Natural and anthropogenic factors driving the distribution of seafloor litter along coastal submarine canyons of southern Italy

Submarine canyons are recognized as accumulation areas for litter and conduits for deep-sea transport. Through the analysis of seafloor videos, we assess litter distribution in canyons of southern Italy, characterized by differences in morphology, connection to sediment sources and anthropogenic pressures. Litter was detected in all canyons, with highly variable concentration ranging from 0.1 to more than 100 items/100 m and a dominance of plastic, indicating a terrestrial or coastal origin for most of the litter. The lowest densities were from Cuma Canyon, incising the continental shelf edge at some distance from the coast and thus detached from major terrestrial inputs, while the highest were reported from the Messina Strait, where rivers draining highly populated areas debouche directly into offshore canyon heads. Canyon head proximity to rivers and densely populated coasts appears to be the main driver of litter abundance, although the uneven distribution between canyons reflects more closely the effect of geomorphological features, sediment transport and oceanographic processes, responsible for different abundances along the canyons’ thalweg. Oceanographic processes and sediment transport can be involved in downcanyon litter transport; the latter is dominant in the case of gravity flows, which may deliver debris to the lowest canyon reaches, so far largely unexplored.