A pioneering technique to detect aggregated patches of plastics floating on the sea surface has been published in Scientific Reports.
Led by scientists at Plymouth Marine Laboratory, Earth observation scientists analysed data from the European Space Agency’s Sentinel-2 satellites to develop this new approach, which demonstrates for the first time that aggregated patches of plastics floating in coastal waters can be detected by satellites.
Using this method, aggregations of plastic particles larger than 5mm (macroplastics) were also distinguishable from naturally occurring floating materials, such as seaweed, driftwood and foam, with an average accuracy of 86% across four case study sites.
This technical challenge, primarily funded by the Natural Environment Research Council’s ACCORD research programme, is the first step towards developing an operational method of detecting floating plastic patches in waters all over the world.
The team ran high-resolution, multi-spectral optical satellite data of coastal waters through an algorithm tuned to highlighting objects floating on the ocean surface, creating the Floating Debris Index (FDI) for the Sentinel-2 Multi-spectral Instrument.
The next stage was to identify floating plastics. Thanks to a collaboration with the University of the Aegean, who shared information on deployed plastic targets for their new study into plastic litter, the team was able to know exactly what Sentinel-2 was ‘seeing’ through the FDI and, therefore, able to build an optical signature for floating plastics. These known plastic detections were supplemented with validated plastics data detected after severe flooding in Durban, South Africa. Once the plastic signatures were established, the team then began the same process for natural debris, such as driftwood, seaweed and seafoam, which are likely to be mixed in with the plastic patches.
With the algorithm development and validation complete, the team began searching for plastics ‘in the wild’. Based on published studies and social media posts, they detected aggregations in two developed countries – Canada (San Juan Islands) and Scotland – and two developing countries – Ghana (Accra) and Vietnam (Da Nang).
Suspected plastics were successfully classified as plastics with an overall accuracy of 86% (San Juan Islands 100%, Accra 87%, Scotland 83% and Da Nang 77%). Less accurate classification resulted from pixels not being sufficiently full with floating debris and a small proportion of suspected plastics being identified as sea foam.
The team will continue to refine the technique to further increase its accuracy in detecting floating plastic patches in turbid coastal waters, and large river systems.
By Jake Frith