Scientists have found a new monster lurking in the deep: plastic. Discarded plastic floating in the ocean has been a recognized issue for decades, but the extent to which it might be polluting beneath the waves has largely been unknown. New research is finding there is more plastic than what appears on the surface.
“We have a very limited understanding of where all the plastic and stuff that’s being put into the ocean ends up,” said Matthias Egger, the lead scientist behind the new study and a researcher with The Ocean Cleanup. “We know roughly there’s tens of millions of tons of plastics going into the ocean. A large part of that should be afloat, but it’s not.”
This mystery is known as the missing plastic problem. Plastic found adrift in the ocean makes up only 1% of what should be out there, though a large portion is thought to circulate in coastal environments. The largest known reservoirs of plastic at sea are giant, swirling “garbage patches” that can stretch over areas twice the size of Texas. Could some of the missing plastic have sunk beneath these massive gyres?
“Almost everything we know about plastics in the ocean is really from the surface,” said Erik van Sebille, an oceanographer and climate scientist at Utrecht University who was not involved with the new research. “This really is the first time that a group has really systematically gone and done a transact and looked at the amount of plastic in the upper 2 kilometers.”
Into the Deep
On a calm day, the Great Pacific Garbage Patch looks like a reflection of the night sky, with shining pieces of plastic speckled across or just below the surface in every direction. Scientists estimate it is home to some 80,000 metric tons of plastic brought together in the North Pacific Gyre by ocean currents. It was there that Eggers and his team dropped nets to sample as much as 2,000 meters below the surface.
The results, published in Scientific Reports, found microplastics at every depth sampled. Analyzing over 12,000 plastic fragments, the researchers found the types of plastics found in the depths matched what was seen on the surface, providing the first evidence of fallout from the garbage patches. Although plastic is usually buoyant, it can start to sink as it becomes mixed with heavier sediment or is colonized by algae and other marine life—a process known as biofouling. The scientists estimated that between 5 and 2,000 meters below the surface, the total mass of plastic pieces smaller than 5 centimeters is 56%–80% of what is seen at the surface. In total, these plastics could compose 10% of the weight of plastics on the surface.
A Drop in the Plastic Ocean
The new research, despite its deep sampling, is still just dipping beneath the surface. There are several thousand meters more to reach the ocean floor, and there’s evidence that there could be a large amount of plastic just nanometers in size—10 to 100 times smaller than what is typically measured in the ocean. Additionally, although transects help showcase the variability in plastic distribution, scientists still don’t understand how currents circulate plastics, particularly subsurface.
However, the findings do reveal a new chapter in the ocean plastics story: what happens to plastic permanently adrift. The results support the theory that most of the missing plastic circulates in coastal regions but also show how plastics are already filtering into the deep ocean. Understanding the amount of subsurface plastic in garbage patches is key to understanding the longevity of these Anthropocene icons.
“It’s really important information for us to be able to say anything or make any predictions,” Egger said. “Let’s say we stopped all emissions into the ocean; how long will these garbage patches still persist?”
—Mara Johnson-Groh ([email protected]), Science Writer