Vertical transport of marine microplastics mediated by filter-feeding organisms

Microplastics, as an increasingly concerning environmental pollutant, transport from the surface waters to the seafloor after entering the ocean and can be buried in deeper sediments through bioturbation. However, the role of marine organisms in this vertical transport remains unclear. In this study, sediment traps were used to quantify the contribution of typical filter-feeding organisms, including sea squirts (Halocynthia roretzi), Pacific oysters (Crassostrea gigas), scallops (Chlamys farreri), and Manila clams (Ruditapes philippinarum), to the vertical transport of microplastics in the water column. The results showed that microplastics were present in feces and pseudofeces of filter-feeding organisms and sank to form biodeposits, significantly enhancing the deposition of microplastics <1000 μm in size and with positive buoyancy (density lower than seawater). Additionally, experiments with Manila clams, a representative benthic filter-feeding species, were conducted to simulate bioturbation processes in sediments. The results demonstrated that exposure to polystyrene microbeads did not significantly affect the physiological indices of clams. The burrowing, movement, and feeding activities of clams facilitated the rapid transport of microplastics to deeper sediment layers (6–8 cm below the surface). These findings highlight the critical role of filter-feeding organisms in the vertical transport of microplastics in the water column and sediments, contributing to a better understanding of microplastic spatial variability and source-sink dynamics.