Seasonal patterns of floating macro-litter across the Western Mediterranean Sea: a potential threat for cetacean species

Marine litter is a major form of pollution in the Mediterranean, but despite legislative requirements, additional information is still needed to evaluate basin-scale amounts, trends, and potential threats for the biota, especially for Habitats Directive listed species, such as cetaceans. Through repeated sampling of floating macro-litter along a fixed transect between Spain and Italy, this study provided detailed data within four marine sectors of the Western Mediterranean Sea, characterized by seasonal variability in oceanographic/ecological conditions and anthropogenic pressure. Moreover, we identified the areas of major overlap between high density of plastic and cetacean sightings. Litter was composed of plastic for more than 65%, with more diverse compositions occurring in the Balearic and Sardinian Sea compared to the semi-enclosed Bonifacio and Tyrrhenian sectors. The average amount of macro-litter ranged between 1.9 and 2.8 items km−2 and reached the highest values in spring/summer in all basins, suggesting a relationship with the increasing of touristic and maritime activities, both in coastal and offshore waters. The Balearic and Bonifacio sectors showed higher amounts and larger accumulation areas for plastic, likely due to a combination of multiple sources and oceanographic processes influencing the distributional patterns. Cetacean sightings were recorded in all sectors with fin whale (Balaenoptera physalus) and striped dolphin (Stenella coeruleoalba) as the most frequent species. The presence of cetaceans in areas with high densities of plastic emerged mostly during spring/summer in the Balearic and Bonifacio sectors, but other specific areas of potential exposure were identified, indicating the need for a dynamic definition of this threat.