[MIS30-06] Behaviors of foamed polystyrene microplastics in a coastal sea
Keywords:Foamed polystyrene, Aquaculture, Sinking factors
Microplastics (0.3–5 mm) have been found in the bottom sediments of coastal seas (Vianello et al., 2013; Matsuguma et al., 2017), but the sinking process (e.g. sinking factors and fluxes) and abundance of microplastics in coastal seas remain unclear. To determine the extent of microplastic pollution in the coastal marine environment, it is necessary to understand the distribution of microplastics in the reservoirs of the coastal marine system, namely, the sea surface, water column, sea bottom, beaches and marine ecosystem. In addition, microplastic fluxes among the reservoirs should be estimated (Hardesty et al., 2017). Understanding the abundance and size of microplastics in a coastal sea is a key factor in determining the spatial distribution and fluxes, because their behaviors in coastal seas will depend on their sizes (e.g. Isobe et al., 2014; Hinata et al., 2017).
This study examined the abundance and size of microplastics as well as their polymer types in the surface water and the bottom and beach sediments of Hiroshima Bay to infer the extent of microplastics in the region. Moreover, this study aimed to examine the shape characteristics, surface states and internal structures of foamed polystyrene (FPS) microplastics in the beach and bottom sediments by using digital microscopy, field emission scanning electron microscopy (FE-SEM) and X-ray computed tomography (X-ray CT) to deduce the sinking and fragmentation process of FPS microplastics.
In Hiroshima Bay, where oyster aquaculture is widespread, FPS was numerically dominant and serious pollution by FPS microplastics was found to be widespread not only in beach sediments as reported by Fujieda and Sasaki (2005), but also in bottom sediments. FPS microplastics in the beach and bottom sediments were three to seven orders of magnitude larger than those in the surface water. This study is the first report of the dominance of FPS microplastics in bottom sediments.
The ratio of microplastic polymers in the surface water was different from that in the beach and bottom sediments: polyethylene (PE) and polypropylene (PP) microplastics were more abundant. This suggests that FPS microplastics are likely to be washed ashore immediately by Stokes drift, surface residual currents and winds because of their much smaller specific gravity (Isobe et al., 2014). The smaller specific gravity of FPS causes much larger upward terminal velocities in sea water compared to those of PE and PP, so FPS particles have much longer residence times on the beaches than PE and PP (Hinata et al., 2017), resulting in the overwhelming dominance of FPS on the beaches.
The average size of FPS microplastics in the bottom sediments was significantly smaller than that of beached FPS particles. This study compared the shape of FPS microplastics in the beach and bottom sediments. FE-SEM images suggest that large amounts of micro- or nanosized FPS fragments are likely to be generated from the margin of one microplastic-size FPS particle. Also, the specific surface area of smaller FPS microplastics with a complex shape is much greater than that of larger FPS particles. X-ray CT images show that FPS microplastics from the bottom sediments had tunnel-like structures, with pores extending from the surface to the inside. Therefore, based on the FE-SEM and X-ray CT images, FPS microplastics in the bottom sediments were found to be susceptible to biofouling and soil deposition.
This study examined the abundance, polymer type, size and shape of microplastics in three reservoirs, that is, the beach and bottom sediments and the surface water of Hiroshima Bay. The spatial distributions of the abundance and size of microplastics among the sampling sites were not examined. In the future, three-dimensional numerical simulations will help us understand the microplastic residence times and the fluxes between reservoirs to determine the extent of microplastic pollution.
This study examined the abundance and size of microplastics as well as their polymer types in the surface water and the bottom and beach sediments of Hiroshima Bay to infer the extent of microplastics in the region. Moreover, this study aimed to examine the shape characteristics, surface states and internal structures of foamed polystyrene (FPS) microplastics in the beach and bottom sediments by using digital microscopy, field emission scanning electron microscopy (FE-SEM) and X-ray computed tomography (X-ray CT) to deduce the sinking and fragmentation process of FPS microplastics.
In Hiroshima Bay, where oyster aquaculture is widespread, FPS was numerically dominant and serious pollution by FPS microplastics was found to be widespread not only in beach sediments as reported by Fujieda and Sasaki (2005), but also in bottom sediments. FPS microplastics in the beach and bottom sediments were three to seven orders of magnitude larger than those in the surface water. This study is the first report of the dominance of FPS microplastics in bottom sediments.
The ratio of microplastic polymers in the surface water was different from that in the beach and bottom sediments: polyethylene (PE) and polypropylene (PP) microplastics were more abundant. This suggests that FPS microplastics are likely to be washed ashore immediately by Stokes drift, surface residual currents and winds because of their much smaller specific gravity (Isobe et al., 2014). The smaller specific gravity of FPS causes much larger upward terminal velocities in sea water compared to those of PE and PP, so FPS particles have much longer residence times on the beaches than PE and PP (Hinata et al., 2017), resulting in the overwhelming dominance of FPS on the beaches.
The average size of FPS microplastics in the bottom sediments was significantly smaller than that of beached FPS particles. This study compared the shape of FPS microplastics in the beach and bottom sediments. FE-SEM images suggest that large amounts of micro- or nanosized FPS fragments are likely to be generated from the margin of one microplastic-size FPS particle. Also, the specific surface area of smaller FPS microplastics with a complex shape is much greater than that of larger FPS particles. X-ray CT images show that FPS microplastics from the bottom sediments had tunnel-like structures, with pores extending from the surface to the inside. Therefore, based on the FE-SEM and X-ray CT images, FPS microplastics in the bottom sediments were found to be susceptible to biofouling and soil deposition.
This study examined the abundance, polymer type, size and shape of microplastics in three reservoirs, that is, the beach and bottom sediments and the surface water of Hiroshima Bay. The spatial distributions of the abundance and size of microplastics among the sampling sites were not examined. In the future, three-dimensional numerical simulations will help us understand the microplastic residence times and the fluxes between reservoirs to determine the extent of microplastic pollution.