Amount, composition, and spatial distribution of floating macro litter along fixed trans-border transects in the Mediterranean basin

Highlights: • The study gave a synoptic estimation of floating macro-litter in Mediterranean. • The average amount of macro-litter was of 2–5 items/km², highest in Adriatic. • Seasonal patterns were shown in almost all study areas. • Plastic accounted for > 80% of litter in all areas and seasons. • No evident permanent aggregation zones were found. - Abstract: Marine litter is a major source of pollution in the Mediterranean basin, but despite legislative requirements, scant information is available for the ongoing assessment of this threat. Using higher size classes as proxy for litter distribution, this study gave a synoptic estimation of the amount, composition, and distribution of floating macro-litter in the Mediterranean. The average amount of macro-litter was in a range of 2–5 items/km², with the highest in the Adriatic basin. Seasonal patterns were present in almost all study areas and were significant in the Ligurian Sea, Sardinian-Balearic basin, and Central Tyrrhenian Sea. Plastic accounted for > 80% of litter in all areas and seasons, with the highest proportion in the Adriatic Sea, Ligurian Sea, and Sicilian-Sardinian Channels; in the Bonifacio Strait, Tyrrhenian Sea, and Sardinian-Balearic basin, litter composition was instead more diverse. Spatial analysis suggested an almost homogeneous distribution of litter without evident regular aggregation zones.