Microplastics spread deep into the ocean, far below the surface

The ocean has long been Earth’s beating heart. It stabilizes the climate, supports half of all primary production, and absorbs a quarter of the carbon dioxide we emit. But beneath its vast, shimmering surface lies a growing threat – microplastics.

These tiny particles result from the breakdown of larger plastic debris and have seeped into every corner of the marine world. While most studies have stayed near the top, a new global investigation dives far deeper.

A landmark study, recently published in Nature, has mapped microplastics from the ocean surface down to the deepest layers. Led by scientists across Japan, Europe, the Americas, and Asia, it reveals the alarming extent to which plastic particles have penetrated the ocean’s core systems.

This new research presents the first complete picture of how microplastics behave vertically, reshaping our understanding of marine pollution.

Most studies looked at the surface

For decades, surface net tows – sampling the top 15 to 50 centimeters (6 to 20 inches) – served as the main method of collecting microplastic data. These approaches, while useful, ignored the majority of the ocean’s volume.

Microplastics, defined as particles between 1 micron and 5 millimeters (0.00004 to 0.2 inches), come in many shapes, densities, and chemical compositions. Their properties affect how they move, interact with organisms, and ultimately where they end up.

With traditional methods limited to surface water, scientists had no clear sense of how deep these particles travel or how long they remain suspended.

Now, a massive dataset of 1,885 vertical profiles from 2014 to 2024 has provided the missing layers. Researchers discovered microplastics in every region and at almost every depth — from tropical gyres to polar seas, and from coastal bays to abyssal trenches.

Microplastics spread deep into the ocean

The findings confirm that microplastics are not just surface drifters. They are now part of the ocean’s interior. Concentrations range from just a fraction of a particle to more than 10,000 particles per cubic meter.

The particles’ size dictates their movement: smaller ones – between 1 and 100 micrometers (0.00004 to 0.004 inches) – disperse widely and descend gradually. Larger ones – between 100 and 5,000 micrometers (0.004 to 0.2 inches) – cluster in the top 100 meters (328 feet), especially within gyres and stratified layers.

Gyres, which are slow-moving oceanic whirlpools, trap large floating plastics. But this study found that their influence extends below the surface. The team observed vertical extension of these accumulation zones into subsurface layers, particularly in the 1–60 meter (3 to 197 feet) range.

The presence of plastic there mirrors the structure of gyres above, showing that the vertical transport of microplastics is shaped by both physics and biology.

Microplastics and the carbon system

The implications go beyond pollution. Plastic particles are now becoming part of the ocean’s carbon pool.

At shallow depths like 30 meters (98 feet), microplastics represent a small fraction – about 0.1% – of the total carbon particles. But this figure jumps to 5% at depths around 2,000 meters (6,562 feet). As organic carbon breaks down during descent, the proportion of persistent plastic-carbon rises.

“Microplastics are not just floating at the surface – they’re deeply embedded throughout the ocean, from coastal waters to the open sea,” said Tracy Mincer, Ph.D., co-author and associate professor at Florida Atlantic University.

Plastic-carbon, known as plastic-C, behaves differently from organic carbon. Unlike organic matter, plastic does not decompose easily. It interferes with natural processes like carbon sinking and microbial cycling.

If plastic-C continues to accumulate in deep-sea ecosystems, it could distort long-term climate models and make radiocarbon dating of ocean samples unreliable.

Coastal zones have more plastic

The study found much higher microplastic concentrations in nearshore waters. Measurements showed up to 500 particles per cubic meter in coastal regions – 30 times higher than in the open ocean.

These hotspots align with expectations: coastlines are close to human activities, sewage outflows, and fishing zones. But biological productivity also plays a role.

Diatoms and other microorganisms in these waters produce siliceous or calcite structures that attach to plastic particles. This adds weight and causes them to sink. In some cases, mineral ballast increases the rate of descent dramatically.

Stratification layers, which occur where temperature or salinity changes rapidly, further influence particle motion by trapping larger plastics.

In these stratified zones, plastic particles settle more slowly and accumulate. Models and observations show that pycnoclines – layers where water density shifts – tend to hold large microplastics, preventing their rapid descent and prolonging their exposure to marine organisms.

