Once in the ocean, where does the plastic *go*?

Nature surveys the plastic in the seas, expects to see things like detergent bottles and Barbies breaking up into tiny “microplastic” particles, and doesn’t. So the question becomes… where does the plastic go?:

More than 5 trillion plastic pieces, with a combined mass of more than 250,000 tonnes, are floating in the ocean, researchers reported on 10 December in PLoS ONE.

On its face, the estimate is shockingly high — but it is still much lower than expected, amounting to less than 1% of the annual global production of plastic, says study co-author Hank Carson, a marine biologist at the Washington Department of Fish and Wildlife in Olympia.

A team led by Marcus Eriksen, research director at the Five Gyres Institute in Los Angeles, California, took samples with fine-mesh nets and visually counted pieces of trash on 24 expeditions through all five subtropical gyres — areas of rotating ocean currents where plastic collects — as well as coastal Australia, the Bay of Bengal and the Mediterranean Sea.

Eriksen’s team found that microplastic accounted for a small proportion of the estimated mass. Pieces larger than 200 millimetres across accounted for 75% of the total mass of plastic surveyed. But large items such as bottles, buoys and plastic bags would be expected to degrade into smaller and smaller pieces of floating trash. So where is all the microplastic?

The question has bedevilled the field for a decade, says Richard Thompson, a marine biologist at Plymouth University, UK, who coined the term ‘microplastic’ and suggested that plastic was breaking down into pieces so small that they slip through sampling nets. Others have proposed that the plastic is being washed ashore, that small particles are being fouled by bacteria and grime until they sink to the sea floor or that they are being eaten and incorporated into animal tissue and faeces — even that some bacteria are able to digest them.

Thompson finds that last hypothesis unlikely. “Polymer chemists say that all the plastic we have ever made is still with us,” he says.