The best places to pick up ocean plastic aren’t the big garbage patches

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Cleanup efforts for ocean plastics should be concentrated close to shore, at the source of the problem, rather than in areas of open ocean where plastic tends to accumulate, according to a study recently published in the journal Environmental Research Letters. Ideally, if plastic collectors were placed offshore near coastal population centers, they could remove nearly one-third of plastic in the ocean over the next 10 years.

In the study, oceanographer Erik van Sebille and undergraduate physics student Peter Sherman, both at Imperial College London, used data on ocean currents and waste management practices in different countries to simulate the entry and circulation of plastic in the oceans from 2015 to 2025.

Then they modeled how much plastic could be removed by floating plastic collectors that sieve out tiny pieces of plastic, or microplastic, drifting on the surface of the ocean. The analysis assumed that each of 29 collectors would capture 45 percent of the plastic bits passing through its vicinity. These parameters are based on a previous study by an organization known as The Ocean Cleanup, which is developing the devices.

The Ocean Cleanup has proposed deploying plastic collectors in subtropical gyres, which are circular currents that tend to trap and concentrate floating plastic. The largest and most dramatic of these is the Pacific Garbage Patch, a conglomeration of plastic that covers a stretch of ocean more than twice the size of the United Kingdom between Hawaii and California.

That proposal has intuitive appeal: if you see a bunch of garbage, pick it up.

But according to the new analysis, cleaning up the Garbage Patch isn’t the most efficient way to get the most plastic out of the ocean, nor the best way to minimize plastics’ harm to marine life.

The best places to put plastic collectors are areas where the most plastic is moving through, not the places where the most plastic ends up, the researchers found. They identified a set of locations for the devices that would result in removal of 31 percent of the ocean’s microplastics by 2025. In contrast, putting all the collectors in the Pacific Garbage Patch would only reduce plastic in the oceans by 17 percent.

The researchers also considered where to put the collectors in order to minimize harm to marine life from plastics. To do this, they mapped net primary productivity in the oceans, which indicates the growth of microscopic floating plants called phytoplankton. Since phytoplankton are the base of the marine food chain, they reasoned, areas with high net primary productivity are likely to be rich in ocean life in general.

This run of the model identified a set of locations for the plastic collectors that would result in a 46 percent decrease in the overlap between plastic debris and net primary productivity by 2025. In contrast, putting all the collectors in the Garbage Patch would only reduce this overlap by 14 percent.

For both goals, maximizing plastic removal and minimizing harm to marine life, the best arrangement would be to place most of the plastic collectors near coastlines – especially off China and in the Indonesian archipelago, because a lot of the plastic in the oceans comes from East Asia.

“It makes sense to remove plastics where they first enter the ocean around dense coastal economic and population centers,” says van Sebille. “It also means you can remove the plastics before they have had a chance to do any harm.”

Last year, van Sebille and a group of Australian collaborators published an analysis suggesting that 90 percent of seabirds have swallowed plastic. Seabird populations are also concentrated around the coasts, he points out.

Despite the impressive plastic removal stats generated by the researchers’ model, the total mass of microplastic on the ocean surface would still increase by 4 percent by 2025, even with the plastic collectors going full-time. That’s because we’re producing ever greater amounts of the stuff, and the more plastic we make, the more of it ends up in the sea. – Sarah DeWeerdt | 2 February 2016

Source: Sherman P. and E. van Sebille. “Modeling marine surface microplastic transport to assess optimal removal locations.” Environmental Research Letters DOI: 10.1088/1748-9326/11/1/014006

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