Scientists Identify 1.9 Million Pieces of Microplastic in Square Meter of Ocean Floor

Researchers writing in Science have reported the highest levels of microplastics observed on the seafloor to date, finding up to 1.9 million fibers and fragments per square meter of ocean bed.

For some time, scientists have struggled to find the "missing" 99 percent of plastic that ends up in our oceans—the polyethylene bags and polypropylene straws that accumulate at the surface accounting for a tiny fraction of total waste. Now an international team of scientists say they have identified a mechanism that transports and dumps at least some of the remaining 99 percent onto the seabed.

While islands of floating plastic—such as the great Pacific garbage patch, estimated to be twice the size of Texas—have garnered a lot of media attention, researchers have identified what they call "microplastic hotspots," which form in a not-too-dissimilar fashion, only thousands of feet below the sea surface.

"Until now we haven't understood where the missing microplastics in the ocean end up and how they are transported," Mike Clare of the National Oceanography Centre told Newsweek. "We know that there are concentrations of plastic on the ocean's surface, but this accounts for less than one percent of the plastic that makes it into the ocean."

He added: "Until now, the rest has been assumed to settle slowly out, like rain or snow in the deep sea. We found instead that currents in the deep sea act like conveyor belts—moving plastic around, locally creating hotspots on the seafloor."

A collection of microplastics and mesoplastic debris
A volunteer of the NGO 'Canarias Libre de Plasticos' (Canary Islands free of plastics) carries out a collection of microplastics and mesoplastic debris to clean the Almaciga Beach, on the north coast of the Canary Island of Tenerife, on July 14, 2018. DESIREE MARTIN/AFP/Getty

The researchers came to these conclusions after analyzing sediment samples collected from the bed of the Tyrrhenian Sea in the Mediterranean, near Italy's west coast. The team studied the contents of the samples, identifying the different types of plastic, and compared the results to models of ocean currents.

The results suggest deep-sea ocean currents driven by temperature and salinity transport fibers and fragments of plastic, as well as nutrients and oxygenated water, across the seafloor. The formation of these currents can cause these plastics to accumulate in certain areas, described in the paper as "microplastic hotspots."

According to the study, the concentration of plastics found in some areas is more than twice that recorded in submarine canyons. The highest concentrations revealed close to two million plastic particles per square meter—a finding lead author Ian Kane of the U.K.'s University of Manchester described as "astounding."

The great bulk of microplastics building up in these hotspots are fibers found in clothing and larger pieces of plastic waste. The researchers did not find a correlation between concentrations of microplastic and proximity to land-based sources of plastic.

Illustration: microplastic hotspots
An illustration showing how microplastics end up on the seafloor. I. Kane et al. 2020

While the research is entirely focused on one site in the Mediterranean, these currents run around the globe and the study's authors suggest the process will play an important role in the distribution of plastic elsewhere. However, more research is needed to determine exactly what kind of effect they are having in other parts of the world.

"Our study focuses in the Mediterranean, where currents create circular gyres (or vortex) that may explain why the microplastics are so concentrated—it is like plastics are trapped in a swirling plughole," said Clare. "In other places, where the margins of the ocean do not create such confinement—such as in the Atlantic and Pacific oceans—these seafloor currents may actually disperse the microplastics.

"There is still much work to be done to understand how these currents control microplastic concentrations on a global scale. But we now know that these currents are very important."

It is largely unknown what kind of impact these hotspots might have on marine ecosystems. But if microplastics are piggybacking on currents that carry oxygenated water and nutrients across the ocean, there is a concern they could affect organisms that ingest or absorb these plastics. Previous studies have suggested the consumption of microplastics can change animal behavior, making periwinkles more vulnerable to attack from predators. Other research suggests microplastics could have a harmful effect on fish health.

Kane told Newsweek the researchers hope their work will help turn the intangible "missing 99 percent" into something "real" by showing where and how it builds up on the seafloor.

"To me, that makes these hotspots a tangible or "real" feature, and the hope is that in much the same way as the seafloor garbage patches, these might capture the public imagination and result in the same sort of push for responsible plastic production, and more effective policies for dealing with waste water," he said.

Microplastics, fragments and fiber
A sample of the type of microplastics found in "hotspots" on the seafloor. Kane et al. 2020

"Very little can be done to deal with these hotspots as they are—a deep seafloor clean up is not a viable option for a number of reasons," said Clare.

"Therefore the solution needs to start at the source. We need to limit the pathways that get plastics to the ocean, switch to more sustainable materials where possible, and keep to the mantra of reduce, re-use, recycle."

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