The linked dynamics of marine microplastic pollution and plastic-related organic pollutants: a coupled modelling approach

Floating plastic litter in the marine environment is not just passively transported by wind and surface currents, rather it is often interacting with both the abiotic and the biotic components of the seascape. These two-way interactions are even more relevant for microplastic, known to act as a vector of other contaminants due to its high surface to volume ratio. Microplastic tends to concentrate organic pollutants due to chemical affinities while also carrying the chemical additives used in the production of its parent polymer. As these chemical interactions may exacerbate microplastic toxicity to marine biota, a thorough assessment of the risk to marine ecosystems caused by microplastic particles and related pollutants of concern demands a joint description of the physical and chemical sides of the problem. . Here we present a novel 2D Lagrangian-Eulerian modelling framework to simultaneously describe in a simple, yet comprehensive way, the Lagrangian dispersal of microplastic on the sea surface, the Eulerian advection-diffusion of selected organic contaminants and the gradient-driven chemical exchanges between microplastic particles and the marine environment. Applying our method to the Mediterranean Sea, we simulate two years of the coupled dispersal of both a target pollutant and microplastic particles. In our framework, each particle actively interacts with the chemical pollution of its surrounding marine water throughout its journey at sea and is finally subject to sinking after colonization by marine organisms. Our results suggest that the presence of microplastic particles acting as vectors might affect the patterns of organic pollution at sea and that particle sinking could regulate the balance on the sea surface layer of plastic-related pollutants. Although simplified, the modelling framework we propose here is intended as a tool to shed some light towards a comprehensive description of the multifaceted threat of marine plastic pollution.