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By Michael Le Page

There’s even more plastic in the Pacific than we thought. At least 79,000 tonnes of plastic are floating in the Great Pacific Garbage Patch. That’s four to sixteen times as much as was estimated by two studies in 2014.

The Garbage Patch is an area of 1.6 million square kilometres between Hawaii and California. There, floating debris – from microscopic particles of plastic to large pieces like ropes and fishing nets – is carried by currents and accumulates. Similar patches exist in other oceans.

Researchers gathered data from aerial surveys and nets towed by ships, and fed it into a computer model. This showed there is around one kilogram of plastic per square kilometre in the outer regions, rising to more than 100kg/km2 near the centre.

Earlier studies probably greatly underestimated the mass of plastic because they were not as comprehensive. For instance, they had to rely on spotting flotsam from boats, rather than on aerial surveys. But the team that carried out the latest study says there has also been a real increase in the mass of plastic.

“The Great Pacific Garbage Patch is getting denser with floating plastic, but is not expanding in terms of surface area,” says team member Laurent Lebreton of The Ocean Cleanup in Delft, the Netherlands, an organisation trying to find ways to remove plastic from the seas.

Part of this increase could be flotsam washed out to sea by the 2011 Tohoku tsunami. Of the objects collected by the team, a third of those with a “made in” label came from Japan.

We still know little about how plastic affects ocean life but there is growing evidence that it is harmful to many creatures – including us. “Floating plastic litter can be ingested or entangle marine life, and carry invasive organisms across oceanic basins,” says Matthew Cole of the Plymouth Marine Laboratory in the UK.

The other mystery is where the rest of the plastic is. 79,000 tonnes is a lot, but based on estimates of how much floating plastic is washed into the sea from land and discarded from ships, the model suggests there should be millions of tonnes in the Great Pacific Garbage Patch alone. Other studies have come to similar conclusions.

The most likely explanations are that the missing plastic is sinking to the ocean floor, perhaps after being eaten and excreted, or is breaking down into pieces so small they are not being detected. Another suggestion is that bacteria in the ocean are evolving the ability to break down plastic.

Journal reference: Scientific Reports, DOI: 10.1038/s41598-018-22939-w