Ocean Garbage Patch Still a Mystery

Jeremy Hsu

Date: 19 August 2010 Time: 08:13 AM ET

Plastic pieces collected in a surface plankton net tow. Pieces are typically millimeter sized fragments of once-larger items.
CREDIT: SEA/Giora Proskurowski

A vast patch of garbage spanning a swath of the Atlantic Ocean has long puzzled scientists who wondered where the plastic bits came from and why there's not more of it.

Now an exhaustive study, resulting in more than 64,000 bits of plastic collected from the Atlantic Ocean over two decades, has allowed scientists to "go through the garbage" and get to the bottom of some of the mysteries.

Scientists have been particularly mystified over why the concentration of plastic in the Atlantic has not increased during the past 22 years, despite both plastic production and plastic trash increasing during that time period. Still, they have their suspicions.

"I think it's certain that the plastic is breaking down into pieces smaller than what we capture in the net," said Kara Lavender Law, an oceanographer with the Sea Education Association at Woods Hole, Mass.

As bacteria and other organisms built up on the plastic, the added weight may have dragged the debris down to lower ocean depths, according to Lavender Law and her colleagues in a study detailed in the Aug. 19 issue of the journal Science.

Sizing up the trash

Ships towing long nets found the plastic pieces floating across hundreds of miles of the North Atlantic during the past 22 years. The nets only snag objects bigger than a third of millimeter, which can include plankton, seaweed and even tarballs from oil.

The sheer scale of the affected area could rival that of the "Great Pacific Garbage Patch," although Lavender Law cautioned that both regions remain poorly defined. For instance, the exact eastern boundary of the Atlantic region remains undiscovered.

"It's entirely possible that it reaches almost all the way across the Atlantic," Lavender Law told LiveScience.

The affected region in the western North Atlantic Ocean and Caribbean Ocean stretches east to west between Cuba and Virginia, where a combination of wind-forced ocean circulation and the so-called Coriolis Effect of the Earth's rotation keep the plastic circling almost endlessly.

What lies on the surface

The term "garbage patch" does not necessarily mean a visible island of trash floating on the waves, researchers said. Only 62 percent of net tows by ships have contained detectable amounts of plastic.

"What we're collecting are really small fragments of plastic from larger consumer items," Lavender Law explained. "If you're on the deck of a ship, you normally can't even see the plastic pieces."

Each half-hour net tow typically turned up just 20 plastic pieces equivalent to about 0.3 grams in all. By comparison, a U.S. nickel weighs 5 grams.

The vast majority of plastic pieces caught in the net turned out smaller than 10 millimeters, Lavendar Law said. She pointed to a companion study published in this week's issue of the journal Marine Pollution Bulletin that includes all the details on the plastic pieces.

But the unusual discovery of a five-gallon bucket harbored a special surprise – trigger fish, which normally live around ocean reefs. That suggested the fish had found both shelter and perhaps food from the accumulated plastic scum on the bucket.

The more common tiny pieces of plastic can also harbor colonies of bacteria that may not typically belong at the ocean's surface.

"We need to ask if microbes are able to use the plastic as food and degrade it, or if the plastic is acting as substrates for [microbial] communities living on them," Lavender Law said.

How trash travels

The origins of all the plastic remain largely unknown, because researchers currently cannot trace it back to the original location or even the original product that a plastic piece came from.

But ocean circulation studies that use satellite-tracked buoys have found that floating plastic can travel from Washington, D.C., or Miami, Fla., to the Atlantic garbage patch within just 40 days.

The amount of plastic reaching the oceans should have grown in recent decades, according to available data. The amount of buoyant plastics in U.S. Municipal Solid Waste increased by 24 percent between 1993 and 2008, and totaled 14.5 million tons in 2008.

That goes back to the case of the missing trash that should have boosted plastic concentrations in the Atlantic Ocean. Future ship surveys may find more of the plastic lurking in the lower ocean depths, or uncover more about how microbes break down the plastic.

"Understanding the size spectrum and the fate of the plastic is a very important direction to go," Lavender Law noted.

Much of that future research rests upon undergraduate students, who used tweezers to pick out the plastic from the goo pulled up by the plankton nets. More than 7,000 students took part in that painstaking work during the Sea Education Association's SEA Semester annual voyages, which last for a total of three months each year.

"I always want to make sure I give full credit to the undergrads," Lavender Law said. "Undergrads with or without science backgrounds can make real contributions."