Ocean lovers are familiar with the image of a vast marine "garbage patch," where garbage swirls in a never-ending gyre. But the same ocean currents that help concentrate plastic and other debris also ultimately help disperse them toward shore, a new study finds.
"Our study has shown the existence of exit routes for debris in the South Pacific Ocean, a region where it was once thought that that drifters can converge but not escape during their lifetimes," said study co-author Christophe Maes, a physical oceanographer at France's University of Western Brittany (UBO) and the French Research Institute for Development (IRD).
The findings, published online in the journal Geophysical Research Letters, show that ocean garbage patches are much more dynamic than previously thought and could complicate ocean trash cleanup efforts.
Winds and the rotation of the Earth combine to create huge vortices in each of the five major ocean basins: North and South Pacific, the South Atlantic, and the Indian Ocean. Year after year, the swirling motions of these "oceanic gyres" sweep in debris to produce polluted stretches of ocean. (The name “garbage patch” is a misnomer. There are no floating landfills in the middle of the ocean, just regions of higher than normal concentrations of plastics, metals and other junk.)
Plastics and metals degrade very slowly over time and can leach harmful chemicals into the ocean. In addition, the floating trash can also contribute to the spread of disease-causing bacteria and viruses. Scientists previously thought that these garbage patches were largely static—that once trash was sucked into the oceanic gyres, it largely remained there.
But no longer. A new study by Maes and his team have revealed the existence of "exit door" currents flowing away from the oceanic gyres. Using computer models to simulate ocean current flow in the Pacific Ocean, the scientists tracked the trajectories of several million virtual particles. Their results revealed the existence of outward bound currents, several hundred kilometers wide, that flow eastward, away from the gyre and toward the coastlines of North and South America.
Juan Baztan, a marine geologist at the University of Versailles in France, welcomed the new information about oceanic gyres. "The more we know about the sources and the distribution, the better it will be for improving monitoring, optimizing cleanup and reducing sources of the pollution," said Baztan, who was not involved in the study.
Erik van Sebille, an oceanographer and climate scientist at the Imperial College London in the UK, said the exit routes are good news for trash cleanup, but not for projects aiming to improve the health of marine life.
"If you're focusing on cleanup on the coastlines, then this will make things easier because a lot of the trash will eventually end up on the beaches, allowing you to concentrate your efforts there," said van Sebille, who also did not participate in the study. "The bad news is that plastic does much more harm near the coastlines than in the open ocean, where there is much less marine life. Coastlines are where the coral reefs and fisheries are."