An international team of experts has discovered surprisingly large concentrations of microplastics in the deep sea. The researchers found up to 1.9 million plastic pieces in a single square meter, which is the highest level of microplastics that has ever been documented on the ocean floor.
Each year, over 10 million tons of plastic waste enters the ocean. Previous studies have established that floating microplastics only account for about one percent of all marine plastic pollution, and the fate of the other 99 percent has remained a mystery.
Study lead author Dr. Ian Kane is a researcher in the School of Earth, Atmospheric, and Environmental Sciences at the University of Manchester.
“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. Kane.
“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.”
The currents cluster the microplastics together and bury them within deep sediment piles, creating large concentrations of particles and fibers that the researchers refer to as “microplastic hotspots.” The hotspots are essentially the deep-sea equivalents of the floating garbage patches that are formed by ocean currents on the surface.
The majority of the microplastics that reach the seafloor are made up of fibers from textiles and clothing. These fibers are not effectively filtered out in domestic wastewater treatment plants, and liberally flow into rivers and oceans.
Once they arrive in the deep sea, microplastics are dominated by bottom currents, which herd the particles and fibers into large sediment drifts on the seafloor. The Manchester-led study provides the first direct evidence of this phenomenon.
The findings will help to predict other microplastic hotspots in deep ocean sediment, and will inform future studies on how microplastics impact marine life.
The team collected sediment samples from the bottom of the Tyrrhenian Sea, which is part of the Mediterranean Sea. The experts combined these samples with calibrated models of deep ocean currents and detailed mapping of the seafloor.
In the lab, the microplastics were separated from sediment and counted under the microscope. To determine what types of plastics were present, the researchers used infra-red spectroscopy.
The analysis revealed that the distribution of microplastics on the seafloor is controlled by ocean currents.
“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 study co-author Dr. Mike Clare. “The results highlight the need for policy interventions to limit the future flow of plastics into natural environments and minimize impacts on ocean ecosystems.”
The study is published in the journal Science.