Millions of Tons of Plastic Piling Up on Ocean Floors

New research estimates that up to 11 million tons of plastic pollution is sitting on the bottom of the world’s oceans.

Every minute, an entire garbage truck’s worth of plastic enters the oceans. With plastic use expected to double by 2040, understanding where all this plastic ends up is crucial for protecting marine life and ecosystems.

Dr. Denise Hardesty, a senior research scientist with CSIRO, Australia’s national science agency, said this is the first estimate of how much plastic waste sinks to the ocean floors, where it accumulates before breaking down into smaller pieces that mix into ocean sediments.

“We know that millions of tonnes of plastic waste enter our oceans every year but what we didn’t know is how much of this pollution ends up on our ocean floor,” Dr. Hardesty said.

“We discovered that the ocean floor has become a resting place, or reservoir, for most plastic pollution, with between 3 to 11 million tonnes of plastic estimated to be sinking to the ocean floor. While there has been a previous estimate of microplastics on the seafloor, this research looks at larger items, from nets and cups to plastic bags and everything in between.”

Alice Zhu, a PhD candidate from the University of Toronto who led the study, said the estimate of plastic pollution on the ocean floors could be up to 100 times more than the amount floating on the ocean surfaces based on recent estimates.

“The ocean surface is a temporary resting place of plastic so it is expected that if we can stop plastic entering our oceans, the amount would be reduced,” Ms. Zhu said. “However, our research found that plastic will continue to end up in the deep ocean, which becomes a permanent resting place or sink for marine plastic pollution.”

The researchers used scientific data to build two predictive computer models to estimate the amount and distribution of plastic on the ocean floors – one based on data from remotely operated underwater vehicles (ROVs) and the other from bottom trawling nets.

Using the ROV data, they estimated 3 to 11 million metric tons of plastic pollution resides on the ocean floors around the world. The ROV results also revealed that this plastic mass clusters around continents – with approximately half (46 percent) of the predicted plastic residing above 200 meters depth. The remaining 54 percent was predicted to be found at ocean depths from 200 meters down to 11,000 meters.

Although inland and coastal seas cover much less area than the open oceans (11 percent versus 56 percent of the Earth’s total area), these areas are predicted to hold just as much plastic mass as the rest of the ocean floors.

“These findings help to fill a longstanding knowledge gap on the behaviour of plastic in the marine environment,” Ms. Zhu said. “Understanding the driving forces behind the transport and accumulation of plastic in the deep ocean will help to inform source reduction and environmental remediation efforts, thereby reducing the risks that plastic pollution may pose to marine life.”

The research was published in the journal Deep Sea Research Part I: Oceanographic Research Papers in an article titled “Plastics in the deep sea – A global estimate of the ocean floor reservoir.” It is part of CSIRO’s Ending Plastic Waste Mission, which aims to change how we make, use, recycle and dispose of plastics.

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