Plastic type changes how it moves

Microplastics are more than just plastic bits – they are chemically, physically, and biologically complex. Their density, color, shape, and biofilm covering affect how they move and whom they interact with.

Some even host pathogens and antibiotic-resistant genes, forming what scientists call the “plastisphere.”

As plastics degrade, their surface properties change. They may absorb pollutants or release additives. Dense polymers like polyester or polyamide fragment more easily and dominate offshore regions.

Buoyant plastics such as polyethylene float longer and are more common near the coast. Polypropylene, often used in packaging, breaks down quickly, which may explain its lower abundance offshore.

These differences make it hard to generalize the fate of microplastics. Even identical polymers behave differently under different conditions, requiring highly customized sampling and analysis methods.

Microplastics size and ocean depth

The study categorizes microplastics into two key groups: small (<100 micrometers) and large (>100 micrometers). Small particles are more uniformly distributed with depth.

Their sinking speeds – between 1 and 0.001 millimeters per second – allow them to pass through pycnoclines and stay suspended longer.

Large microplastics, by contrast, either remain at the surface or settle quickly to the seafloor. Observations show a sharp two-order-of-magnitude drop in large microplastics with depth. This supports model predictions and indicates strong size-dependent sorting.

In some offshore sites, microplastics were found even in the Mariana Trench at 6,800 meters (22,310 feet). Along the Korean coast, researchers estimate that over 3 trillion microplastics float between 0.33 and 4.75 millimeters (0.013 to 0.187 inches) in size.

These particles join the estimated 171 trillion floating plastics already documented across the globe.

Gaps in microplastic measurements

Despite growing evidence, microplastic research still faces major gaps. Sampling techniques vary widely. The mesh size used, the depth intervals sampled, and the detection tools employed all influence results. Using a 10-micron mesh will capture many more particles than a 500-micron one.

The study stresses that without standard protocols, comparing datasets becomes nearly impossible. Some researchers use full chemical imaging, while others rely on visual inspection, which is unreliable below 300 microns.

Subsampling adds further bias. In one case, extrapolating from a single subsample inflated results by over 600%.

To move forward, scientists must harmonize data collection and develop continuous monitoring technologies. Autonomous sensors, high-resolution imaging, and collaboration across marine science disciplines are vital.

Models often miss plastic movement

Most ocean plastic models rely on surface data and assume particles are buoyant spheres. This oversimplification leads to discrepancies. Observations show a much more complex scenario, with both dense and buoyant plastics interacting with marine life, aggregates, and variable ocean currents.

Key mechanisms – like faecal pellet transport, biofouling, or marine snow formation – are often ignored in models.

Yet these processes strongly influence sinking rates and horizontal dispersal. Plastic fragments caught in marine snow can sink hundreds of meters per day, creating sudden drops in surface concentrations.

Still, some model predictions hold up. For example, gyre centers show smaller particles than their edges – a pattern confirmed by this global study. But many models underestimate deep transport and fail to capture polar accumulation, where ocean currents and river inputs deliver plastics even to the Arctic.

Scientific and policy priorities

The study outlines several priorities for future research. These include refining vertical flux estimates, improving plastic age dating, and understanding the role of microplastics in altering nitrogen cycling. Long-term datasets are especially needed to observe changes over seasons and decades.

“Microplastics are becoming a measurable part of the ocean’s carbon cycle, with potential consequences for climate regulation and marine food webs,” said Mincer.

“This work sets the stage for taking the next steps in understanding the residence time of plastic in the interior of the ocean.”

Policies must address not only plastic production and surface cleanup but also deep-sea contamination. Since these particles are nearly impossible to retrieve, prevention becomes the only viable strategy.

Microplastics are part of the ocean’s legacy

This research rewrites what we know about plastic pollution. Microplastics now permeate the ocean’s very structure. They are not surface scars but deep imprints, shifting chemical balances and carbon cycles.

The ocean remembers everything. And increasingly, it remembers us by the plastic we leave behind. Scientists have now traced that memory from the top to the abyss.

The next step is ours to take – armed with knowledge and pressed by urgency – to protect what lies beneath.

The study is published in the journal Nature.

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