Quantification of Microplastics in North-Western Mediterranean Harbors: Seasonality and Biofilm-Related Metallic Contaminants

The Mediterranean Sea is one of the most impacted basins in terms of microplastics pollution. Land-based activities are the major sources of plastic litter to the ocean, with harbors probably representing significant hotspots. In the framework of the SPlasH! project (Stop alle Plastiche in H2O, Interreg Marittimo project), microplastics were sampled in three north-western Mediterranean harbors during summer and winter. In this study, the areal concentrations of microplastics ranged from 5576 to 379,965 items·km−2. A decreasing gradient was observed from the inner to the outer zones of the studied harbors, pointing out these enclosed systems as hotspots regarding microplastic pollution. During summer, the areal concentrations of microplastics were higher than in winter, which could be explained by an enhancement of port activities leading to MPs production. The investigation of microplastics size classes distribution in the surface waters revealed that microplastics within the size range between 300 µm and 500 µm were the least represented. In this study, we assessed trace metal (Pb, Fe, Cu, V, Cd and As) bioaccumulation by the biofilm which developed on the surface of microplastics. The results highlighted that concentrations within the biofilm were higher than those in the surrounding waters. This result strongly suggested trace metal bioaccumulation on microplastics through biofilm formation. When trace metal concentrations were normalized over the corresponding surface of microplastics and macroplastics, higher values were obtained for microplastics, evidencing their enhanced capacities to bioaccumulate contaminants when compared to macroplastics.