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

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