Discarded plastics—such as water bottles, fishing nets, and grocery bags—have been identified in the far reaches of the ocean, both on the surface and in places as deep as the Marianas Trench.
Most of this garbage has been found laboriously: Cameras towed underwater have snapped images, and humans have peered over the sides of boats—or even swum through the debris.
Easy to see #oceanplastic floating on the surface virtually everywhere on a dive yesterday in Asia. Periodically there are thicker accumulations as you see me swimming through in this video. And there would be even more if not for a lack of rain recently. pic.twitter.com/bf0qku00im
— H Fisk Johnson, Ph.D. (@HFiskJohnson) February 24, 2019
Now scientists have used satellite imagery to pinpoint aggregations of floating plastic debris off the coasts of Scotland and Canada, a technique that opens up wide swathes of the remote ocean for analysis, the researchers suggest. Their results were presented at the General Assembly of the European Geosciences Union in Vienna, Austria.
A New Application
Lauren Biermann, a marine satellite scientist at Plymouth Marine Laboratory in Plymouth, United Kingdom, and her colleagues used imagery from the Sentinel-2A and Sentinel-2B satellites, platforms intended to image Earth’s landforms. These satellites, orbiting roughly 780 kilometers above Earth, were never designed for marine applications, Biermann said. But their frequent overpasses—the satellites repeatedly image the same patch of Earth every few days—and high spatial resolution (10 meters) make them perfect for imaging discarded plastics near coastlines.
Using sightings of plastic debris reported in the literature and on Twitter, the researchers focused on two areas: Gabriola Island, British Columbia, Canada and the eastern coast of Scotland near Edinburgh. They collected Sentinel images of these regions and compared them with reference measurements of how water, floating plants (for example, Sargassum seaweed), and plastics reflect and absorb light.
Biermann and her collaborators then estimated the relative contributions of these different materials to each pixel. Plastics exhibit a spectral peak in the near infrared, and vegetation emits at certain wavelengths because of its photosynthetic activity, said Biermann.
“There are distinct differences that we can use to determine what is what.”
A Promising Monitoring Tool
Biermann and her colleagues inferred that aggregations of plastics—probably water bottles, polystyrene, and packaging—were present off the coasts of Canada and Scotland.
It’s critical to do follow-up fieldwork to validate these findings, however, said Biermann. That’s because one possible source of confusion might be marine creatures: Some of the plastic debris measured near Scotland might have, in fact, been northern gannets, large seabirds common along the shorelines of the Atlantic Ocean.
This work is promising, said Stefanie Rynders, an oceanographer at the National Oceanography Centre in England not involved in the research, but follow-on research is necessary. “Provided they can do the ground truthing, it will be a useful monitoring tool, both for natural ecosystems and man-made pollution.”
In the future, Biermann and her colleagues hope to automate their analysis. Right now it takes half a day to manually process a single image, she said. By developing an algorithm to pinpoint pixels that likely contain plastics, this work could be expanded to encompass coastal regions around the world.
“What we’d like to do eventually is build a global hot spot map,” said Biermann.