Microplastics as vectors of contaminants in the Mediterranean Sea: A modelling description

The toxicity effects caused by microplastics to marine species may be exacerbated by the potential of the former to act also as vectors of other contaminants, such as organic pollutants that concentrate on the organic surface of marine plastic litter, as well as the chemical additives used in the production of plastics. To achieve a deeper understanding of the advection-diffusion processes involving both plastic and the pollutants it carries, we provide a simple yet comprehensive modelling framework to account for both the distribution of microplastics at sea and their chemical interactions with the marine environment, focusing here on the Mediterranean Sea. Surface dispersal patterns of microplastics are obtained with Lagrangian particle-tracking simulations, where plastic particles are released from the most impacting sources of pollution, i.e., mismanaged plastic waste originating from coastal areas, the watersheds of major rivers, and maritime activities. These model inputs are made time- and space-varying through a variety of proxy data (from Earth Observations to GPS ship tracking and national socioeconomic data). During their transport by surface currents, as provided by oceanographic reanalysis products, simulated plastic particles interact with the surrounding environment in response to chemical gradients. The description of the advection-diffusion of primary pollutants in the sea is here obtained with an Eulerian model, whose dynamics is coherently affected by the chemical interaction with particles. While providing further understanding of the distribution of microplastics at a Mediterranean-wide scale, our modelling approach also contributes to assessing their role as a vector of other pollutants of concern in the marine environment.