Bioturbation of microplastics in the coastal zone: The role of the lugworm, Arenicola marina

Sediments are an important reservoir and sink for microplastics (MPs) in coastal waters but the role of bioturbating organisms on MPs distribution and cycling is not well understood. In this study, the elemental content and concentrations and characteristics of MPs in casts of the conveyor belt-feeding lugworm, Arenicola marina, have been compared with equivalent measures in intertidal surface and subsurface (∼20 cm) sediments, representative of the worm's food and its vertical extent of burrowing, respectively. Little differences in elemental content were observed between the three sample categories, consistent with non-selective feeding and vertical sediment homogenisation. MPs were dominated by negatively buoyant regenerated (rayon-cotton) anthropogenic fibres (up to 4.5 n g^-1 dw), polyvinyl chloride fragments (about 2 n g^-1) and distinctively brittle, orange particles (up to 11.2 n g^-1) whose precise composition was unknown. Differences in the concentration or size of MP fibres and (non-orange) fragments were not evident among the sample categories, but orange fragments were significantly more abundant and smaller in casts. These observations suggest that, for most shapes and types of MP, deposition on the sediment surface and egestion in casts are balanced by subducted and buried MPs, but that processing of more brittle particles may result in a redistribution by number and size. More generally, the role of a broader array of bioturbating invertebrates could be highly significant in the redistribution and retention of MPs in the intertidal zone and the modification of land-sea fluxes of anthropogenic particles.