- Plastic water bottles and snack-food packaging can be found in the deepest parts of the oceans, a new study has found.
- By poring over the three decades of deep-sea videos, researchers have found that fragments of plastic made up one-third of the debris, of which, 89 percent were single-use items such as plastic bags and water bottles.
- However, how all that plastic reaches the deep sea and affects deep sea creatures is still unclear.
Plastic water bottles and snack-food packaging have made their way to the deepest parts of the world’s oceans, drifting to depths of nearly 11,000 meters (36,100 feet), according to a new study published in Marine Policy.
Sanae Chiba, the study’s lead author and a scientist at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), said she was unsurprised by the discovery.
“The finding of deep-sea plastic itself is not really new but it is recent[ly] that we realised plastic debris are ubiquitous not only [at the] surface … but also bottom of the deep sea,” she said.
JAMSTEC has for years deployed manned submersibles and remotely operated vehicles (ROVs) to survey the ocean floor. Chiba and her colleagues revisited footage collected between 1984 and 2014 from locations in the North and South Pacific — including the Mariana Trench, the deepest part of the world’s ocean — the Indian Ocean, and the Atlantic Ocean. All the videos collected are archived online at the Deep-sea Debris Database (DsDD).
Poring over the three decades of videos, the researchers found a variety of man-made litter, from metal and scraps of cloth, to bits of fishing net and rubber. But fragments of plastic made up one-third of the debris. Of these, 89 percent were single-use items such as plastic bags and water bottles.
“While most of the deep-sea debris (plastic) surveys have been conducted in the areas shallower than 4,000 meters [13,100 feet], our study is the first scientific report on single-use plastics in the ocean deeper than 6,000 meters [19,700 feet],” Chiba wrote in an email to Mongabay.
One of the most remarkable visuals in the database is that of a plastic bag embedded in sediment in the Mariana Trench. The video was recorded in 1998 at a depth of 10,898 meters (35,750 feet).
Video: Plastic bags lying in the Mariana Trench at 10,898 meters. Video shot in 1998. (copyright Japan Agency for Marine-Earth Science and Technology).
But how does a plastic bag make its way to the deepest point on Earth?
The United Nations Environment Programme (UNEP) estimates that 11 million tonnes of plastics enter the ocean every year, pumped in from overflowing landfills, polluted rivers, stormwater drains and even strong winds.
Chiba’s study suggests that plastic can go deeper than other debris. Of all the debris in the ocean, the percentage of only plastic increased with depth. In fact, at dive sites below 6,000 meters, 52 percent of all litter turned out to be plastic, mostly single-use.
Chiba said tracking the journey of the plastic from land to sea and into the ocean’s depths was complicated.
“We can roughly guess the horizontal transportation route of plastics,” she said, referring to how it ends up in the sea. She cautioned that this guess was based on the assumption that single-use plastics move in the same way as water, which is not the case.
How that plastic then reaches the deep sea is even harder to figure out.
“Accuracy of current 3D ocean model is limited and [it] can only roughly estimate the transport and accumulation of plastics in [the] entire water column,” Chiba said. “In addition, we need to know accurately how much plastics enter the ocean, from which sources.”
Chiba also said scientists understood little about how a plastic bag started breaking apart once it entered the ocean, and where the tiny, often invisible fragments called micro-plastics ended up.
This is vital to know because plastics release toxic chemicals when they degrade, which can affect marine life. Crabs and sea cucumbers can ingest them while feeding off the sea floor. Fish and even corals can take in bits of plastic in the water.
Working up the food chain, the plastic can make its way to humans. But given that most marine life, including species consumed by humans, live no deeper than 200 meters (660 feet), is plastic likely to have any effect at depths greater than 6,000 meters?
There isn’t much life at these depths, but what little exists is unique and complex because of the environment it survives in, according to Thomas Linley, a deep-sea ecologist at the University of Newcastle, U.K., who was not involved in Chiba’s study.
“It is hard not to focus on the incredible depth and therefore pressure these animals are exposed to, as much as an African bull elephant standing on your thumbnail in the deepest areas,” he said.
While much of the deep sea is flat-plain habitat, a layer known as the abyssal zone that extends from 4,000 to 6,000 meters, oceanic trenches like the Mariana Trench are steep valleys that go even deeper, from 6,000 to 11,000 meters. The latter constitute what’s known as the hadal zone.
“So where the abyssal plain is like one huge habitat, the hadal trenches are like tiny islands, [and cover] less than 1 percent of the earth’s surface,” Linley said. Each of these hadal islands can harbor its own set of unique endemic fauna.
Video: Several sea anemones settled on a heap of plastic bags and cans at 3848m in the Nankai Trough. Video shot in 2003 (copyright Japan Agency for Marine-Earth Science and Technology).
How plastics affect creatures in these islands is unclear.
“The interaction of animals with their environment and each other is so complex, it’s very difficult to predict what might happen, and some of the results can be unexpected,” Linley said.
In Chiba’s study, 16 percent of the videos showed plastic in the vicinity of deep-sea organisms such ray-finned fish and sea anemones, which were often attached to the plastic. The videos even showed deep-sea molluscs entangled with plastic bags in the Japan Trench, at a depth of 8,046 meters (26,400 feet).
While soft plastics like bags can harm the animals by smothering them, hard plastics can give some animals like anemones an unfair advantage, Linley said. This is because sea anemones, much like barnacles, float in the water until they find a hard surface, typically rocks, to settle on. One of the study’s findings was that that even in places with no rocky surfaces, sea anemones were able to settle down on hard plastic. “This could potentially spread them into new areas as an invasive species,” Linley said.
Recognizing the magnitude of the problem, the United Nations Environment Assembly passed resolutions in December last year to tackle marine plastic pollution. Several countries have pledged to implement bans on single-use plastics nationally, but the resolutions are non-binding. China, India and the U.S., which are hotspots of plastic use, have declined to set any specific targets.
Nonetheless, the researchers are optimistic. Chiba, who is also on deployment to the United Nations Environment Program (UNEP), said there was global momentum to deal with the issue. “It just takes times to reach global agreement on the legally binding target with specific timeline,” she said.
Compared to other problems like ocean acidification and global warming, Chiba said she believed the plastic threat was simpler to understand. “Plastic pollution is caused by direct human activities,” she said, adding that this made the problem much more manageable and understandable.
Linley agreed, adding that single-use plastics, which were the biggest problem, were a much easier fix. “It can be overwhelming to hear nothing but negative environmental news,” he said, “but I really believe in the ability of people to make a sudden change for the better.”
Citation
Chiba, S., Saito, H., Fletcher, R., Yogi, T., Kayo, M., Miyagi, S., … & Fujikura, K. (2018). Human footprint in the abyss: 30 year records of deep-sea plastic debris. Marine Policy.
Green, D. S., Boots, B., Blockley, D. J., Rocha, C., & Thompson, R. (2015). Impacts of discarded plastic bags on marine assemblages and ecosystem functioning. Environmental science & technology, 49(9), 5380-5389.
Woodall, L. C., Sanchez-Vidal, A., Canals, M., Paterson, G. L., Coppock, R., Sleight, V., … & Thompson, R. C. (2014). The deep sea is a major sink for microplastic debris. Royal Society open science, 1(4), 140317.
