Comparison of the macro-, meso- and microplastic pollution in French riverbanks and beaches using citizen science with schoolchildren

<title>Abstract</title> Rivers are the major source of anthropogenic litters entering the ocean, especially plastic debris that accumulate in all ecosystems around the world and pose a risk to the biota. Reliable data on distribution, abundance and types of stranded plastics are needed, especially on riverbanks that have received less attention than beaches. Here, we present the citizen science initiative <italic>Plastique à la loupe</italic> (Plastic under the magnifier), that compares for the first time the distribution of different litter sizes (macrolitters, meso- and microplastics) over 81 riverbanks and 66 beaches sampled in France between 2019 and 2021. A total of 149 classes (3,113 schoolchildren) from middle class 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 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 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 <italic>Plastique à la loupe</italic> 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.