Marine litter is an emerging environmental threat affecting all world’s oceans including the deep seafloor, where the extent of the phenomenon is still largely unknown. We report the spatial patterns of macro-litter distribution within the Messina Strait’s channels (Central Mediterranean), focusing on the transfer mechanisms responsible for its emplacement, a key information to better understand litter distribution. Litter is patchy but pervasive on all surveyed channels, reaching densities up to ~200 items/10 m, the highest reported for the deep sea until now. Litter is often arranged in large accumulations formed by hundreds of land-sourced items, mixed to vegetal and coarse-grained debris, indicating an emplacement from sedimentary gravity flows. Such impressive amount of litter can be explained by the superposition of a very efficient source-to-sink sedimentary transport and a strong urbanization of the coastal area. These findings point out that macro-benthic litter pollution is a major, often overlooked, threat for deep-sea ecosystems. Further explorations are thus required in similar marine settings to fully understand the magnitude of the problem, since they may represent the largest litter hotspots in the deep-sea.

Scleractinian cold-water coral (CWC) reefs are key habitats for benthic fauna as they enhance spatial heterogeneity and biodiversity. Understanding their environmental and ecological dynamics has therefore important implications for biodiversity conservation. This is especially true for the Mediterranean Sea, where living cold-water coral reefs are rare. In this study, we present a quantitative analysis of the CWC assemblages from Cabliers Coral Mound Province, located in the Alboran Sea (westernmost Mediterranean). The province extends for 25 km, with some mounds rising up to 140 m from the surrounding seafloor and being partly topped by living CWC reefs. The observed megabenthic species were quantified through video analysis of three Remotely Operated Vehicle (ROV) dives (280 -485 m water depth) and their distribution was related to mound geomorphic characteristics and seafloor terrain parameters, extracted from a high-resolution Autonomous Underwater Vehicle (AUV) multi-beam bathymetry. The pronounced abundance and size of scleractinian CWCs among the observed assemblages, makes Cabliers the only known coral mound province in the Mediterranean Sea with currently growing reefs. Within these reefs, several recruits and juveniles of the sebastid Helicolenus dactylopterus were observed, confirming the use of such habitats as nursery grounds by some commercially valuable fish species. The qualitative comparison between the fauna of Cabliers and Atlantic coral mounds suggest that the number of species associated with CWC mounds worldwide is even higher than previously thought. This finding has important implications for the conservation and management of CWC habitats in different geographic regions.

No abstract