Oceans of rubbish

Human litter has worked its way into remote and inaccessible parts of the ocean where even humans haven’t managed to penetrate. A large-scale European seafloor survey of 588 seafloor transects, photos and video taken over 10 years at depths ranging from 35 metres to 4.5 kilometres, found the presence of bottles, plastic bags, fishing nets and other types of human litter at all sample locations, from the continental shelf of Europe to as far as the Mid-Atlantic Ridge some 2,000 km away from land.

Litter present in all marine habitats

“This survey has shown that human litter is present in all marine habitats, from beaches to the most remote and deepest parts of the oceans,” says one of the researchers, Professor Kerry Howell, in a press release from Plymouth University’s Marine Institute. “Most of the deep sea remains unexplored by humans and these are our first visits to many of these sites, but we were shocked to find that our rubbish has got there before us.”

The study area included the northeast Atlantic Ocean, Arctic Ocean and Mediterranean Sea, with sampling from continental shelves, continental slopes, submarine canyons, seamounts, banks, mounds, ocean ridges and deep basins. Most of the sampling was done using remotely operated vehicles (ROVs), towed camera systems or trawls – as it is difficult for humans to explore first-hand at great depths.

Nearly half of all litter is plastic

The researchers reported in their published paper that the highest litter density occurs in submarine canyons, whilst the lowest density can be found on continental shelves and on ocean ridges. “Plastic was the most prevalent litter item found on the seafloor. Litter from fishing activities (derelict fishing lines and nets) was particularly common on seamounts, banks, mounds and ocean ridges.”

Plastic was found in 98% of trawls and accounted for 41% of the litter. The published paper highlighted the mobility of plastics originating from coastal and land sources and being carried along continental shelves and slopes into deep water. Stormwater drains, sewers, rivers and wind play a role in moving litter to the coast and the sea.

In Plymouth University’s press release, Dr Veerle Huvenne, Seafloor and Habitat Mapping Team Leader at the National Oceanography Centre, Southampton, explained that submarine canyons form the main connection between shallow coastal waters and the deep sea. “Canyons that are located close to major coastal towns and cities, such as the Lisbon Canyon offshore Portugal, or the Blanes Canyon offshore Barcelona, can funnel litter straight to water depths of 4,500 m or more.”

Derelict fishing gear accounted for a further 34%. Glass (particularly bottles), metal, clinker (burnt coal residue), wood, paper/cardboard, fabric and pottery were also found.

Marine biologist from the Spanish Research Council and co-author of the research Dr Eva Ramirez-Llodra says the clinker deposits on the seafloor were an interesting discovery. “This is the residue of burnt coal that had been dumped by steam ships from the late 18th century onwards. We have known that clinker occurs on the deep-sea bed for some time, but what we found was the accumulation of clinker is closely related with modern shipping routes, indicating that the main shipping corridors have not been altered in the last two centuries.”

The effects of litter on sea creatures

Millions of tonnes of litter wind up in the sea every year and have been documented to cause problems for marine mammals and fish when mistaken for food and eaten, for example, eaten plastic can cause choking or block digestive systems or give animals a sense of feeling full and they can starve to death. Nets and lines can entangle coral or entangle, injure and drown marine wildlife – a process known as ‘ghost fishing’. Ironically, toxins found in various litter, especially plastics, can enter the food chain and eventually come back to affect humans and other predators.

The survey researchers say their results “highlight the extent of the problem and the need for action to prevent increasing accumulation of litter in marine environments”.

The international study involved 15 organisations across Europe and was led by the University of the Azores, Portugal. Christopher Pham, from the university’s Departamento de Oceanografia e Pescas, was the lead author of the survey report. The study was a collaboration between the Mapping the Deep Project led by Plymouth University and the European Union-funded HERMIONE Project (Hotspot Ecosystem Research and Man’s Impact on European Seas), co-ordinated by the National Oceanography Centre, Southampton. Other project partners included the University of Southampton and the British Geological Survey.

Results of the survey were published in the 30 April 2014 edition of the open-access journal PLoS ONE.