Measuring the depth of plastic in oceans

An international research team, led by The University of Western Australia, has reported the first ever high-resolution vertical profiles of plastic pollution in the so-called ‘ocean garbage patches’. Their results have been published in the journal Biogeosciences.

The researchers explained that millimetre-sized plastics, which are abundant across the world’s ocean surface, “can be mixed within the upper water column by turbulent transport” such as wind and waves. The team sought to investigate the depth profile and physical properties of the buoyant plastic debris, using a measuring device called a multilevel trawl at 12 sites within the North Atlantic Gyre.

The researchers were able to measure plastic concentrations in 10 layers simultaneously, down to a depth of 5 m. They found that when the wind was stronger than 10 knots, more than half of the 0.5 to 1 mm particles were underwater. When there was no wind, this figure was about 20%.

“Our results show that plastic concentrations drop exponentially with water depth, and decay rates decrease with increasing Beaufort number,” the study authors said. “Furthermore, smaller pieces presented lower rise velocities and were more susceptible to vertical transport.”

The study was sponsored by UWA and The Ocean Cleanup, whose founder, Boyan Slat, said the results are good news to those developing technologies to extract plastic from oceanic garbage patches.

“Almost all plastic was on or very close to the surface, meaning it’s within reachable distances for a clean-up operation,” Slat said.

The Ocean Cleanup’s next expedition studying the vertical distribution of plastic in world’s oceans will begin on 5 March.