A new study by Dutch researchers says plastics pollution in oceans may not be settled as deep as previously thought.
The study from the University of Utrecht in the Netherlands, "Global mass of buoyant marine plastics dominated by large long-lived debris" in the publication Nature Geoscience takes a more precise look at the amount of plastics measured in surface water between 1980 and 2020.
"This model is more accurate," said lead author Mikael Kaandorp. "Until now, scientists were mainly looking at measurements of amounts of plastic in the upper layer of the water surface. We added counts of beach clean-ups in various places around the world, and observations of large floating plastic objects on the water, amongst other things. Those pieces are relatively scarce, but because they are heavy, they make up a large portion of the total amount of plastic in the ocean."
Results show that the largest fraction of plastic mass is located at the ocean surface. This is in stark contrast with the widely held assumption that only 1 percent of the total amount of plastic in the oceans floats on the surface. That theory anticipates that there is a "missing sink" of plastics pollution in deeper waters.
The study found, instead, that the total amount of buoyant marine plastic litter is much higher than previous estimates, at around 3,000 kilotons to 3,400 kilotons. It also found the majority of plastic mass is contained in large plastic items — around 90 percent to 98 percent — and that these constitute most of the total buoyant plastic mass.
"Our mass estimate for particles between 5 [millimeters] and 200 mm, 700 kilotons, is much higher than previously estimated (30.6 kilotons)," the authors wrote. "This is even more so for particles larger than 200 mm, where our estimate of 2,500 kilotonnes vastly exceeds the previously estimated 202.8 kilotonnes."
The difference can be explained by the very broad size of intervals used in previous studies, where large particles are likely to be under sampled, causing an underestimation of the total mass, the scientists claimed.
The study also estimates that about 460-540 kilotonnes enter the oceans each year, which is lower than previous estimates. This means that the new model rejects theories that much of the plastic enters the oceans, then sinks from view. Instead, it maintains that about 60 percent of plastics remains floating on the surface.
"Our finding of a lower plastic input into the marine environment and a higher standing stock means that the residence time of plastics in the marine environment is much higher than previously estimated," the scientists said. They calculated that in a scenario where there is a sudden stop of new plastics entering the marine environment in 2025, only 10 percent of the plastic mass would be removed within two years.
The study focused only on polyethylene, polypropylene and polystyrene which make up the majority of items in the ocean's surface, deeper layers, and beaches. It did not consider polymers denser than seawater such as PVC and PET, estimated to make up about 35 percent to 45 percent of the plastic mass entering the marine environment.