Mussels facilitate the sinking of microplastics to bottom sediments and their subsequent uptake by detritus-feeders

Microplastics (MP) are omnipresent contaminants in the oceans, however little is known about the MP transfer between marine compartments and species. Three connected laboratory experiments using the filter-feeding mussel Mytilus galloprovincialis and the omnivorous polichaete Hediste diversicolor were conducted to evaluate whether the filtering action by mussels affects the vertical transfer of MP of different sizes (MP = 41 μm; MP = 129 μm) and densities (polyamide = 1.15 g cm; polypropylene = 0.92 g cm) across compartments and species with different feeding modes. Mussels significantly removed MP from the water column by incorporating them into biodeposits. This effect was particularly evident for the MP, whose deposition from the water column to the bottom was enhanced (about 15%) by the action of mussels. The incorporation of MP into faecal pellets increased the particles' sinking velocity by about 3-4 orders of magnitude. Conversely, the MP presence significantly decreased the depositional velocities of faecal pellets, and the magnitude of this effect was greater with increasing MP size and decreasing density. The MP incorporation into mussels' biodeposits also more than doubled the amount of MP uptake by H. diversicolor. We conclude that detrital pathways could be a transfer route of MP across marine compartments and food webs, potentially affecting the distribution of MP in sediments and creating hot-spots of bioavailable MP.