[M-IS17] Ocean Plastics, an earth science perspective

[MIS17-P07] Fragmentation rate of ocean microplastics: an estimate based on their size distribution

*Hiroki Takeda^1,2, Atsuhiko Isobe³ (1.Kyushu University, 2.Keio University, 3.Kyushu University Research Institute for Applied Mechanics)

Keywords:Ocean Microplastic, Distribution, Microplastic Size

In recent years, many observations of ocean microplastics have been conducted (Law, 2019). In particular, the data are accumulating around Japan. Microplastic surveys have been undertaken continuously in East Asia and Antarctica (e.g. Isobe et al. 2020). As a result, we have established observation methods for microplastics and clarified that the East Asian area is a hotspot for microplastics. Observations in the Setonaikai Sea, where the surface layer is relatively calm, have also revealed a mechanism by which microplastics are selectively transported according to their size (Isobe et al. 2014). Furthermore, it is now known that drifting microplastics move through a fracturing process while riding the ocean currents and Stokes drift (Isobe et al. 2014).
It is thought that the ocean current field around Japan may be related to the size distribution of microplastics. Therefore, the purpose of this study was to clarify the characteristics of the size distribution and to estimate the fracture rate, which were obtained from microplastics observations around Japan. Especially in the Japan Sea, we found that the size of microplastics tends to be smaller downstream of the Tsushima Warm Current flowing through the Japan Sea.
Furthermore, the study conducted a microplastic drifting simulation incorporating the crushing processes of landfall and redrift. The results showed that the microplastics off the coast of North kyushu were repeatedly washed ashore and re-drifting in the Tsushima Current. From the advection velocity toward downstream and the size change rate, we also estimated that the crushing rate of microplastics is about 8.5 × 10^-2 mm in 100 days.