Lagrangian tracking of river microplastics in the Mediterranean Basin

The Mediterranean Sea, a semi-enclosed basin with highly anthropized coastlines, intense marine traffic and significant river discharges, has been identified as a plastic pollution hotspot. However, the quantification of sources, transfers and accumulations remains variable, and simulated marine plastic cycles are still incomplete. In this study, we applied a recent river microplastic source scenario (Weiss et al., 2021, Science) to Mediterranean river basins. This enabled Lagrangian dispersion simulations to be initiated using high-resolution 3D current fields (including atmospheric, tidal, wave and river forcing) performed with the SYMPHONIE hydrodynamic model and its Lagrangian module (Weiss et al., 2024a,b, ESPR). Modeled concentrations of floating and sinking particles were analyzed, simulating a wide range of vertical velocities. A coherent regional 3D dispersion scenario allowed to establish a mass balance of microplastic fluxes, from river sources to coastal stranding in the different sub-basins. Results revealed a massive export of floating particles from the northwestern to the southeastern sub-basins, with residence times ranging from 1-3 weeks in dissipative zones to 11 weeks in convergent zones. Comparison of modeled and observed surface stocks suggested the need to introduce missing sources and sinks, as fragmentation or sedimentation, and to reduce stranding probabilities (by about 30%). A seasonal analysis of the microplastic dispersion from the Rhône River plume (the largest freshwater discharge in the Mediterranean) in the SYMPHONIE simulations highlighted the influence of hydrodynamic conditions on particle transfer. It included dispersion patterns on the continental shelf of the Gulf of Lion and the frontal zone from the Pyrenees to the North Balearic fronts, demonstrating the role of fine-scale circulation in shaping concentration gradients.