Single-use plastics dominate debris on the North Pacifics deep ocean floor

By doing video surveillance deep in the ocean, researchers working in the North Pacific have discovered the densest accumulation of plastic waste ever recorded on an abyssal seafloor, finding that the majority of this waste is single-use packaging.

The study, published March 29 in Marine Pollution Bulletin, highlights the huge environmental burden of single-use plastic, and also sheds more light on what happens to plastic once it goes into the sea.

"The majority of plastic debris that [ends] up in the ocean [is] missing," said Ryota Nakajima, a marine biology researcher at the Japan Agency for Marine Earth-Science and Technology. "Each year, more than 10 million tons of plastics make their way into the ocean, but the abundance of plastics floating on the ocean surface represents merely a few percent of the plastics in the ocean."

More precisely, between 4.8 million and 12.7 million tons of plastic makes its way into the ocean every year. Plastic in the ocean poses a huge threat to the safety of the environment, particularly when it deteriorates into microplastics. Plastic disrupts habitats, is mistaken for food by animals and spreads volatile toxins through the water.

However, most plastic waste is deep in the sea, leaving many people blissfully unaware of the harm it causes.

"The abundance of plastic debris leaking into the ocean continues to increase, but the floating plastic debris on the surface of the ocean eventually is transported into the deeper water," Nakajima said. "This will foster illusions in your mind that debris is not increasing. But the truth is, plastic debris accumulates on the deep-sea floor as undying garbage."

Nakajima and his colleagues sought to track some of it down in the oceans around Japan, which he described as "a hotspot of floating microplastics."

According to a 2015 report from Science on plastic ocean pollution, 12 of the top 20 countries with the most poorly managed plastic waste are in Asia, with China being the worst.

The proximity of these plastic-producing countries to the Kuroshio ocean current, one of the largest in the world, allows excessive waste to accumulate within the recirculation gyres, areas of the current where water moves in a continuous circle.

From there, the plastic eventually sinks and comes to rest on the ocean floor. It was on the abyssal seafloor underneath one of these recirculation gyres that Nakajima and his colleagues searched for plastic by diving 5,800 meters in a submarine and capturing footage.

They found a massive accumulation of waste.

"Our data confirm that view that the debris problem in the marine environment is serious even in the abyssal zone," Nakajima said, adding that bottom-dwelling plastic "is ubiquitous on the abyssal plain."

To understand better what kinds of plastic are trapped by currents like the Kuroshio, the team was also interested in characterizing the type of waste found on the abyssal seafloor.

They found that more than 80% of the debris was single-use plastics such as bags, food packaging and water bottles. And the overall density of plastics found in the area, 4,561 items per square kilometer, was the largest ever recorded on any abyssal seafloor by two orders of magnitude.

This waste has been accumulating for decades. The team found an old package for steak from 1984 that still appeared visually intact, with vivid colors. And because plastic takes so long to biodegrade, particularly in the ocean, this waste is unlikely to go anywhere any time soon and the pile will only continue to increase in size as the current keeps moving.

"Plastic debris on the deep seafloor will most likely persist for at least a century," Nakajima said.

The team is now planning to look for plastics in the other recirculation gyre of the Kuroshio current, which is located south of Japan. The researchers also hope that their work will encourage others to search for missing plastic in the oceans around Asia.

"The journey to find 'missing plastics' has just begun," Nakajima said. "Further investigations into the deep-sea floor lie downward of ocean currents sourced from the massive waste producers of Asian countries will cut through the mystery of the missing plastics."

The study, "Massive occurrence of benthic plastic debris at the abyssal seafloor beneath the Kuroshio Extension, the North West Pacific," published March 29, 2021 in Marine Pollution Bulletin, was authored by Ryota Nakajima, Masashi Tsuchiya, Akinori Yabuki, Shuhei Masuda, Tomo Kitahashi, Yuriko Nagano, Tetsuro Ikuta, Noriyuki Isobe, Heather Ritchie, Kazumasa Oguri, Satoshi Osafune, Koichi Iijima, Takao Yoshida, Sanae Chiba and Katsunori Fujikura, Japanese Agency for Marine-Earth Science and Technology; Haruhiko Nakata and Takuya Yamauchi, Kumamoto University; and Kiichiro Kawamura and Maki Suzukawa, Yamaguchi University.