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Physical oceanography

The end of the line for ocean plastics

Source: Cózar, Andrés, et al. “The Arctic Ocean as a dead end for floating plastics in the North Atlantic branch of the Thermohaline Circulation.” Science Advances 3.4 (2017): e1600582. 10.1126/sciadv.1600582

Oceans of plastic

Plastic pollution in the ocean a massive environmental problem. The ‘Great Pacific Garbage Patch’, a pile of trash accumulating in the stagnant centre of the subtropical Pacific ocean gyre, isa symbol of pollution of the world’s oceans. This problem often invoked images of of mountains of garbage so thick that you could walk on it, but in reality, most of the plastic pollution in the open ocean has broken down into tiny fragments that are carried along with the ocean currents.

While these tiny pieces of plastic may be less obvious to humans, and you could cruise over them on a boat unnoticed, when these tiny plastic pieces and fibers are consumed by marine animals they have the potential cause enormous damage to marine ecosystems. However, outside of the subtropical ocean gyres, there is relatively little known about plastic accumulation in other parts of the world’s oceans.

Microplastics seen during the 2014 Northwestern Hawaiian Islands marine debris removal mission. From the National Oceanic and Atmospheric Administration (http://oceanservice.noaa.gov/facts/microplastics.html)

Drifting northward

The Arctic Ocean, which is rapidly changing in response to global warming and the melting of sea ice, seems like an unlikely place for plastic to accumulate because it is so far away from major human populations. But an ocean circulation model, which predicts where plastic pollution will be carried by ocean surface currents, predicts plastic being carried by strong currents from the North Atlantic to one corner of the Arctic Ocean, the Greenland and Barents Seas. Here the plastic would run into a dead end and become trapped.

Researchers hypothesized that plastic pollution enters the ocean in highly populated coastal areas, accumulates in the middle of subtropical gyres before being carried by the ocean circulation from the North Atlantic subtropical gyre, northward along the western side of the Atlantic, through the gap between Scotland and Iceland and into the Greenland and Barents Seas.

Nobody had collected plastic samples in the Arctic Ocean to see if this hypothesis could be correct until 2013, when the Tara Ocean expedition completed a circumpolar navigation of the Arctic on board the R/V Tara, measuring the amount of plastic in the ocean along the way. The scientists used nets to collect plastic samples at 42 locations, weighed the plastic samples and recorded the size and type of plastics using a microscope. An international team of researchers from Europe, Saudi Arabia, Japan and the US combined forces to analyze the newly collected measurements and compare them to measurements of ocean plastic pollution from around the world.

Figure 1 from Cózar et al. 2017. The top panels show the frequency of shipping in the Arctic (left), human population density (middle) and ocean surface salinity (right). The lower map shows the measured concentration of plastic pollution throughout the Arctic Ocean, with the Atlantic surface ocean circulation patterns showing the high concentration of plastic in the Greenland and Barents Sea where the circulation runs into a dead end. The lower right panel shows the total plastic weight and abundance of types of plastics along the ship’s circumpolar track.

Plastic pile up

The samples from around the Arctic confirmed what the researchers had suspected: there were high concentrations of plastic pollution in the Greenland and Barents Seas, and very little plastic elsewhere in the Arctic. In fact, 95% of the plastic accumulated in the Arctic Ocean was found in these this region.
Now that the researchers had confirmed that plastic debris is accumulating in the Barents Sea, they were able to compare the size and type of plastic from the Arctic to samples from other parts of the world’s oceans. If the range and size of plastics was similar, this would be evidence that the plastics could have come from the same source. The researchers found that although there was less plastic accumulation in the Arctic compared to the subpolar ocean gyres, the size and types of plastics were very similar. When instead they compared the Arctic to the Mediterranean Sea, they found bigger differences.

Figure 2 from Cózar et al. 2017. The distribution of different types and sizes of plastic in the Arctic Ocean compared with the subtropical gyres and the Mediterranean Sea shows that the Arctic is more similar to the subtropical gyres, but has fewer very large plastic pieces, suggesting that the plastic has had more time to break down.

The changing Arctic

Although very few people live along the edges of the Arctic Ocean, it is has become an increasing hotspot of shipping in recent decades as the retreating sea ice opens up new shipping routes and extends the length of the ice-free season. It is possible that more ships in the Arctic could have contributed to the hotspot of plastics in the Arctic, but the number of ships in the Arctic is low compared with many other parts of the Oceans and there is no reason to believe the ships in the Arctic are worse polluters than everywhere else.
With all of this evidence combined, the researchers conclude that is it most likely that the plastic was brought to the Arctic from remote sources via the ocean currents, but that shipping activity could be contributing. As Arctic sea ice continues to disappear, increasing human presence in the Arctic Ocean may contribute to marine pollution in the future. Arctic ecosystems are already facing an uncertain future as rapid warming causes changes in ice cover and ocean conditions. The impacts of this plastic pile up on Arctic ecosystems is unknown, and this study provides strong motivation to investigate these effects.

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