Reliable data on distribution, abundance and types of stranded plastics are needed, especially on riverbanks that have received less attention than coastal beaches. Here, we present the citizen science initiative Plastique à la loupe (Plastic under the magnifier), that compares for the first time the distribution of different litter sizes (macrolitter, meso-and microplastics) over 81 riverbanks and 66 coastal beaches sampled in France between 2019 and 2021. A total of 147 school classes (3,113 schoolchildren) from middle to high school collected, sorted and enumerated 55,986 pieces of plastic to provide a baseline of the current pollution by stranded debris at the national level. Single-use plastics (mainly food-related items) were very abundant on riverbanks (43 %), whereas fragmented debris dominated the macrolitter on coastal beaches (28 %). Microplastics were always higher in number compared to mesoplastics and macrolitter, with polystyrene and polyethylene found in equivalent proportions on riverbanks while polyethylene dominated microplastics on coastal beaches.

Tracing the source of plastic items was possible only for a small proportion of the numerous collected items, mainly for identifiable macrolitter and microplastic pellets. This study lays out the foundations for further works using the Plastique à la loupe citizen science initiative in France and additional comparisons to other studied habitats worldwide, which can be used by scientists and policy-makers for future litter monitoring, prevention and clean-up strategies.