Accumulation of debris on the deep sea floor off the French Mediterranean coast

Abstract

The distribution and abundance of large marine debris were investigated on the continental slope and bathyal plain of the northwestern Mediterranean Sea during 3 oceanographic cruises undertaken in June 1994, July 1995 and April 1996. Different types of debris were enumerated, particularly pieces of plastic, plastic and glass bottles, metallic objects, glass and diverse materials including flshing gear. The results showed considerable geographical variation, with concentrations ranging from 0 to 78 pieces of debris ha-' In most stations sampled, plastic bags accounted for a very high percentage (more than 70%) of total debrls. In the Gulf of Lions, only small amounts of debris were collected on the continental shelf. Most of the debris was found in canyons descending from the continental slope and in the bathyal plain, with high amounts occurring to a depth of more than 500 m. An additional cruise was undertaken using the manned submersible Cyana (provided by IFREMER). Sixteen dives (46.07 km distance on the floor) conducted in canyons off Marseilles and Nice (France) ranging from 40 to 1448 m in depth, enabled us to locate accumulation areas. All these results are interpreted in terms of hydrological conditions.

Global Distribution, Composition and Abundance of Marine Litter

“…The geographic distribution of debris on the ocean floor is strongly influenced by hydrodynamics, geomorphology and human factors (Galgani et al 1996;Pham et al 2014). Moreover, there are notable temporal variations, particularly seasonal, with tendencies for accumulation and concentration of marine litter in particular geographic areas (Galgani et al 1995).…”

“…Only few studies have assessed debris below 500 m depth (June 1990;Galil et al 1995;Galgani et al 1996Galgani et al , 2000Galgani and Lecornu 2004;Keller et al 2010;Miyake et al 2011;Mordecai et al 2011;Bergmann and Klages 2012;Wei et al 2012;Pham et al 2013Pham et al , 2014Ramirez-Llodra et al 2013, Schlining et al 2013Fischer et al 2015;Vieira et al 2014);Galgani et al (2000) observed trends in deep-sea pollution over time off the European coast with an extremely variable distribution and debris accumulating in submarine canyons. Miyake et al (2011) recorded debris down to 7,216 m depth in video surveys from the Ryukyu Trench.…”

“…Litter was primarily composed of plastic and accumulated in deep-sea trenches and depressions. Accordingly, several authors (Galgani et al 1996;Mordecai et al 2011;Pham et al 2014) concluded that submarine canyons may act as a conduit for the transport of marine debris into the deep sea. Recent studies conducted in coastal deep-sea areas along California and the Gulf of Mexico (Watters et al 2010;Schlining et al 2013;Wei et al 2012) confirmed this pattern.…”

“…Rivers can also transport waste far offshore because of their high flow rate and strong currents (Galgani et al 1995(Galgani et al , 1996(Galgani et al , 2000. Alternatively, small rivers and estuaries can also act as a sink for litter, when weak currents facilitate deposition on shores and banks (Galgani et al 2000).…”

“…Accordingly, even in the shallow subtidal abundance and distribution patterns can differ substantially from the adjacent strandlines with plastics being the most important fraction at sea. In general, bottom debris tends to become trapped in areas of low circulation where sediments are accumulating (Galgani et al 1996;Schlining et al 2013;Pham et al 2014). The consequence is an accumulation of plastic debris in bays, including lagoons of coral reefs, rather than in the open sea.…”