Microplastics form extensive deep-sea drifts

More than 10 million tonnes of plastic waste enter the oceans every year. Until now, it has been thought that these decompose into microplastics, which sooner or later sink vertically down to eventually settle on the seabed. In the light of recent studies, it turns out that this is not true.

A British team of scientists has shown that microplastics get caught up in powerful bottom currents that, among other things, are responsible for supplying oxygen and nutrients to deep-sea ecosystems. The researchers also concluded that this creates huge drifts that threaten to overtake these ecosystems.

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Researchers led by geologist Ian Kane of the University of Manchester and geologist Michael Clare of the National Oceanography Centre in Southampton, studied seafloor currents in the Tyrrhenian Sea and Mediterranean Sea off the west coast of Italy, operating within a global network of deep-sea flows driven by salinity and temperature differences.

^{Picture: Ian Kane}

More than half of the plastics are said to sink immediately, while others are pulled down over time after they become covered in algae and sediments of organic matter. Their descent to the bottom is not vertical, however, as along the way they are likely to be driven by currents along deep-sea canyons and become part of sediment ‘avalanches’ until the currents weaken.

Drifts, which can be many kilometres in diameter and hundreds of metres high, form at points that are likely to coincide with those where ecosystems such as deep-sea coral reefs have formed.

The scientists took sediment samples from a depth of several hundred metres. They separated and counted microplastics under a microscope in the laboratory, using infrared spectroscopy to identify the types of polymers present.

In a single 5cm thick layer, occupying just 1m², they discovered as many as 1.9 million microplastic particles and this is the highest level recorded on the seabed.

Scientists say that most microplastics from the seabed are fibres from clothing and textiles. They threaten marine organisms because they can be ingested by them, and even if they were originally non-toxic, harmful toxins can accumulate on their surface.

“The cheap plastic products we take for granted ultimately have to end their lives somewhere. Clothes that might entertain one season in your wardrobe remain at the bottom of the sea for decades to centuries, potentially harming these unique and still poorly understood creatures that inhabit the deep sea.” – say Kane and Clare

Source: science.sciencemag.org