Microplastic pollution in surface waters of urban canals in a highly urbanized city (Dunkirk, Northern France): influence of dry and wet periods on discharge to the sea

Microplastic (MP) pollution is a growing environmental concern, with urban waterways constituting critical pathways for transporting MPs into marine ecosystems. This study investigated the distribution, abundance, diversity, and flux of MPs in six urban canals influenced by urban and industrial activities in the city of Dunkirk, during dry and wet (rainy) periods. Plastic abundance was higher during the wet period (18.25 ± 22.16 particles/m) compared to the dry period (8.14 ± 7.76 particles/m), reflecting increased inputs from surface runoff and stormwater discharge. Spatial trends revealed distinct site-specific patterns, with the most urbanized sites (S1 and S6) exhibiting the highest abundances. Fibers were the dominant type accounting for over 75% of particles at all sites. PET was the main polymer in urban sites, while PE and PP were more prevalent in industrial zones. The estimated annual flux of MP at the outlet ranged from 19.21 × 10 ± 26.12 × 10 particles/year/m (dry period) to 23.97 × 10 ± 19.41 × 10 particles/year/m (wet period), revealing the significant contributions of urban canals to coastal pollution. These results highlight the need for improved source control and stormwater management to better assess and mitigate MP pollution in urban aquatic systems.