Recent research shows the ocean may contain more plastic debris than previously thought. Here, samples researchers pulled up from the Sargasso Sea. (Credit: Kukulka et al.)

The ocean is filled with more plastics than previously thought, according to a new study.

Tiny plastic fragments not only float on the ocean’s surface, but are also temporarily pushed beneath the top layer of water by the tumult caused by maritime winds, according to the new research. The higher the wind speed, and the more turbulent the gusts, the more mixing that occurs between the upper and lower ocean layers.

Traditional measurements of plastic marine pollutants only account for the top layer of water, since plastic is buoyant.  Researchers studying the plastic patches typically drag a mesh net behind a boat at a depth of 25 centimeters (10 inches) to collect debris, then collect and sort the plastic catch. But these samples taken at the ocean’s surface underestimate the amount of plastic debris present in the world’s oceans, reports a team of scientists led by Tobias Kukulka, of the University of Delaware, and Giora Proskurowski, with the University of Washington.

Kukulka and his colleagues wanted to look deeper, and scour lower depths for debris in a process called subsurface towing. With this method, researchers use a special net that can be lowered to specific depths, in this case five, 10, and 20 meters (16, 33 and 66 feet), while still closed. They then open it to collect debris only at those depths.

“This process allows us to sample only a ‘discrete’ depth, without exposing the open net to other depths,” Proskurowski said.

In the open ocean, plastic breaks into millimeter-sized fragments. (Credit: Kukulka et al.)

Combining data from this and previous studies, researchers constructed a model that included wind-induced mixing of ocean layers, and discovered that the real amount of plastic debris in the ocean is an average of 2.5 times the amount found by only surface towing. In particularly windy conditions, when even more of the plastic has been mixed out of the surface layer, researchers predicted that total plastic content can be as much as 27 times the amount measured at the surface. The research was published this week in Geophysical Research Letters, a journal of the American Geophysical Union.

Kukulka and his team trawled about 600 miles east of Bermuda in the Sargasso Sea, a 2 million square mile area of water where ocean currents converge and concentrate plastic debris. They chronicled their research in a series of YouTube videos. Some of the team’s members, working with other researchers, had also previously found subsurface plastic three to five meters beneath the surface while trawling in the Pacific Ocean, a finding that the group reported at the AGU-sponsored 2010 Ocean Sciences Meeting in Portland, Ore.*

Most of the plastic clogging the world’s oceans is in the form of millimeter-sized fragments, Proskurowski said. These tend to be highly resistant to environmental degradation because of their manufactured durable nature and can pose a threat to marine life. Some aquatic animals ingest stray particles of plastic, which can injure or poison them because of chemicals present within the material. Floating plastic can also transport invasive species between different environments. Removing the debris from the ocean – especially at greater depths – can prove to be a difficult task.

“Our work emphasizes the difficulty of cleaning up the small plastic particles – they are not just at the surface but also in the shallow subsurface,” Proskurowski said.

While researchers don’t yet have enough data to estimate the total amount of plastic in the world’s oceans, Proskurowski said, the pollution problem runs deeper than previously reported.

Kukulka, T., Proskurowski, G., Morét-Ferguson, S., Meyer, D., & Law, K. (2012). The effect of wind mixing on the vertical distribution of buoyant plastic debris Geophysical Research Letters, 39 (7) DOI: 10.1029/2012GL051116

-Eric Villard, AGU science writing intern

*This post has been corrected, due to an editing error. The researchers had presented other, related material at AGU’s 2010 Ocean Science meeting, but not the results described here.