Relationships between sediment size distribution and microplastic abundance and characteristics along the strandline of a sandy embayment (Whitsand, Southwest England)

Beach sediments taken from 1 m² areas of strandline across an embayment in southwest England (Whitsand) have been analysed for grain size distribution and elemental content. Large (1-5 mm) and small (< 1 mm) microplastics were isolated by sieving and flotation in NaBr solution, respectively, and characterised by size, shape, colour and polymeric makeup. Sediments displayed varying median diameters and degrees of sorting but distributions were always positively skewed. Concentrations of Fe, K, Mn and Ti were relatively invariant, but Ca concentrations exhibited heterogeneous distributions across the bay. Large microplastics were largely composed of polyolefin-based pre-production pellets, bio-beads and fragments whose numbers were correlated with each other. Positive and inverse relationships between beads and sediment skewness and sorting, respectively, suggests that deposition of this type of plastic is favoured where sediment is well-sorted and contains a high proportion of fine material. Small microplastics were dominated by fragments and fibres <200 μm in size composed of a broader array of polymers (including epoxy resin, polyetherimide and polyvinyl alcohol). Fibres exhibited an inverse relationship with bead and pellet abundance but there was no evidence of dependency on grain size distribution, and their presence is attributed to entrapment in interstitial spaces between sediment grains. Compositional differences between large (1-5 mm) and small (< 1 mm) microplastic fragments suggests they are derived from different sources, with the former coupled with pellet and bead deposition and the latter small enough to be retained in interstitial spaces with fibres. However, a positive relationship between mean (small) fragment size and median sediment diameter suggests that their retention is more constrained by interstitial space than fibres. The study provides evidence that microplastics of different size and shape behave differently but are, ultimately, related to or controlled by sediment size distribution in the coastal littoral zone.