Microplastic abundances in the surface and subsurface strandline sediments along two beaches, the Norfolk coastline, UK

Globally, it is well established that beaches act as both sinks and sources for macro and microplastic. However, most studies only investigate the plastic found on the surface of the beach sediments neglecting the lower beach sediments that may also act as a sink for microplastics. Quantifying the abundance and morphology of microplastics within beach sediments at depth is important for local, regional and global estimates of beach microplastic. This data is essential for effective monitoring, management and policy of coastal environments. This is the first UK study to systematically examine depth-resolved microplastic concentrations and morphologies in beach sediments along the Norfolk coastline, focusing on West Runton and Cart Gap during the summer months (June–August). Beach sediment samples were collected from surface sediments and at 15 cm depth from both strandline and back-beach environments at each site. Microplastic abundance, morphology, and colour were analysed, while anthropogenic litter was assessed using OSPAR methodology. A Pollution Load Index (PLI) was also applied to evaluate spatial patterns of contamination. Results showed that microplastic concentrations were higher in surface sediments (67.6 ± 26.6 MPs/100 g dry weight) than at 15 cm depth (58.1 ± 24.2 MPs/100 g) and strandline sediments contained consistently higher concentrations than back-beach sediments at both sites (West Runton: 70.5 ± 29.9 vs 62.3 ± 27.4 MPs/100 g dw; Cart Gap: 61.8 ± 27.1 vs 56.7 ± 16.5 MPs/100 g dw). Pollution Load Index data supported these spatial contamination trends. The concentration of microplastics increased over the summer months (June–August) and microfibres accounted for over 94% of particles recovered and likely originated from textile-related sources. These results suggest that tidal processes may enhance microplastic accumulation at the strandline. Sediment characteristics also influenced microplastic retention, with the pebble and shingle sediments at West Runton trapping more particles and reducing reworking compared to the more dynamic sandy conditions at Cart Gap. Tourism over the summer months increased textile based microplastics in the beach sediments. Anthropogenic litter analysis identified plastic as the dominant material type, particularly food packaging and fishing-related debris. The study highlights the need for further research into the transport, burial, and reworking of microplastics to improve monitoring strategies and support effective coastal pollution management.