Long-term observations of microplastic and mesoplastic distribution on sandy beaches in north-east Taiwan: Impact of typhoons on spatial and temporal variability.

Plastic pollution in coastal environments poses ecological risks, yet long-term monitoring data and insights into the impacts of extreme weather events remain limited. This study examines the spatiotemporal distribution of microplastics and mesoplastics on sandy beaches, with a focus on long-term trends and the influence of typhoons. Samples were collected from two beaches in northern Taiwan over a 20-month period, spanning 8-9 sampling sessions per site. A dense grid sampling approach (50 × 50 cm quadrats) was used across three transects. Post-typhoon microplastic abundances peaked at 5080.6 pcs/kg dry weight (d.w.) at Xialiao Beach, where the average concentration was 115.3 pcs/kg d.w., compared to 18.1 pcs/kg d.w. at Longmen Beach. Typhoon events increased plastic particle accumulation, primarily along storm lines and backshore areas, but long-term accumulation was not observed, indicating dynamic deposition and removal processes. These findings emphasize the need for long-term monitoring through repeated sampling campaigns conducted over an extended period, and high-resolution monitoring using a dense sampling grid, to accurately assess plastic pollution. Single-event studies risk misrepresenting pollution levels due to spatial and temporal variability.