Dr. Miquel Canals, CRG Marine Geosciences, University of Barcelona, Spain
Galderic Lastras, CRG Marine Geosciences, University of Barcelona, Spain
Xavier Tubau, CRG Marine Geosciences, University of Barcelona, Spain
Jesús Rivera, Instituto Español de Oceanografía, Madrid, Spain
Xavier Rayo, CRG Marine Geosciences, University of Barcelona, Spain
David Amblas, Scott Polar Research Institute, University of Cambridge, UK
Anna Sanchez-Vidal, CRG Marine Geosciences, University of Barcelona, Spain
Antoni M. Calafat, CRG Marine Geosciences, University of Barcelona, Spain
Litter peak concentrations in submarine canyons of the Northwestern Mediterranean Sea and the critical role of hydrodynamic processes in their dispersal
In this contribution we report litter abundance, type and distribution in three large submarine canyons of the NW Mediterranean Sea, namely Cap de Creus, La Fonera and Blanes canyons based on direct seafloor observation with a Remotely Operated Vehicle (ROV). One common feature of the investigated canyons is that the three of them have their heads deeply INCISEd on the continental shelf, with their heads at short distance (<4 km) from the shoreline. The main aim of the work performed was determining the relationships between active hydrodynamic processes and litter distribution, thus going beyond previous, mostly descriptive studies on litter occurrence in the deep-sea.
Litter was monitored using the Liropus 2000 ROV at water depths ranging from 140 to 1731 m. Most litter consisted of plastic objects (72%), which were followed by lost fishing gear of different types (17%) and metal objects (8%). La Fonera and Cap de Creus canyons showed the highest mean concentrations of litter ever seen on the deep-sea floor, with 15,057 and 8090 items per km2, respectively. The largest concentrations (i.e. “litter hotspots”) of litter were found on canyon floors at depths in excess of 1000 m. In contrast, litter objects were much less abundant on the canyon walls.
The analysis of litter objects in the studied canyons and their potential sources suggest a land source for the majority of the debris. The land-to-ocean transport of litter is assumed to involve wind transport, river discharge and direct dumping along the coastline, with coastal urban and industrial concentrations representing a permanent source of litter, in contrast with a seasonal component essentially driven by tourism and associated activities during summer months.
The finding of litter hotspots formed by mixtures of land-sourced litter items but also natural debris such as sea urchin carcasses requires efficient transport processes down to the deeper canyon reaches. Such hotspots may also involve marine-sourced litter (e.g. fishing gear) that enhance the trapping efficiency of further debris by such concentrations.
High-energy, down canyon near-bottom flows associated to seasonal dense shelf water cascading and severe coastal storms are known to occur in the investigated canyons where peak near-bottom current velocities in excess of 1 m s-1 have been measured in situ. Since these flows are seasonal, and occur with different intensities from one year to another, we hypothesize that the transfer of large amounts of light litter object to the deep reaches of the investigated canyons occurs episodically during relatively short periods of time (days to several weeks). In between “litter transfer events”, debris tend to accumulate on the shallower continental shelf and canyon heads, which behave as temporary “debris accumulation zones” that discharge at the occasion of such high-energy events.
Theme 4: Physical and anthropogenic disturbance in submarine canyons, conservation and marine policy
Submarine canyons, litter dispersal, hydrodynamic processes, Northwestern Mediterranean