Published in Philosophical Transactions of the Royal Society A the research is the culmination of a trilogy examining the fate of microplastics in the ocean. The team combined expertise in marine geochemistry environmental modeling and fluid dynamics to simulate long-term plastic transport from the ocean's surface to the seafloor.
Their results indicate large plastic items degrade at the surface gradually breaking into smaller particles over decades. These fragments then attach to marine snow - organic material that aids sinking - and make their way to the deep sea over long periods. The model suggests degradation is the main factor limiting removal of surface plastics with approximately 10 percent of original material remaining after one hundred years.
Lead author Dr Nan Wu stated "People often assume that plastic in the ocean just sinks or disappears. But our model shows that most large buoyant plastics degrade slowly at the surface fragmenting into smaller particles over decades. These tiny fragments can then hitch a ride with marine snow to reach the ocean floor but that process takes time. Even after 100 years about 10 percent of the original plastic can still be found at the surface."
Co-author Prof Kate Spencer explained "This is part of our wider research that shows how important fine and sticky suspended sediments are for controlling microplastic fate and transport. It also tells us that microplastic pollution is an intergenerational problem and our grandchildren will still be trying to clean up our oceans even if we stop plastic pollution tomorrow."
Prof Andrew Manning added "This study helps explain why so much of the plastic we expect to find at the ocean surface is missing. As large plastics fragment they become small enough to attach to marine snow and sink. But that transformation takes decades. Even after a hundred years fragments are still floating and breaking down. To tackle the problem properly we need long-term thinking that goes beyond just cleaning the surface."
The study also demonstrates that the ocean's biological pump - its natural mechanism for transporting carbon and particles - may be at risk of being overwhelmed as plastic production grows potentially threatening biogeochemical cycles.
Funding for the work was provided by the Lloyd's Register Foundation Queen Mary University of London HR Wallingford Ltd and the EU INTERREG Preventing Plastic Pollution project.
Research Report:Coupling fragmentation to a size-selective sedimentation model can quantify the long-term fate of buoyant plastics in the ocean.
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