By Sid Perkins, Science News
PORTLAND, Ore.—Recent studies show that the oceans may hold more “garbage patches” of fine plastic flotsam than scientists realized and that the fragments extend well below the sea surface.
Most of these items are the size of fingernail clippings or smaller. They are the wave-shattered remnants of items such as rubbish, abandoned fishing gear and floats from fishing nets and scientific instruments. These plastic bits are especially common in a region of the Pacific Ocean southwest of California that is sometimes called the Great Pacific Garbage Patch.
Recent cruises reveal that there’s more garbage in this patch than often meets the eye, oceanographer Giora Proskurowski of the Sea Education Association in Woods Hole, Mass., reported February 24 at the American Geophysical Union’s Ocean Sciences meeting.
Scientists often tow fine mesh nets behind their boats to conduct a census of floating debris, Proskurowski said. But if researchers tow their nets just at the surface, especially on windy days, they’re likely finding only a fraction of the debris that’s actually present.
On a calm day, debris floats to the ocean’s surface and is readily collected there. But when winds roil the seas, the team’s data show, wave action could briefly send items that are barely buoyant, such as tiny bits of plastic, down as much as 20 meters below the surface. Even during a light breeze, when the ocean’s surface is only dotted with whitecaps, plastic bits can temporarily be deep-sixed, Proskurowski noted.
For example, during one tow — which the researchers conducted when wind speeds were just under 28 kilometers per hour (about 17 miles per hour) — a net towed along the surface caught 431 bits of plastic, while one towed simultaneously at a depth of five meters trapped 240. In similar circumstances, the researchers estimate, the waters between one and 10 meters deep hold as much plastic as the top meter of ocean does.
He and his colleagues collected their data on six cruises through the northwestern portions of the Great Pacific Garbage Patch between June 2004 and July 2009. The researchers estimate that part of that region — a whopping 3.5-million-square-kilometer swath about twice the size of Alaska — contains more than 20,000 bits of floating plastic per square kilometer.
Large swaths of the western North Atlantic also hold prodigious amounts of plastic debris, Kara Lavender Law, who also is an oceanographer at the Sea Education Association, reported at the meeting. On research cruises between the Gulf of Maine and the Caribbean from 1986 through 2008, researchers conducted more than 6,100 surface tows that collected more than 64,000 pieces of plastic, she said.
As in the Pacific, the vast majority of the plastic bits scooped from the North Atlantic were tiny: Analyses of a sample containing about 750 fragments showed that most of these barely buoyant bits were less than a centimeter across and weighed less than 0.15 grams.
About 83 percent of the pieces were collected between the latitudes of 22°N (approximately the latitude of central Cuba), and 38°N (Philadelphia). In general, the highest concentrations of floating plastic occurred in the portion of that area where surface currents converged and flowed at less than two centimeters per second. That area roughly corresponds with the Sargasso Sea, a seaweed-choked region notorious for becalming sailing ships.
Computer simulations reveal that oceanic garbage patches may be more common than even scientists generally recognize, said Nikolai Maximenko, an oceanographer at the University of Hawaii in Manoa. The pattern of ocean currents, chronicled by the movements of scientific instruments set to drift in the world’s oceans, show several large areas in the world’s oceans where currents are slack and garbage could accumulate. “Some of these areas are like a black hole,” he noted at the meeting. “Once things are trapped there, they never escape.”
Two areas particularly suited to trap flotsam are near South America, one west of central Chile and the other stretching from Argentina across the Atlantic nearly to South Africa. But few if any oceanographic cruises have trolled for plastic debris in these two regions — or collected much other data, for that matter. One probable reason, Maximenko noted, is that scientists and fishermen have largely avoided these regions because they aren’t biologically productive.