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Scientists from the Department of Geography and Environmental Science at Queen Mary University of London have developed a simple model to show how buoyant plastic can settle through the water column and they predict it could take over 100 years to remove plastic waste from the ocean's surface.

in Philosophical Transactions of the Royal Society A: Mathematical, ÌÇÐÄÊÓƵical and Engineering Sciences, the study is the third and final paper in a trilogy that explores the long-term fate of microplastic in the ocean. It builds on earlier research featured in Nature Water and Limnology & Oceanography, offering a complete picture of how plastic pollution moves from ocean surface to seafloor.

The study was led by researchers from the Department of Geography and Environmental Science at Queen Mary University of London, in collaboration with HR Wallingford Ltd. It combines expertise in marine geochemistry, fluid dynamics, and environmental modeling to simulate how plastics move from the ocean surface to the deep sea over time.

The research reveals that even if all plastic inputs into the ocean were stopped immediately, fragments of buoyant plastic debris would continue to pollute the ocean surface and release microplastics for more than a century.

Using a model that simulates the slow breakdown of large plastic particles and their interaction with marine snow (sticky organic material that helps transport debris to the deep sea), the researchers show that the degradation process is the limiting factor in removing plastic from the surface.

Dr. Nan Wu, the paper's lead author from the Department of Geography and Environmental Science at Queen Mary University of London, said, "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."

The findings help explain the persistent mismatch between the amount of buoyant plastic entering the ocean and the relatively small amounts observed at the surface. This is often referred to as the 'missing plastic' problem.

Prof Kate Spencer, co-author and project supervisor from the Department of Geography and Environmental Science at Queen Mary University of London, said, "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, co-author and Principal Scientist at HR Wallingford and Associate Professor at the University of Plymouth, said, "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 model also shows that the biological pump, the ocean's natural conveyor belt for carbon and particles, may become overwhelmed as plastic production increases. If microplastic concentrations continue to rise, there is a risk they could interfere with ocean biogeochemical cycles.