Every year, 8 million tonnes of plastic enter the ocean. Images of common household waste swirling in vast garbage patches in the open sea, or tangled up with whales and seabirds, have turned plastic pollution into one of the most popular environmental issues in the world.
However, for at least a decade, the biggest question among scientists who study marine plastic has not been why plastic in the ocean is so abundant, but why it is not. What scientists can see and measure, in the garbage patches and on beaches, accounts for only a tiny fraction of the total plastic entering the water.
So where is the other 99 percent of ocean plastic? Unsettling answers have begun to emerge.
Illustration: Constance Chou
What people commonly see accumulating at the sea surface is “less than the tip of the iceberg, maybe a half of 1 percent of the total,” said Erik van Sebille, an oceanographer at Utrecht University in the Netherlands.
“I often joke that being an ocean plastic scientist should be an easy job, because you can always find a bit wherever you look,” Van Sebille said, but added that the reality is that maps of the ocean essentially end at the surface and solid numbers on how much plastic is in any one location are lacking.
It is becoming apparent that plastic ends up in huge quantities in the deepest parts of the ocean, buried in sediment on the seafloor and caught like clouds of dust deep in the water column.
Perhaps most frighteningly, it could fragment into such small pieces that it can barely be detected, Royal Netherlands Institute for Sea Research biogeochemist Helge Niemann said.
At this point it becomes “more like a chemical dissolved in the water than floating in it,” Niemann said.
The 444km of coastline that runs from the narrow mouth of San Francisco Bay, past the open water of Monterey Bay to the scenic mountains and redwood forests of Big Sur, is the land border of the US’ largest national marine sanctuary. To anyone visiting the beaches near Santa Cruz or driving the coastal highways, it appears remarkably unspoiled. That is not the whole story.
Over the past two years, scientists from the nearby Monterey Bay Aquarium Institute have been using customized remote-controlled submersibles to take samples of near-invisible plastic drifting far below the surface.
“Just because you don’t see it, doesn’t mean it isn’t there,” said Anela Choy, a professor of oceanography at the University of California, San Diego and the lead researcher on the project.
Below what she called the “skin surface” of the ocean, the submersibles carefully filter seawater and take a snapshot of what is in it.
Her team found that at a depth of 200m, there were nearly 15 bits of plastic in every liter of water, similar to the amount found at the surface of the so-called garbage patches. The remote samplers were still finding plastic at their maximum depth of 1km.
However, that was just the start of the hunt.
“After two to three years of work, the honest truth is we have only one set of samples from one portion of the world’s entire ocean,” Choy said.
The group’s work is among the first to count the exact amount of plastic below the ocean surface and to show that plastic waste is abundant at lower depths.
Scientists have speculated about this for years.
Richard Thomson, the oceanologist who first coined the term “microplastic” in 2004 to describe difficult-to-capture bits less than 2mm in length, has suggested that large amounts could be found in the deep ocean and sea floor.
A 2017 paper from Van Sebille’s group said that, based on the amount of plastic entering the ocean and the potential ways it is known to sink, 196 million tonnes of plastic might have settled from the surface into the deep ocean since 1950.
The next steps are to show where the plastic comes from and to ascertain how it moves from the surface, where it is relatively easy to both find and track, to the depths.
The conventional view is that it is very hard to track ocean microplastics back to their source, but even very small bits of plastic do not necessarily look the same. By examining how laser light scatters when it hits different bits of plastic, researchers can create a fingerprint.
For example, the plastic found in Monterey Bay did not resemble the plastics used in local fishing equipment, but was mostly polyethylene terephthalate, a polymer used in disposable packing, indicating that it probably came from land.
How plastic descends to the deep ocean is, for the most part, a mystery. Due to its low density, most commercial plastic floats. It needs help to get below the surface. Plastic can become attached to ocean detritus that sinks, or fragment under the sun or waves, or find its way into something’s stomach.
Choy’s team identified two kinds of animals, red crabs and translucent, filter-feeding creatures called giant larvaceans, which consume plastic and move it to deeper water — either by eating it near the surface and expelling it lower down or, in the case of the larvaceans, in a layer of mucus they periodically discard and let sink.
This sort of unwitting animal transit has been observed in many species.
A 2011 study examining plastic in fish in the North Pacific Ocean estimated that they ingested about 12,000 tonnes per year.
In a later paper, Van Sebille’s group said that if the number held across the entire ocean, 100,000 tonnes of plastic could be inside animals at any one time.
The search for the missing maritime plastic has opened new frontiers of research. A decade ago the discovery of microplastics sparked a radical shift in the conception of plastic pollution. Scientists revealed the existence of billions of pieces of plastic almost too small to see, definitely too small to catch and easily eaten by the tiniest sea creature.
Now, they are making startling new discoveries about the extent of plastic pollution.
On a cool, gray day in London in June last year, Alexandra ter Halle, a researcher at Paul Sabatier University in France, was on a sailboat just below Tower Bridge taking samples of water from the Thames. It was the crew’s first stop on a tour of 10 European estuaries and the other scientists on board were doing familiar work, counting microplastic particles with microscopes and characterizing the bacteria in the samples.
However, Ter Halle’s samples would have to wait until she was back at her university, where she has specialized equipment for the detection of nanoplastics — plastics that have broken down to sizes of less than a thousandth of a millimeter, smaller than a single cell
Two years ago her group was the first to detect these particles in seawater. Ter Halle employs techniques similar to those used by forensic scientists to detect chemicals at crime scenes: The samples are ignited into a gas, bombarded with electrons and separated across an electric field to measure their weight and charge. They cannot be conventionally seen, only detected.
Nanoplastic research is still in its infancy, but laboratory tests have shown that unlike microplastics, nanoplastics are small enough to accumulate within the bloodstreams and cell membranes of a range of organisms, even passing the blood-brain barrier in a test on Japanese rice fish, and cause various toxic effects, including neurological damage and reproductive abnormalities.
“This question of where is all the plastic in the sea. For 40 years we sought out plastic we could see. Now we reach the nanoscale, which is very particular, very reactive, and we have to begin again,” Ter Halle said.
The huge amounts of plastic on the ocean surface were what originally sparked public and scientific interest in the plastic problem. In this way, they acted like a buoy, pointing the way to something much larger beneath the surface.
The deep ocean is “the world’s largest habitat,” Choy said.
Humanity is only now beginning the accounting of how much of its plastic has ended up there.
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