Settling mechanisms of microplastics in an estuary system in the northeastern Japan

Despite numerical simulations demonstrate accumulation of buoyant microplastics in the oceanic surface water, field studies reported lower abundance of them than those predicted. One possible sink of buoyant microplastics from the upper layers is settling to the seafloor. Experimental studies examining potential mechanism reducing buoyancy of microplastics show that microplastics can be incorporated into rapidly sinking fecal materials or large aggregate of phyto-detritus. It is, however, not clear what mechanism is reducing its buoyancy in natural marine environments. In this study we collected settling microplastics by time-series sediment trap at Otsuchi Bay, a small embayment locating northeastern Japan, to estimate settling flux of land-derived microplastics to estuarine sediment and to reveal incorporation process of microplastics into settling materials. Three months moorings were conducted at near the river mouth of Otsuchi River and Kozuchi River inflowing to Otsuchi Bay from September to December 2018 and from February to May 2019. With fecal pellets and mineral like particles synthetic fibers and fragments appeared in collected sediments. In poster presentation seasonal variability of the settling flux of microplastics and the type of them will be discussed.