Influence of Particle Size and Fragmentation on Large-Scale Microplastic Transport in the Mediterranean Sea

Microplastic particles move three-dimensionally through the ocean, but modeling studies often do not consider size-dependent vertical transport processes. In addition, microplastic fragmentation in ocean environments remains poorly understood, despite fragments making up the majority of microplastic pollution in terms of the number of particles and despite its potential role in mass removal. Here, we first investigate the role of particle size and density on the large-scale transport of microplastics in the Mediterranean Sea and next analyze how fragmentation may affect transport and mass loss of plastics. For progressively smaller particle sizes, microplastics are shown to be less likely to be beached and more likely to reach open water. Smaller particles also generally get mixed deeper, resulting in lower near-surface concentrations of small particles despite their higher total abundance. Microplastic fragmentation is shown to be dominated by beach-based fragmentation, with ocean-based fragmentation processes likely having negligible influence. However, fragmentation remains a slow process acting on decadal time scales and as such likely does not have a major influence on the large-scale distribution of microplastics and mass loss over periods less than 3 years.