Millions of tons of plastic are entering the ocean every year. But where it all ends up is still a mystery. A new study estimates between 3 and 11 million metric tons of it are lying on the seafloor. That’s up to a hundred times the amount thought to be floating on the surface.
“We’re just starting to crack the plastic-fate puzzle,” said environmental analytical scientist and doctoral student Alice Zhu of the University of Toronto, first author on the study, which was recently published in Deep Sea Research, Part I: Oceanographic Research Papers.
Zhu and her colleagues collated data from 41 plastic surveys spanning every major ocean in the world, going back 3 decades. They targeted large pieces of plastic (bigger than 5 millimeters) sitting on the ocean floor. They then used variables suspected to affect the distribution of plastic in the ocean, including the depth and slope of the seafloor, shipping intensity, fishing effort in the area, and distance to shore from the sample, to predict where in the ocean the world’s plastic is ending up.
Ocean currents were not considered as a variable because large, negatively buoyant plastics were assumed to fall close to their source.
The data showed that plastic debris clusters around continents, close to human populations and shipping routes.
The new statistic is one coauthor Denise Hardesty of Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) hopes will shock people into action.
“I think we often expect that the ocean takes care of itself and that it’s not something we need to think or worry about,” she said. “But so much of our food security, and the enjoyment of our lives, is connected to the ocean.”
Wide Margins
“Everyone is talking about plastics, but there are still some pretty fundamental unknowns.”
Independent microplastics researcher Jennifer Brandon, who was not involved in the study, said the research is important and timely. “Everyone is talking about plastics, but there are still some pretty fundamental unknowns.” Where it all ends up is one. (Countries met last week to negotiate a global plastic pollution treaty, aiming to have an agreement by the end of 2024.)
Though estimates of the amount of plastic at the sea surface exist, this is the first study that has attempted to quantify the amount on the seafloor.
Miquel Canals, a marine geologist with the University of Barcelona who was not involved in the study, commended the research but noted the wide margin of error in the results. “It would be desirable to narrow the overall range,” he wrote in an email.
Wide error margins are typical of current oceanic plastic research and are symptomatic of how much we still have to learn, Brandon said. Previous studies have predicted between 4 and 12 million metric tons of plastic might be entering the sea every year. A more recent study came to a much more conservative, but still wide-ranging, estimate of 0.13 to 3.8 million metric tons.
Narrowing down such large error margins is crucial, Zhu said, to balancing the budget of plastic pollution in the marine environment and identifying whether there are any “missing” plastics reservoirs.
Stemming the Flow
Dense plastics, such as fishing nets, fall to the seafloor immediately, whereas lighter plastics may float for some time until they accumulate enough barnacles and other organic growth to cause them to sink. Such pieces of plastic can travel far from their source.
In 2018, the discovery of a plastic bag lying in the Marianas trench—the deepest point on Earth’s surface—sparked fears a torrent of plastic might be reaching the abyssal region of the ocean.
The new study, however, suggests that around half the plastic in the ocean is concentrated in water shallower than 200 meters. That offers a glimmer of hope—we might one day be able to safely retrieve some of it, at least in areas that are accessible to divers. “It’s falling close to where we live,” Zhu said, “so there is hope that we can clean a lot of this up with the technology that we have.”
“We need to turn off the tap on plastic.”
The clock, however, is ticking. In shallower waters, plastics degrade quickly, breaking down to form microplastics that can persist in the environment indefinitely.
“Once they become microplastics they are much harder to quantify and clean up,” Brandon said. Her own research has shown how microplastics off the California coast have become part of the sedimentary record—a perpetual, toxic marker of the Anthropocene.
Zhu also emphasized that reducing plastic use and stopping it from getting into the sea must be prioritized. “It’s infinitely easier to prevent plastic from entering the environment in the first place than to clean it up after the fact,” she said. “We need to turn off the tap on plastic.”
—Bill Morris, Science Writer