Where’s the 99% of ocean plastic that we can’t see?

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. 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. What we commonly see accumulating at the sea surface is “less than the tip of the iceberg, maybe a half of 1% of the total,” says Erik Van Sebille, an oceanographer at Utrecht University in the Netherlands. But, he adds, the reality is that our 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, says Helge Niemann, a biogeochemist at the Royal Netherlands Institute for Sea Research, it could fragment into such small pieces that it can barely be detected. For the past two years, scientists from the nearby Monterey Bay Aquarium Institute have been using customised remote-control submersibles to take samples of the near-invisible plastic drifting far below the surface. “Just because you don’t see it, doesn’t mean it isn’t there” says Anela Choy, a professor of oceanography at the University of California San Diego, and thelead researcher on the project. Richard Thomson, the oceanologist who first coined the term “microplastic” in 2004 to describe difficult-to-capture bits under 2mm in length, has suggested that large amountscould be found in the deep ocean and sea floor. Plastic can become attached to ocean detritus that sinks, or fragment under the sun or waves, or find its way into something’s stomach. Watch: Royal weddings to novichok: The decade in headlines (Sky News) Choy’s team identified two kinds of animals, red crabs and translucent, filter-feeding creatures called giant larvaceans, which consume plastic and moving 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 mucous they periodically discard and let sink. A 2011 study examining plastic in fish in the north Pacific Ocean estimated that they ingested around 12,000 tons a year. In a later paper Van Sebille’s group noted that if the number held across the entire ocean, 100,000 tons of plastic could be inside animals at any one time. On a cool, gray June day in London, Alexandra Ter Halle, a researcher with 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, andcharacterising the bacteria in the samples. Ter Halle’s samples, though, would have to wait until she was back at her university, where she has specialized equipment for the detection of nanoplastistics – plastics that have broken down to sizes below a thousandth of a millimeter, smaller than a single cell. 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.