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

[MIS17-P04] Formation of ‘subsurface plastic maximum’ in coastal waters

*Miho Yoshitake¹, Atsuhiko Isobe² (1.Kyushu Univ., 2.Kyushu University)

Keywords:microplastics, vertical distribution

Less dense microplastics (MPs) consisted mainly of polyethylene and polypropylene are considered to float around sea surface. In fact, previous studies such as Reisser et al. (2015) observed the concentrations of these buoyant MPs in the different depths and found highest concentrations at the sea surface regardless of size and wind speed. MP observations, therefore, have been conducted mainly at the sea surface using manta trawls and neuston nets suitable for on-board surveys. However, we here present a quite different result from previous understanding especially for very small MPs (< 300μm) using observations in Korean coastal waters in summer (partly presented in Song et al., [2018], Yoshitake et al., [JGR-Oceans, in revision]). By analyzing surface (~ 0.2 m), middle (average: 9.2 ± 6.3 m) and bottom (average: 19 ± 13) seawater samples, we found a lot of very small MPs not only around the sea surface but also in the middle and bottom water. In particular, these ‘subsurface plastic maximum’ was clearly formed under the condition with the wind speed higher than around 3.0 m/s. As the wind speed becomes higher, although relatively large MPs (> 300 μm) existed still around the sea surface, very small MPs especially around 100-μm size formed a subsurface maximum around 20-m depth. Moreover, this subsurface plastic maximum was coincided with the bottom of the surface mixed layer (just above the pycnocline) suggested from the vertical profile of water temperature.