Establishing standardized protocols for characterising marine plastic litter in submarine canyons: implications for regulation and conservation

Litter pollution is a global concern, occurring in all oceans, including deep benthic habitats, where the extent of the problem remains largely unexplored. Characterization of litter on river banks and beaches have been standardized in the OSPAR convention. In the deep sea, marine litter assessments result from "punctual” observations on the seabed by remotely operated vehicle (ROV) imaging, generally conducted during biodiversity survey cruises, or by trawling, with limited data in rocky areas and vulnerable marine ecosystems. Additionally, creating protocols with gaps makes temporal and geographical comparison difficult.

Submarine canyons host a wide variety of marine species, particularly with the presence of reef-building cold-water corals that provide habitat for many species. But canyons are also a major sink for waste debris that represent a threat to deep-sea biodiversity. Based on the OSPAR litter surveys, we propose a new protocol to standardize quantification of marine litter in canyon ecosystems, by conducting repeated 100-meter transects using ROV video capture.

This protocol was tested at three sites in the Lacaze-Duthiers Canyon (Gulf of Lion, Western Mediterranean Sea), along a longitudinal gradient from 300 to 600 m depth, where a high density of cold-water coral reefs occurs. We observed an average of 13 000 ± 6 000 items.km^-2, which increases by more than 150 times the previous values obtained with other protocols. These data however agree better with measurements made in close areas after a significant effort (> 60 dives compared to 3 dives here). A significant part of the marine litter found at these sites appeared to be fishing-related plastics and, to a lesser extent, bags, mostly entangled in coral reef structures.

This easy-to-implement deep-sea protocol is a promising tool for the assessment of the descriptor 10 of the European Marine Framework Directive (MFD), which ensures that marine debris do not harm habitat condition.