The key role of canyons in funnelling litter to the deep sea: A study of the Gioia Canyon (Southern Tyrrhenian Sea)

Litter is a significant environmental issue, threatening all ocean environments including the deep sea, where little is known about the extent of the threat. This study collected underwater videos on shelf-indenting canyons (the Gioia and Petrace Canyon) and adjacent sectors to characterize benthic litter and its distribution patterns. A high density of litter was present in the canyon thalwegs (up to 56.3 items/100 m), where ~95 % of observed debris was concentrated. Litter primarily comprised of plastic (~90 %). The presence of household objects intermingled with vegetal material indicates a strong influence of land-based sources for litter in these areas. Sedimentary gravity flow processes transported natural and anthropogenic debris downslope and may determine its burial beneath the seafloor. Another litter transport mechanism is the presence of light items, observed while drifting above the seafloor under the action of down-canyon bottom currents. This phenomenon was frequently documented within Gioia Canyon, where up to 74 drifting items were counted in one hour of video. Conversely, a lower litter concentration was found along the continental margin, where items were mostly fishing-related. Findings from this study confirm that canyons act as conduits for land-sourced debris and litter transport from coastal to deep-sea environments. The funnelling of a large amount of plastic to the deep sea through multiple processes indicates that the magnitude of this pollution may be greater than expected. This pollution could concentrate in canyons along continental margins and possibly at the mouths of canyons at the base of the slope. This issue requires urgent attention, as understanding of deep-sea litter distribution is still limited and potential impacts to benthic ecosystems are poorly understood.