This study investigated the abundance, composition, and vertical distribution of microplastics (MPs), as well as the influence of sediment properties on MPs abundance, while also comparing analytical methods for MPs quantification between microscopy-based counts and micro-Fourier Transform Infrared Spectroscopy (µFTIR) counts. Sediment samples from the Belgian Part of the North Sea and the Wadden Sea were analysed across three depth intervals (0–1 cm, 1–2 cm, and 2–3 cm). MPs abundance was substantially higher in the Wadden Sea, where fine-grained sediments appeared to enhance MPs retention. MPs abundance generally decreased with increasing sediment depth, although size-class composition did not differ significantly across sediment layers. Distinct spatial differences in MPs shape and colour were observed, with the Wadden Sea showing greater colour heterogeneity (11 colours) and a dominance of film-type MPs, whereas fibres and fewer colour types (4 colours) dominated the Belgian Part of the North Sea. Methodological comparison revealed significant differences between microscopy- and µFTIR -derived MPs counts, although the magnitude of these differences varied between stations. Overall, the findings highlight the combined influence of sediment characteristics, vertical sediment structure, and analytical methodology on observed MPs distributions, and emphasise the need for standardised approaches in future monitoring studies.
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