under the mixing conditions cited above. They could thus contribute to surface sampling. Winter neutrally-buoyant microplastics are more homogenously mixed during the winter but are under the stratified layers during summer.