Chaotic 3D Ocean Currents May Be Trapping Microplastics Beneath the Surface, New Study Reveals
The world’s oceans are already known to be heavily polluted with plastic, but most of what we understand comes from what we can see floating at or near the surface. A new scientific study suggests that a much more complex and hidden system of microplastic accumulation may exist beneath the ocean surface, driven by chaotic three-dimensional currents that create invisible traps for tiny plastic particles.
Researchers from the Woods Hole Oceanographic Institution have developed a detailed theoretical framework explaining how microplastics can accumulate below the surface in stable structures known as attractors, even when the surrounding water appears to be in constant motion. Their findings were published in Chaos: An Interdisciplinary Journal of Nonlinear Science, a journal focused on complex and nonlinear systems.
Why Subsurface Microplastics Are So Hard to Track
Microplastics, defined as plastic particles smaller than 5 millimeters, are now found throughout the global ocean. While surface features like the Great Pacific Garbage Patch have been widely studied, scientists still know very little about where microplastics go once they sink or become suspended below the surface.
The main challenge is scale. The ocean is vast, and direct sampling of subsurface microplastics is extremely sparse. Ships can only collect samples from limited locations and depths, making it difficult to build a complete picture. Because of this, theoretical models and laboratory analogs have become essential tools for understanding how plastics might move and concentrate in three dimensions.
Modeling the Ocean Using a Rotating Cylinder
To tackle this problem, researchers Larry Pratt and Irina Rypina turned to a classic experimental setup used in oceanography and atmospheric science: a rotating cylinder filled with fluid.
In their model, the main body of the cylinder rotates at a constant speed, while the lid spins at a different rate. This difference in rotation creates a circulation pattern where fluid spins upward in the center and spirals downward along the edges. This type of motion closely resembles large-scale ocean eddies, which can span hundreds of kilometers in real-world oceans.
These eddies are common features in ocean circulation and are already known to play a role in transporting heat, nutrients, and biological material. The researchers used this setup as an idealized representation of ocean flow, allowing them to study particle motion in a controlled but realistic way.
How Chaos Changes Everything
The real breakthrough came when the researchers introduced a small but critical change: tilting the lid of the rotating cylinder.
This tilt disrupts the otherwise smooth and predictable flow, transforming it into a chaotic three-dimensional system. Instead of following neat circular paths, fluid trajectories become tangled, overlapping, and highly sensitive to initial conditions. This is where chaos theory comes into play.
In these chaotic flows, the researchers observed the emergence of donut-shaped circulation structures embedded within the larger current. These structures are not random; they act as stable attractors, meaning particles that enter their influence tend to remain there over time.
What Are Microplastic Attractors?
In this context, attractors are stable pathways or loops within the fluid that particles naturally converge toward. Rather than dispersing evenly, microplastics can become trapped within these structures, circulating endlessly along twisted, closed loops that move both upward and downward through the water column.
The study found that multiple attractors can exist within a single eddy, creating several localized zones of accumulation rather than one large patch. Each attractor resembles a tubelike structure winding around the circular current, with particles spiraling through it in three dimensions.
This means microplastics may be accumulating far below the ocean surface, in places that current sampling methods rarely reach.
Why Microplastics Don’t Perfectly Follow the Water
At first glance, it might seem logical to assume that microplastics simply move wherever the water moves. In reality, the situation is more complicated.
While viscous drag quickly forces small particles to move almost like the surrounding fluid, microplastics still have mass and inertia. This slight mismatch causes them to slowly drift away from pure fluid trajectories. Over time, this deviation becomes significant enough to push particles into attractors rather than letting them circulate freely.
The researchers used the mathematics of inertial particle dynamics to explain how this process works and why particles eventually settle into stable accumulation zones.
What This Means for the Real Ocean
Although the rotating cylinder is a simplified model, the underlying physics applies to real ocean conditions. Ocean eddies, especially those with complex three-dimensional motion, are likely to host similar attractor structures capable of trapping microplastics below the surface.
This could help explain why field measurements often find fewer microplastics than expected based on how much plastic enters the ocean each year. Many particles may be hidden in subsurface layers, circulating within chaotic flows that are difficult to detect.
Understanding where these accumulation zones exist could help scientists design better sampling strategies, targeting specific depths and regions instead of relying on surface measurements alone.
Current Limitations of the Theory
The researchers are clear that their model has limitations. The theory assumes that microplastics are small, spherical, and slightly buoyant, which is rarely true in reality. Most microplastics have irregular shapes, varying densities, and complex surface properties.
Another missing piece is small-scale turbulence, which is common in the ocean and could disrupt or modify attractor behavior. Adding these factors will be an important step toward making the model more realistic.
Despite these challenges, the theory successfully explains experimentally and numerically observed particle behavior, giving researchers confidence that the core mechanism is sound.
Why This Research Matters Beyond Plastics
The implications of this study go beyond microplastics alone. Similar attractor mechanisms may influence the movement of plankton, marine larvae, pollutants, and even nutrients in the ocean.
Understanding how chaotic flows organize particles in three dimensions could improve models of marine ecosystems, carbon cycling, and pollutant transport. It also highlights how much remains hidden beneath the ocean surface, even in systems we think we understand well.
A Step Toward Smarter Ocean Monitoring
The ultimate goal of this research is practical. By identifying the physical mechanisms that cause subsurface accumulation, scientists hope to predict where microplastics are most likely to be found. This could make future ocean surveys more efficient and help build a clearer picture of plastic pollution’s true scale.
As researchers continue to refine the model by incorporating turbulence and realistic particle shapes, the invisible world of subsurface microplastics may finally come into sharper focus.
Research Paper:
Pratt, L. J., & Rypina, I. I. (2025). Modeling microplastic accumulation under the ocean surface. Chaos: An Interdisciplinary Journal of Nonlinear Science.
https://doi.org/10.1063/5.0288722
