From shallow to deep water: A large-scale assessment of seafloor marine macrolitter in the Italian seas by ROV-imaging.

The seafloor is the ultimate sink for most litter worldwide. Although significant efforts have been made over the past two decades to assess the impact of litter on marine ecosystems, collecting quantitative data using standardised, comparable protocols remains limited. A large-scale dataset, derived from over 56 oceanographic surveys conducted between 2007 and 2023 using remotely operated vehicles (ROVs) in the Italian seas (Mediterranean Sea), enabled a comprehensive census of seafloor marine litter across shallow to bathyal zones (40-2129 m). The explorations primarily focused on rocky areas, including continental shelf reliefs, canyons, and offshore seamounts. The ROV-imaging dataset enabled the collection of important information on litter quantity, composition, patterns of geographic and bathymetric distribution, effects on benthic communities, and relationships with several environmental and anthropogenic variables. 10,982 items were recorded, and litter abundance reached 16.1 items 100 m-2 (average 1.5 ± 0.1 items 100 m-2), with significant local differences in quantity and composition. Only 8.6% of the investigated sites were litter-free, while 1.0% had abundances exceeding 10 items 100 m-2. Fishery-related litter (mainly lines and ropes) was the most common type, accounting for ≈70.8% of the total items. This work provides the first comprehensive picture of seafloor marine litter in the Italian seas. In addition, it shows that 87% of sites exceeded the European threshold (expressed as litter density) for seafloor marine litter, recently defined under the Marine Strategy Framework Directive Descriptor 10. The results underscore the need for urgent, targeted measures to manage marine pollution and reduce inputs of environmental litter.