Researchers have found the highest levels of microplastic recorded on the seafloor ever, with up to 1.9 million pieces in a thin layer covering just 1 square meter.
It is estimated that over 10 million tons of plastic waste enter the oceans each year. Floating plastic waste at sea has caught the public’s interest thanks to the ‘Blue Planet Effect‘ seeing moves to discourage the use of plastic drinking straws and carrier bags. Yet such accumulations account for less than 1% of the plastic that enters the world’s oceans.
The missing 99% is thought to occur in the deep ocean, but until now it has been unclear where it actually ended up. Published this week in the journal Science, the research showed how deep-sea currents act as conveyor belts, transporting tiny plastic fragments and fibers across the seafloor.
These currents can concentrate micro-plastics within huge sediment accumulations, which they termed ‘micro-plastic hotspots‘. These hotspots appear to be the deep-sea equivalents of ‘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.
“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 mainly consists of fibers from textiles and clothing. These can not be effectively filtered out in domestic wastewater treatment plants and thus easily enter rivers and oceans.
In the ocean, they either settle out slowly or can be transported rapidly by episodic turbidity currents 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 that can preferentially concentrate fibers and fragments within large drifts of sediment.
These deep ocean currents are found to carry oxygenated water and nutrients and that means that seafloor microplastic hotspots can also house important ecosystems that can consume microplastics.
This study provides the first direct link between the behavior 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 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.
I.A. Kane el al., “Seafloor microplastic hotspots controlled by deep-sea circulation,” Science (2020): Science