Seafloor microplastic hotspots controlled by deep-sea currents

New research has revealed the highest levels of microplastic yet recorded on the seafloor, with up to 1.9 million pieces in an area of just one square metre.

Published this week in the journal Science, this study shows how deep-sea currents act as conveyor belts, transporting tiny plastic fragments and fibres across the seafloor. These currents can concentrate microplastics within huge sediment accumulations, which the authors of the research term ‘microplastic hotspots’. These hotspots appear to be the deep-sea equivalents of the so-called ‘garbage patches’ formed by currents on the ocean surface.

Over 10 million tons of plastic waste enters the oceans each year. Floating plastic waste at sea has piqued the public interest, yet such accumulations account for less than 1% of the plastic that enters the world’s oceans. The missing 99% is instead thought to exist in the deep ocean, but until now it has been unclear where it actually ended up.

Dr Mike Clare of the National Oceanography Centre, who was a co-lead on the research, stated: “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. The results highlight the need for policy interventions to limit the future flow of plastics into natural environments and minimise impacts on ocean ecosystems”.

The lead author of the study, Dr Ian Kane of the University of Manchester, said “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”. Dr Kane added “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.”

Microplastics on the seafloor dominantly comprise fibres from textiles and clothing. These are not effectively filtered out in domestic wastewater treatment plants, and easily enter rivers and oceans. In the ocean they either settle out slowly, or can be transported rapidly by episodic turbidity currents – powerful underwater avalanches – that travel down submarine canyons to the deep seafloor (see the group’s earlier research in Environmental Science & Technology). Once in the deep sea, microplastics are readily picked up and carried by continuously flowing seafloor currents (‘bottom currents’) that can preferentially concentrate fibres and fragments within large drifts of sediment.

These deep ocean currents also carry oxygenated water and nutrients, meaning that seafloor microplastic hotspots can also house important ecosystems that may consume or absorb the microplastics. This study provides the first direct link between the behaviour of these currents and the concentrations of seafloor microplastics and the findings will help to predict the locations of other deep-sea microplastic hotspots and direct research into the impact of microplastics on marine life.

The team collected samples from the seafloor of the Tyrrhenian Sea (off the west coast of Italy) and combined these with calibrated models of deep ocean currents and detailed mapping of the seafloor. In the laboratory, the plastics were separated from sediment, counted under the microscope, and further analysed using infra-red spectroscopy to determine the plastic types. Using this information the team were able to show how ocean currents controlled the distribution of microplastics on the seafloor.

This research forms part of an international research project at the University of Manchester (UK), the National Oceanography Centre (UK), IFREMER (France) and the Universities of Bremen (Germany) and Durham (UK). 

This project is part of wider research by the Microplastics Research Group at the National Oceanography Centre that aims to understand the extent and implications of microplastic pollution on the health of marine ecosystems: https://noc.ac.uk/science/microplastics. 

The open-access article can be downloaded at the following link: https://science.sciencemag.org/content/early/2020/04/29/science.aba5899

Microplastics are delivered to the ocean through rivers carrying industrial and domestic wastewater, carried down submarine canyons by powerful avalanches of sediment (turbidity currents) and then transported on the seafloor by ‘bottom currents’ and deposited in sediment drifts. Other microplastics sink from the ocean surface and can also be picked up and carried by bottom currents.