Microplastic Transport Trends in a Tide-Dominated Coastal Lagoon: Insights from Arcachon Bay (SW France)

Understanding the transport of microplastics in coastal and estuarine areas is critical for assessing their global distribution and ecological impact. These regions act as dynamic interfaces where microplastics can accumulate, transform, and be transported to the open ocean or coastal sediments. In this study, we investigate the seasonal transport dynamics of microplastics in Arcachon Bay (SW France), a tide-dominated coastal lagoon characterized by 70% intertidal flats, using a numerical Lagrangian model combined with in-situ observations. Modeled trajectories were validated against Lagrangian drifter data under various conditions, showing remarkable agreement, including beaching and refloating processes at sandbars during ebb and flood tides, respectively. In-situ observations of microplastic concentrations and properties, collected in April 2019 from the sea surface, water column, and intertidal bottom sediments at single times across five stations, were used to set up the numerical model and to contextualize and discuss the simulation results. Trajectories of the three prevalent particle categories (low-density fragments, PET fibers, and rubber fragments) were simulated from major sources (rivers, sewage, port, and fishing areas) across two contrasting seasons, with trends compared to in-situ observations. The analysis is expected to reveal distinct seasonal transport pathways influenced by particle properties and hydrodynamic conditions, providing insights into dispersal patterns, retention zones, and potential hotspots for microplastic accumulation. The modeling results aim also elucidate transport patterns suggested by localized observations, such as the presence of hotspots of low-density particles at the sea surface within the channels, the greater abundance of fibers and rubbery particles outside the bay, the role of intertidal channels in flushing or retaining different particle types, and the combined influence of source location (e.g., sewage) and hydrodynamics on these distributions.