Microplastic hotspots on the seafloor

The ocean currents that transport nutrients and oxygen to organisms on the deep-sea floor also carry microplastics there, creating hotspots of up to 1.9 million pieces per square metre, according to a study published in the journal Science.

This is the most reported for any seafloor environment in the world, write Ian Kane, from the University of Manchester, UK, and co-authors. 

Every year, eight million metric tonnes of plastic end up in the world’s oceans, which have blown out to at least five trillion particles weighing more than 250,000 tonnes.

People have been shocked by photos of the ocean’s “garbage patches” where gyres have swept buoyant rubbish, mostly plastic. But plastics accrued on the water’s surface only account for around 1% of the estimated total in the world’s oceans.

The other 99%, of which 13.5% is estimated to be microplastics, is thought to end up in the deep sea, but where it is and how it gets there hasn’t been clear until now.

To investigate this, the European team collected sediment samples from the seafloor of the Tyrrhenian Sea in the Mediterranean and linked them to calibrated models of deep ocean currents and detailed mapping of the seafloor.

Back in the lab, they separated the microplastics from sediment, counted them under the microscope and used infra-red spectroscopy to verify the plastic types.

They found the plastics can either settle slowly or be rapidly swept to the deep seafloor by sporadic turbidity currents – powerful underwater avalanches – that travel down through submarine canyons.

Once there, they are picked up and carried by continuously flowing bottom currents that deposit them in concentrated patches of debris – most of which are comprised of fibres from textiles and clothing.

“This concentration exceeds the highest levels previously recorded,” the team writes, “including those from deep-sea trenches, and is more than double that documented in submarine canyons.”

They found no relationship between the location of microplastic concentrations and their land sources, thus providing important insights to inform predictions of microplastic locations and investigations of their impact on deep sea ecosystems.

“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,” says study co-lead Mike Clare, from the University of Southampton Waterfront Campus.

“The results highlight the need for policy interventions to limit the future flow of plastics into natural environments and minimise impacts on ocean ecosystems.”