Stranded in the high tide line: Spatial and temporal variability of beached microplastics in a semi-enclosed embayment (Arcachon, France).

Abstract Coastal environments are a predominant ultimate destination of marine debris, becoming a key focus of studies assessing microplastic (MP) contamination. Here, we described the visible fraction of MP (from 0.5 to 5 mm) that washed up during the high tide at different sites of a semi-enclosed mesotidal bay and investigated the main abiotic factors driving MP beaching. Three contrasted beaches of the Arcachon Bay (SW France) were monitored on a monthly basis during 2019. Samplings were made along a 100 m longitudinal transect at the high-water strandline (4 quadrats of 0.25m2) and at an intermediate tidal range. Each sampled particle was characterized by morphometric data (e.g. size, shape, color, roughness) and polymer identification was performed by ATR-FTIR technique. Results show that MP concentration was higher on the beach located at the mouth of the bay (36.0 ± 39.2 MP.m-2) than at the back and the outside of the bay (respectively 2.7 ± 4.4 and 1.7 ± 2.4 MP.m-2). This may be related to the strong currents at the entry of the embayment and the beach orientation, exposed to predominant winds. Beached MP were mainly pre-production pellets and fragments as they represented respectively 49% and 39% of all analyzed shapes. Polymers with low density were particularly abundant. Polyethylene represented 69% of all the particles while polypropylene accounted for 17% and polystyrene for 10%. We also observed that MP were mostly washed up when wind, waves and river flow were more intense. Analysis suggest that wind direction and speed are key factors influencing beaching as strong onshore wind enhance this process.