Microplastics in coastal urban sediments: Discrepancies in concentration and character revealed by different approaches to sample processing

Abstract

Intertidal, silty sediment samples have been collected from three coastal locations with different uses and anthropogenic signatures in the vicinity of Plymouth, southwest England, and analysed for microplastics (MPs) by two independent means. Firstly, MPs were counted and characterised directly on unprocessed dried sediment under a stereo microscope, and secondly MPs were isolated from sediment by flotation in ZnCl2 solution and filtration before analysis. Direct counting resulted in average (± one standard deviation) numbers of MPs per g of dry sediment of 0.77 ± 0.16 at a marina-harbour, 0.58 ± 0.30 under a busy road bridge and 0.79 ± 0.43 adjacent to country parkland. After flotation and filtration, concentrations were reduced to 0.24 ± 0.11, 0.18 ± 0.06 and 0.48 ± 0.38 MP g-1, respectively. Observations were attributed to hetero-aggregation of small fibres with settling sediment during flotation, and the presence of MPs (including paints) that were too dense to float or that had aggregated or agglomerated with denser sediment and construction material in situ. The findings have implications for the efficacy of flotation procedures, accurate estimations of MP concentrations in sediment and the representativeness of MPs by type, and inter-site comparisons of MPs that are widely reported in the literature.