Research Progress on Source-to-Sink Transport Processes of Marine Microplastics

Microplastics in marine environment are global environmental issue and challenge and have received an extensive international concern. At present, most of researches focus on the investigation of microplastic abundance in the ocean surface water, and there is insufficient understanding of the distribution and transport processes of microplastics in the deep-sea environment. This paper reviewed marine microplastic studies carried out in the last decade, and summarized the source, global distribution and transport processes of microplastics. Field investigations showed that both surface water and water column were important accumulation areas for microplastics, while deep-sea surface sediments were final sinks for microplastic deposition and accumulation. Transport of microplastics to the deep sea included two modes: vertical settlement and lateral transport. Laboratory simulation showed that the sinking rate of microplastic particles in the ocean changed between 300 and 1 000 meters per day, and the sinking process was not solely controlled by particle physical properties such as particle density, but also influenced by ocean dynamic process, biological action and marine snow aggregation. Microplastics deposited on the seafloor could migrate laterally towards the deep sea with resuspended sediments, which were related to internal waves, deep-sea turbidity current or climatic events. However, there remain the key knowledge gaps in uncertain speed and quantity of microplastics moving to the deep sea, which is not conducive to the comprehensive understanding of the microplastic transport process from source to sink. Therefore, it is recommended to observe the vertical sinking flux of microplastics with layered sediment traps in order to study the source-to-sink transport processes of microplastics in deep-sea environment.