An international research project has revealed the highest levels of microplastic ever recorded on the seafloor, with up to 1.9 million pieces in a thin layer covering just one square metre.
Over 10 million tons of plastic waste enters the oceans each year. 99% is instead thought to occur in the deep ocean, but until now it has been unclear where it actually ended up. New research has now shown 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 researchers termed ‘microplastic hotspots’. According to a statement, 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.
“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 are mainly comprised of fibres from textiles and clothing. These are not effectively filtered out in domestic waste water 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 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 can 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.
Image credit: GD Taber, flickr/Creative Commons