Underwater avalanches capable of moving at speeds of up to eight metres-per-second have been found responsible for moving vast quantities of microplastics into the deep sea, transporting plastic pollution from the continental shelf to depths of more than 3,200 metres.

These are the findings of a new study, published today in the journal of Environmental Science and Technology and led by researchers from both The University of Manchester and the UK’s National Oceanography Centre (NOC).

The study has gone new lengths to showcase the extent of the pervasiveness of the microplastics crisis. Studies have shown that more than 10 million tonnes of plastic waste enter the ocean each year, yet while striking images of floating debris have driven effort to curb pollution, this visible waste is actually less than 1% of the total plastic pollution within it.

Scientists have long-suspected that it was these underwater avalanches – known as turbidity currents – playing a major role in distributing microplastics across the seafloor. It was the team of at The University of Manchester, however, who were first to demonstrate this through their research on ‘Microplastic Hotspots’ in the Tyrrhenian Sea.

It wasn’t until now the actual process had been observed or recorded in a real-world setting.

The findings highlight the significant threat that microplastics pose to the environment and the marine ecosystem, underlining the urgent need for stronger pollution controls.

“Microplastics on their own can be toxic to deep-sea life, but they also act as ‘carriers’, transferring other harmful pollutants such as PFAs, ‘forever chemicals’, and heavy metals which makes them an environmental ‘multi-stressor’ which can affect the entire food chain,” said Dr Peng Chen, lead author on the study at the University of Manchester.

The research focused on Whittard Canyon in the Celtic Sea, a land-detached canyon over 300km from the shore. Through a combination of in-situ monitoring and direct seabed sampling – supported by the RRS James Cook and RRS Discovery – the team were able to witness a turbidity current in action, moving a “huge plume” of sediment at over 2.5 metres per second at over 1.5km beneath the water’s surface.

Samples taken directly from the flow revealed that these powerful currents are not only carrying just sand and mud, but a significant quantity of microplastic fragments and microfibres, too.

Further analysis found that the microplastics on the seafloor are mainly comprised of fibres from textiles and clothing, which are not effectively filtered out in domestic wastewater treatment pants and easily enter rivers and oceans.

Dr Ian Kane, geologist and environmental scientist at the University of Manchester, said: “These turbidity currents carry the nutrients and oxygen that are vital to sustain deep-sea life, so it is shocking that the same currents are also carrying these tiny plastic particles.

“These biodiversity hotspots are now co-located with microplastic hotspots, which could pose serious risks to deep-sea organisms. We hope this new understanding will support mitigations strategies going forwards.”

With the publication of the study come fresh calls from those involved in the research for robust policy interventions to limit the future flow of plastics in natural environments and minimise their impacts on ocean ecosystems.

According to Dr Mike Clare of the National Oceanography Centre, this study has helped scientists “connect microplastic transport pathways” in the deep-sea and “find the missing microplastics.’

The team behind the study are to now focus their efforts on better understanding the effect that microplactics have on marine organisms, for example sea turtles and deep-sea fauna.

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