Sea Education Association/Skye Moret
In the journal, Science, researchers led by the Sea Education Association (SEA) in Woods Hole, Massachusetts published results from a 22-year-long study of plastic accumulation in the western North Atlantic and Caribbean Sea. Over 60 percent of the 6136 surface plankton net tows the group pulled from the water between between 1986 and 2008 contained plasticover 64,000 pieces in total. The fragments were hand-picked with tweezers, and archived, mostly by undergraduate researchers.
The group reports finding the greatest concentration of plastic fragments between the latitudes of 22 and 38 degrees Northroughly equivalent to the length between Cuba and Marylandwhich corresponds to the location of the North Atlantic Gyre. The study helps define the northern and southern boundaries of the Atlantic plastic soup, but it's still not clear how far it extends eastward.
Plastic CollectorsThe largest sample collected during a single 30-minute tow, taken in May of 1997 at 24.6 degrees North and 74.0 degrees West, was 1069 pieces. Extrapolated, that's the equivalent of 580,000 pieces per square kilometer. The average concentration observed near 30 degrees North, the midpoint of the high concentration region, was about 20,300 pieces per square kilometer, give or take about 2300.
Most of the plastic collected at this point, says lead author Kara Lavender Law, an oceanographer at SEA, is probably post-consumer plasticthat is, fragments of common household items people use every day. Data from earlier in the study reflects a larger concentration of industrial resin pellets, the raw material of the consumer plastics industry. But the group notes a significant decrease in the concentration of these tiny pellets over the course of the study, likely due to voluntary programs the plastics industry initiated in the 1990s to prevent or recapture spilled material.
This large data set sheds light on some important questions, says Miriam Goldstein, an oceanographer at the Scripps Institution of Oceanography who studies the impact of plastic debris in the Pacific Ocean, and who was not part of this study. "One thing that is really difficult in working with this issue is just the lack of data, and they have a really amazing data set here," Goldstein says. One particularly valuable aspect of the study, she says, is that the researchers combined their data with numerical modeling, and data from drifting buoys designed to help track ocean currents, to show that plastic debris that enters the Atlantic Ocean, from, say, the eastern U.S. seaboard, has an excellent chance of ending up in the North Atlantic Gyre. This fact wasn't necessarily obvious, given the small size of so many of the fragments.
Where Is All the Plastic Going?To look for trends over the 22-year period, the SEA group analyzed the data collected in the most "spatially consistent, annually repeated cruise track," according to the paper. But even though there was a fourfold increase in discarded plastic in the U.S. between 1980 and 2008, the concentration of debris in the Atlantic does not seem to have increased over that time. This poses the question: Assuming plastic entering the ocean has followed a similar trend, where is it all going?
At least some of the plastic is probably being consumed by animals. "We know that marine organisms eat plastic, but we don't have a sense of who, how much, or what happens to it once they eat it," Lavender Law says.
An additional possibility is that some of it has broken down into fragments too small to be captured in the tow nets, made of mesh with holes merely 1/3 of a millimeter in size. "We don't know how small they get in the ocean, and it is very hard to develop a method to look at that," Lavender Law says.
It's also a possibility that a general shift in the types of plastics entering the ocean is affecting the amount of observable debris. A companion study analyzed the collected plastic fragments and showed that 99 percent were less dense than seawater, and the majority were made polyethylene and polypropylene, materials known to float in seawater. Common soft drink and water bottles, meanwhile, are made of polyethylene terephthalate (PETE), which is denser than seawaterand could be sinking. The surface tow nets only collect debris floating near the surface. "Even if it became just a little bit denser, it might be out of reach of our net," Lavender Law says.
Finally, perhaps organisms that grow on the plastic fragments are changing their density enough to make them sink out of reach, or even all the way to the ocean floor. "But then what happens once it sinks? That's the next question," Lavender Law says. To date, there have been no studies of the plastic accumulation on the seafloor offshore, below the areas where a high concentration of floating debris is observed.
Getting an idea of how much plastic has accumulated over such a long period of time in the Atlantic is crucial to the research field, says Goldstein, but the uncertainties it raises are just as key. "Why isn't there a trend in time? In terms of controlling marine debris, these are really important questions," she says.