Ocean currents in the deep sea are creating microplastic ‘hotspots’ which host around 1.9 million tiny pieces of debris per square metre, scientists have revealed.
US researchers say slow-moving currents near the bottom of the Mediterranean direct the flow of microplastics, creating plastic ‘hotpots’ in sediments of the deep sea.
These currents also supply oxygen and nutrients to deep sea creatures, which are likely ingesting toxic microplastics and other microfibres from textiles.
Up to 1.9 million pieces of plastic per square meter is one of the highest reported values for any seafloor setting globally.
While plastic on the surface of our oceans has been well-documented, researchers wanted to investigate the plastic make-up of the mysterious ocean floor.
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A collection of microfibres – micro particles from clothing – even thinner than a human hair. Scale is 1,000 μm (micrometres), or one millimetre
The team analysed sediment samples from the Tyrrhenian seafloor off the western coast of Italy in the Mediterranean to determine the type of plastic present in the deep sea
‘It’s unfortunate, but plastic has become a new type of sediment particle, which is distributed across the seafloor together with sand, mud and nutrients,’ said Dr Florian Pohl, of the Department of Earth Sciences at Durham University, one of the study authors.
‘Thus, sediment-transport processes such as seafloor currents will concentrate plastic particles in certain locations on the seafloor, as demonstrated by our research.’
Microplastics, small pieces of plastic and fibres less than five millimetres in length, can come from plastic bottles, packaging, cosmetics and micro-beads in toiletries.
These and microfibres, from textiles and clothing, are small enough to pass through the filter systems in domestic waste water treatment plants.
A collection of microplastic fragments. Microplastics come from cleaning or personal care products, car and truck tires, as well as from larger plastic products such as bottles and plastic bags
They can also come from fishing nets and other sources, such as shipping and oil and gas industries.
More than 10 million tons of plastic waste enters the oceans each year, but plastic that floats on the surface of the sea accounts for only 1 per cent.
The rest is thought to be present in the deep ocean, but until now it has been unclear how it’s distributed there.
To find out more, the research team used high-resolution data from sediments sampled from the Tyrrhenian Sea in the Mediterranean, off the western coast of Italy.
The team analysed the sediment samples from the Tyrrhenian seafloor to determine the type of plastic present in the deep sea.
They then developed models of deep ocean currents, showing how these movements controlled the distribution of microplastics on the seafloor.
The tiny fragments had been hard to locate because currents of deep-sea ‘conveyor belts’ had dragged them deep into the ocean through submarine canyons, the team explained.
They evade filtering at sewage treatment plants and easily enter rivers and oceans.
In the ocean they either settle slowly, or are flushed away by powerful underwater avalanches, known as ‘turbidity currents’, which travel down submarine canyons to the deep seafloor.
Microplastics were focused within depths of 600 to 900 meters, where slow-moving currents have the greatest interaction with the seafloor, the team found.
Artist’s impression of the seafloor microplastic hotspots controlled by deep-sea circulation, driven by episodic ‘turbidity currents’
The fine-grained particles are transported by currents to the bottom of the ocean, resulting in large sediment accumulations called ‘contourite drifts’, they said.
All of the team’s ocean samples were found to contain microplastics, most of which were fibres, while the highest concentrations occurred on the mounds of the contourite drifts.
‘Almost everybody has heard of the infamous ocean “garbage patches” of floating plastic, but we were shocked at the high concentrations of microplastics we found in the deep seafloor,’ said Dr Ian Kane, of the University of Manchester, who led the study.
A solitary microplastic fibre, around 0.1mm in length. Microfibres can be released into rivers and seas when we wash our synthetic fabrics at home
‘We discovered that microplastics are not uniformly distributed across the study area – instead they are distributed by powerful seafloor currents which concentrate them in certain areas.’
Once in the deep sea, the microplastics are readily picked up and carried by continuously flowing seafloor currents that merge the waste within large drifts of sediment – eventually poisoning marine life and entering the food chain.
The scientists hope their findings, published in the journal Science, will help direct research into the impact of microplastics on marine life and help predict the locations of other deep sea plastic hotspots.
Policymakers should act now to limit the future flow of plastics into the oceans to reduce damage to marine life.
‘Our study has shown how detailed studies of seafloor currents can help us to connect microplastic transport pathways in the deep-sea and find the ‘missing’ microplastics,’ said Dr Mike Clare, of the National Oceanography Centre in Southampton.
‘The results highlight the need for policy interventions to limit the future flow of plastics into natural environments and minimise impacts on ocean ecosystems.’