Highway runoff and recreational activities as primary sources of microplastics in the watershed of the SW Atlantic Mar Chiquita coastal lagoon (MAB-UNESCO).

Plastic waste pollution is one of the most significant global challenges today, and it is urgent to reduce, mitigate, and assess its levels in various environmental matrices and priority areas. Coastal aquatic environments act as sink areas, where the watershed collector effect could lead to microplastic deposition. The Mar Chiquita coastal lagoon (Mch, Buenos Aires, Argentina) lies within a protected area receiving water from a 10,000 km² watershed with diverse land uses. This study aims to determine the occurrence and abundance of microplastics (0.1-5 mm) in surface sediments from 18 sampling sites, including the upper, middle, and lower basin areas, as well as sites within the coastal lagoon. Microplastics were extracted using a high-density solution (>1.6 g/cm), counted, and categorized by type, shape, size, and chemical composition. Results showed total microplastic abundances ranging from 0 to 2468 items per kilogram of dry weight. The higher abundance trends were associated with areas of high traffic and human settlement, specifically in streams and the coastal lagoon, respectively. Common thermo/duroplastics particles (MPs; 5-100%) and tire wear particles (TWPs; 0-95%) represented the main microplastic groups observed in the Mch basin, while Paint particles (Pps) contribution was lower (0-15%). MPs were predominantly composed of polyethylene terephthalate (32%) and polyethylene (24%), while TWPs mainly consisted of polyisoprene and polybutadiene, and PPs were primarily associated with epoxy resins or acrylic paints. This work represents the first baseline assessment of microplastic pollution in the sediments of this priority conservation area on the southwestern Atlantic coast.