Monthly survey of microplastic beaching in the Arcachon Bay (France) and relationship with environmental factors

, Lefebvre Charlotte, Clerandeau Christelle, Le Bihanic Florane, Jalón Rojas Isabel, Morin Bénédicte, Lecomte Sophie, Cachot Jérome.

Since the last two decades, a great number of studies have described microplastics (MP) contamination in different marine ecosystems. These plastic particles have a size inferior to 5 mm and their ubiquity raise environmental, economic and societal issues. In the Arplastic project, we described the visible fraction of MP (from 0.5 to 5 mm) that beached with the high tide line. Three beaches located outside, at the mouth and at the back of the Arcachon Bay (SW France) were studied. During 2019, samplings were done each month at an intermediate tidal coefficient (around 80) and along a 100m longitudinal transect at the high-water mark. Every 25m, a quadrat of 50 cm length sides was laid on the high-water mark and all trapped MP were visually sorted from organic and mineral matter. Then, morphometric data were recorded for each particles (size, shape, color, roughness,..) on stereomicroscope and polymer identification was made by ATR-FTIR. Moreover, data on wind, waves, river flow and currents were analyzed to investigate the main environmental factors driving MP beaching. We found that MP were more abundant on the beach located at the mouth of the bay (36.0 ± 39.2 MP.m-2) than on back and outside stations (respectively 2.7 ± 4.4 and 1.75 ± 2.4 MP.m-2). This may be due to strong currents occurring at the entry of the embayment. Primary and secondary MP were both mainly reported as pellets and fragments represented respectively 43 % and 34 % of all recorded shapes. Polymers with low density were particularly abundant. Polyethylene represented 63 % of all the particles while polystyrene accounts for 19 % and polypropylene for 13 %. We also observed that MP were mostly dropped off on the beach during autumn and winter period when wind, waves and river flow were more important

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