Microplastic conveyor belts: Scientists find “shocking” seafloor pollution levels

04 May 2020 --- An international research team has discovered the highest levels of microplastic ever recorded on the seafloor. Up to 1.9 million pieces were identified in a thin layer covering one square meter of the Mediterranean Sea. The “shocking” findings indicate that the tiny particles – approximately the width of a human hair – are not uniformly distributed across the study area but distributed by powerful seafloor currents, which concentrate them in certain areas. Dr. Mike Clare of the National Oceanography Centre, who was a co-lead on the research, has called on policy interventions to limit the future flow of plastics into natural environments.

Over 10 million tons of plastic waste enter the oceans each year. Floating plastic waste at sea has caught the public’s interest in part due to the “Blue Planet Effect,” which has led to initiatives to discourage the use of plastic drinking straws and carrier bags, such as the EU’s Single-Use Plastics Ban. Yet such accumulations account for less than 1 percent of the plastic that enters the world’s oceans, the research team indicates.

The “missing 99 percent” is thought to occur in the deep ocean but until now it has been unclear where it ended up. Published in the journal Science, the research conducted by The University of Manchester, National Oceanography Centre (UK), University of Bremen (Germany), IFREMER (France) and Durham University (UK) shows how deep-sea currents act as conveyor belts, transporting tiny plastic fragments and fibers across the seafloor.

Submerged garbage patches
Deep-sea currents can concentrate microplastics within huge sediment accumulations, which the researchers termed “microplastic hotspots.” These hotspots appear to be the deep-sea equivalents of the so-called “garbage patches” formed by currents on the ocean surface.

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,” says lead author of the study, Dr. Ian Kane of The University of Manchester.

“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.”Kane et al. (2020) Seafloor microplastic hotspots controlled by deep-sea circulation.

Microplastics on the seafloor are mainly comprised of fibers 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.

Once in the deep sea, microplastics are readily picked up and carried by continuously flowing seafloor currents (“bottom currents”) that can preferentially concentrate fibers and fragments within large drifts of sediment.

Marine life impacts
These deep ocean currents also carry oxygenated water and nutrients, meaning that seafloor microplastic hotspots can also house important ecosystems that can consume or absorb the microplastics. The study provides the first direct link between the behavior of these currents and the concentrations of seafloor microplastics. The researchers believe that 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 sediment samples from the seafloor of the Tyrrhenian Sea (part of the Mediterranean Sea) and combined these with calibrated models of deep ocean currents and detailed mapping of the seafloor. In the laboratory, the microplastics were separated from sediment, counted under the microscope, and further analyzed using infra-red spectroscopy to determine the plastic types. Using this information, the team was able to show how ocean currents controlled the distribution of microplastics on the seafloor.

“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 minimize impacts on ocean ecosystems,” explains Dr. Clare.

“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. Thus, sediment-transport processes such as seafloor currents will concentrate plastic particles in certain locations on the seafloor, as demonstrated by our research,” adds Dr. Florian Pohl of the Department of Earth Sciences, Durham University.

An alarming issue
A recent study funded by the Australian Cooperative Research Centre for Contamination Assessment and Remediation of the Environment found that opening plastic packages can catapult up to 75,000 microplastics across three meters.

The EU is increasingly recognizing the magnitude of the microplastic problem. In March, The European Commission adopted a new Circular Economy Action Plan in which it addresses intentionally added microplastics and develops labeling and regulatory measures on unintentionally released microplastics.

In April, Finland-based research facility VTT revealed that it is developing a method that uses nanocellulose structures for the early identification of microplastics before they enter waterways.

By Joshua Poole