Quantification of the vertical transport of microplastics by biodeposition of typical mariculture filter-feeding organisms

The tremendous loss of microplastics from the sea surface and the low density of microplastics found in the water column and sediments indicate that the oceans have mechanisms capable of transporting microplastics from the surface to the seafloor. These include physicochemical processes and biological influences from marine organisms that drive the vertical migration of microplastics. Little is known, however, about the biological processes involved in the deposition of plastics in the marine environment. A considerable number of mariculture filter-feeding organisms can consume substantial amounts of suspended substances in the water column, and these organisms are ideal candidates for depositing microplastics. In this study, we analyzed microplastic abundance in typical mariculture filter feeders, i.e., ascidians (Halocynthia roretzi), oysters (Crassostrea gigas), scallops (Chlamys farreri) and clams (Ruditapes philippinarum), quantified the number and characteristics of the microplastics they deposited in situ, and further compared microplastic biodeposition rates. Microplastics were present in feces and pseudofeces and sank to form biodeposits rather than accumulating to significant levels in organisms. Microplastics were found in significantly higher numbers in the biodeposits of mariculture organisms than in the control deposits (p < 0.01). The shape and color of the microplastics in the sediments were not impacted by the presence of organisms (p > 0.05), but the deposition of <1000 μm and positive-buoyancy (less dense than seawater) microplastics was significantly increased in the biodeposits (p < 0.05). The highest microplastic biodeposition rate was found in scallops (1.14 ± 0.07 items·ind·d or 0.5 ± 0.03 items·g·d). These results suggest that mariculture filter-feeding organisms have important biodepositional functions that influence the fate of microplastics through the transfer of microplastics from the surface to the seafloor. This study could contribute to a better understanding of the biological plastic pump mechanisms in oceans.