Two sides of the same coin: A coupled modelling description of the dynamics of microplastics and associated contaminants in the Mediterranean Sea

, Guerrini Federica, Mari Lorenzo, Casagrandi Renato.

Floating plastic litter in the marine environment is hardly found as clean fragments, rather it is often colonized by marine organisms and contaminated with organic pollutants, as mounting evidence highlights. Those interactions between plastics and both the biotic and the abiotic components of the seascape are even more relevant for microplastics, due to its high surface-to-volume ratio: they affect the dynamics of microplastics at sea (as biofouled particles tend to sink) and may exacerbate its toxicity to marine biota. A thorough assessment of the risk caused by microplastics and other pollutants of concern to marine ecosystems thus demands for a joint description of these two kinds of pollution. Here we illustrate a novel modelling framework to simultaneously describe in a simple, yet comprehensive, way the transport of microplastics in the sea surface, the advection-diffusion of selected organic contaminants, and the gradient-driven chemical exchanges between microplastic particles and the marine environment. Focusing on the Mediterranean Sea, we simulated two years of microplastic-related pollution (our coin) by coupling the dispersal of a target pollutant (through Eulerian advection-diffusion, one side of the coin) and of microplastic particles (using a Lagrangian framework, the other side of the coin). The two sides make the coin, because each particle can chemically interact with the surrounding environment while being subject to current-driven movement before eventually sinking. Over the simulated period, we found that the patterns of organic pollution at sea may significantly be affected by the presence of plastic particles acting as vectors, and consequently that particle sinking plays a significant role in regulating the balance on the sea surface layer of both microplastics and the pollutants they carry. Although simplified, the modelling framework described here moves a first, yet useful step towards a comprehensive quantification of the multifaceted threat posed by plastic-related marine pollution.

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