Plastic waste generated by human activities has now reached the deepest point in the Mediterranean; the 5,112 metre-deep Calypso Deep – an iconic location within the depths of the Ionian Sea.

A total of 167 objects – made up mainly of plastics but including glass, metal, and paper – have now been identified at the bottom, of which 148 are marine debris and 19 others are of possible anthropogenic origin. 

The results of the study – presented in an article published in Marine Pollution Bulletin and conducted by researchers at the University of Barcelona – represent, they say ‘one of the highest concentrations of marine plastic and litter ever detected at great depths.’

To reach the depths of the Calypso Deep, the research team – comprising Miquel Canals, from the University of Barcelona’s Faculty of Earth Sciences; Georg Hanke, from the European Commission’s Joint Research Centre, and Victor Vescovo, from Caladan Oceanic, among others – used a high-tech manned submarine called the Limiting Factor.

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.

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.

The study describes the inner part of the trench as ‘kidney-shaped’, measuring approximately 20km by 5km in area.

In answer to how the plastic litter found at the bottom of the Deep come to reach this point, Miquel Canals, professor at the Department of Earth and Ocean Dynamics and director of the University of Barcelona Chair on Sustainable Blue Economy, believes the source to be both terrestrial and marine – arriving there by being transported on ocean currents, as well as direct dumping.

“Some light waste, such as plastics, comes from the coast, from where it escapes to the Calypso Deep, just 60 kilometres away,” he said. “Some plastics, such as bags, drift just above the bottom until they are partially or completely buried, or disintegrate into smaller fragments.

“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’.”

The shape of the trench combined with the weaker currents that move through – moving at around two centimetres per second – only aid in the accumulation of debris at its bottom.

Meanwhile, the currents carrying floating debris do so mainly from the southern Ionian Sea and marine areas further south. Surface eddies also tend to form, concentrating debris inland. It’s when these eddies are located over the Calypso Trench, some debris tends to fall slowly to the bottom. Surface currents have also been found to transport debris from the Adriatic Sea to the north, through the Strait of Otranto and from the waters off north-western Greece.

The debris – plastics, metal, and glass – identified by the international team has elicited from them a dire warning of the ‘need to implement global policy actions’ to reduce our waste, as well as to encourage changes in citizens’ consumption habits and waste reduction ‘in order to protect seas and oceans around the world.’

Critical to the study was the cutting edge technology found aboard the deep-submergence vehicle, the Limiting Factor, built by Triton Submarines. This vehicle, which is deployed from specially prepared mother ships, is capable of carrying two passengers to the deepest ocean trenches. During the inspection on the seabed, this unique technological device moves slowly – at around 1.8 kilometres per hour – to obtain good quality images.

In the Calypso Deep, the Limiting Factor was able to cover a distance equivalent to 650 metres in a straight line, during a 43-minute stay near the bottom.

“On each dive, more time is usually spent descending and ascending the vehicle back to the surface than inspecting the bottom. Each complete dive usually takes a few hours,” said Canals.

The technology used by the Limiting Factor has made it possible to calculate the density of marine litter at the bottom of the trench, although no significant impacts on marine life have been detected – being particularly impoverished in biodiversity terms within this chasm of the Ionian Sea. The images have only so far revealed the presence of Coryphaenoides mediterraneus, a fish of the macrouridae family, and the decapod Acanthephyra eximia.

“However, in places with more biodiversity, there are different types of interaction between the debris deposited on the bottom and the organisms, such as ghost fishing, burial, hooking, or ingestion, but also the use of debris as a substrate where animals can grow, hide or lay their eggs,” said Canal.

The first evidence of debris on the world’s seabed dates back to 1975, in the Skagerrak Strait in the North Atlantic. Canyons and seamounts are the major accumulation points for debris, which can be buried, crumbled, or moved by gravity, ocean currents, and other oceanographic factors.

Today, the Mediterranean is a marine region particularly affected by this environmental problem. In 2021, a study already identified the Strait of Messina as the area with the highest known density of marine litter in the world.

“The Mediterranean is an enclosed area, surrounded by humanity, with intense maritime traffic and widespread fishing activity. The evidence provided by our research should shake up global efforts, and in particular in the Mediterranean, to mitigate waste dumping – especially plastics – in the natural environment and ultimately in the sea, in line with the UN Global Plastics Treaty,” said Canal.

Unlike beaches or the coastline, where populations congregate and litter occurs at large scale, the ocean floor is still a largely unknown area. This makes it difficult to raise social and political awareness about its conservation.

The belief among this research team is that a joint effort is now needed between scientists, communicators, journalists, the media, and other people with social impact to raise such awareness and make the invisible visible.

“The problem is there, and it has an enormous scope – even if it is not directly visible,” said Canals. “We should not forget about it.”

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