Spatial and Temporal Distribution of Large (1–5 mm) Microplastics on the Strandline of a Macrotidal Sandy Beach (Polzeath, Southwest England) and Their Association with Beach-Cast Seaweed

Microplastics (MPs) are ubiquitous and persistent contaminants of the marine environment, but a clear understanding of their cycling, fate, and impacts in coastal zones is lacking. In this study, large MPs (1–5 mm) were sampled spatially and temporally from the strandline of a macrotidal, sandy beach (Polzeath) in southwest England. MPs encompassing a diversity of sources were categorised by morphology (foams, nurdles, biobeads, fragments, fibres, films) and quantified by number and mass, with a selection analysed for polymer type. A total of about 17,600 particles of around 350 g in mass were retrieved from 30 samples over a period of five months, with an abundance ranging from 35 and 2048 per m2. The space- and time-integrated average mass of MPs on the beach strandline was about 2 kg and was dominated (>90%) by fragments, nurdles, and biobeads of polyethylene or polypropylene construction. Nurdles, biobeads, fragments, and, to a lesser extent, fibres were correlated with strandline seaweed abundance, which itself was correlated with previous storm activity. Relationships with seaweed abundance were also supported by visible associations of these MP morphologies with macroalgal deposits through entanglement and adhesion. These observations, coupled with a lack of MPs below the sand’s surface (50 cm depth), suggest that the majority of MPs are transported from an offshore stock with floating organic debris, resulting in a transitory strandline repository and a habitat enriched with small plastics.