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Here’s what you’ll learn when you read this story:
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Millions of tons of microplastics flow into the ocean every year, but tracking the movement and accumulation of those microscopic polymers is difficult.
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A new study builds a theory of how spherical microplastics interact with the dynamic flows of a 3D environment, such as an ocean.
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The resulting model suggests that microplastics tend to form twisted, closed loops beneath the surface that spiral upwards and downwards.
The creation at the beginning of the 20th century of the very first synthetic polymers, also known as plastics, has blossomed into a full-blown environmental and health crisis more than a century later. NASA estimates that over eight million tons of microplastics flow into our oceans every year, and these microscopic beads can be found everywhere from the hurricanes that buffet coastlines to our brains. But to understand the impact of microplastics on marine environments, it helps to know exactly where they’re accumulating.
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Previous research has used the satellites that are part of the Cyclone Global Navigation Satellite System to track the movement of microplastics across Earth, but finding concentrations of them below sea level—as opposed to the easy-to-spot, surface-dwelling concentrations like the Great Pacific Garbage Patch—can be immensely difficult.
In a new study published in the journal Chaos, scientists Larry Pratt and Irina Rypina from the Woods Hole Oceanographic Institution in Massachusetts turned to 3D modeling to understand how microplastics move in chaotic environments like the ocean. Because oceans are so vast, gathering sampling data isn’t an option. So the researchers delved deep into modeling how tiny particles move in a 3D fluid, and found that microplastics tend to eventually form an “idealized eddy” (circular current) that closely resembles a kind of closed-loop tornado of ecological destruction.
To approximate ocean currents on a laboratory scale, Pratt and Rypina used a rotating cylinder, where the body rotates at one speed while the lid spins at a different rate. According to the researchers, the resulting circulation closely resembles oceanic movements across hundreds of kilometers.
“If you just threw a small particle into the water with some arbitrary velocity, viscous drag would rapidly bring its motion close to that of the fluid,’ Pratt said in a press statement. “So, to a first approximation, the microplastic particles are just following the fluid trajectories.”
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However, because microplastics have their own inertia, they can disrupt these flows. By working out some mathematical calculations, the authors devised a theory that microplastics below the ocean surface tend to form into multiple attractors—stable states within otherwise chaotic systems—that resemble closed-loops spiraling upwards and downwards. While this movement may hold true for certain microplastics, the authors note that varying parameters can alter these flows.
“The main thing we need to consider is the effects of small-scale turbulence. The theory is valid for spherical particles, but most microplastics in the ocean have very irregular shapes,” Pratt said in a press statement. “In the immediate future, we’re hoping that the theory will inform sampling strategies and lead to a better understanding of where plastics might be accumulating.”
Microplastics will continue to ravage the world, impacting both human and marine health. But if scientists can get a snapshot on where these troublesome plastics are hiding, it could help ensure that future conservation efforts have the most impact possible.
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