Waste generated by human activities has now reached the deepest point in the Mediterranean: the 5,112-metre-deep Calypso Deep in the Ionian Sea. A total of 167 objects - mainly plastics, glass, metal and paper - have been identified at the bottom, of which 148 are marine debris and 19 others are of possible anthropogenic origin. These results represent one of the highest concentrations of marine litter ever detected at great depths.
These findings are presented in an article published in Marine Pollution Bulletin . The article's main authors are Miquel Canals, from the University of Barcelona's Faculty of Earth Sciences; Georg Hanke, from the European Commission's Joint Research Centre (JRC); François Galgani, from the French Research Institute for Exploitation of the Sea (IFREMER), and Victor Vescovo, from the American company Caladan Oceanic.
To reach the bottom of the trench - the most critical aspect of the entire study - the team used a high-tech manned submarine, the Limiting Factor, a deep-submergence vehicle (DSV). The images provided by the Limiting Factor confirm that, in addition to accumulating on coasts, surface waters and shallower bottoms, marine litter also reaches the deepest and most remote points of the Mediterranean, a sea that is particularly affected by human activities.
Marine debris in the deepest Mediterranean Sea
The Calypso Deep is a depression located 60 kilometres west of the Peloponnese coast in Greece, within the so-called Hellenic Trench, with several similar but shallower depressions. Located in an area of high seismicity due to active faults, it is surrounded by fairly steep, stepped relief and has slopes of thousands of metres, and a virtually flat bottom. The inner part of the trench, more than 5,000 metres deep, is kidney-shaped and measures approximately 20 km by 5 km.
But how did the rubbish get so deep? The debris at the bottom of the Calypso Deep "comes from various sources, both terrestrial and marine. It could have arrived by various routes, including both long-distance transport by ocean currents and direct dumping", explains Miquel Canals, professor at the Department of Earth and Ocean Dynamics and director of the UB Chair on Sustainable Blue Economy. "Some light waste, such as plastics, comes from the coast, from where it escapes to the Calypso Deep, just 60 kilometres away. Some plastics, such as bags, drift just above the bottom until they are partially or completely buried, or disintegrate into smaller fragments" he says.
"We have also found evidence of the boats' dumping of bags full of rubbish, as revealed by the pile-up of different types of waste followed by an almost rectilinear furrow. Unfortunately, as far as the Mediterranean is concerned, it would not be wrong to say that "not a single inch of it is clean", warns the expert. The Calypso Trench traps and accumulates the anthropogenic materials that reach it at the bottom: "It is a closed depression, which favours the accumulation of debris inside it. The weak currents in the trench - about two centimetres per second and, exceptionally, 18 - also facilitate the deposition of light debris at the bottom".
