The Big Unknown: Microplastics Beneath the Ocean Surface

1. Key Points

• Microplastic distribution is size-dependent: small microplastics^※1 presenting a more uniform distribution throughout the water column, while larger microplastics tend to be trapped by seawater density^※2 gradient interfaces.

• Mid-gyre plastic accumulation zones stretch into the ocean’s interior but are concentrated within the top 100 m and predominantly consist of larger microplastics.

• Plastic carbon depleted in 14C^※3, becoming significant in deep waters, can skew 14C carbon dating, complicating our understanding of ocean circulation, carbon cycling and past climate conditions.

2. Overview

M-Plastic Group, Marine Biodiversity and Environmental Assessment Research Center, Research Institute for Global Change, Dr. Shiye Zhao and Dr. Ryota Nakajima, together with another 12 collaborators from ten different universities and institutes, provide a global view of microplastic distribution in the oceanic water column beneath the sea surface.
These findings were published in " Nature " on May 1(Japan Time).

3. Background

The ocean is massive—it holds 1.3 billion cubic kilometers of water and reaches an average depth of 3,688 meters, nearly as tall as Mount Fuji (3,776 meters). It’s the largest habitat on Earth and plays a vital role in regulating our planet’s climate. One key function? Absorbing human-made CO₂, the main driver of global warming. This makes the ocean a critical player in the fight against climate change.

The Growing Plastic Problem: Every year, 9 to 14 million tons of plastic end up in the ocean, from large debris (like water bottles and packaging) to tiny microplastics (1 micrometer to 5 millimeters Fig.1, ref.1). Because they’re so small, microplastics are everywhere, posing a serious threat to marine health.

For the past 50 years, scientists have mainly studied microplastics on the ocean surface, using nets to collect samples from the top 20-50 centimeters. But microplastics don’t just float. They come in different shapes, sizes, and densities—some sink, some drift, and others remain suspended in the water column. Our research in the South Atlantic Subtropical Gyre found high quantities of microplastics from 10 meters all the way down to 5,500 meters (ref.2,ref.3). Yet, research on what happens to microplastics below the surface remains limited (Fig.2).

Why Does This Matter? The ocean isn’t just a big puddle—it’s full of life. As microplastics move through the water, they disrupt natural processes. Marine animals mistake them for food, potentially leading to bioaccumulation and biomagnification, which can harm the entire food web. Microplastics also interfere with the ocean’s ability to store carbon, disrupting the biological carbon pump, a collection of various processes that helps trap CO₂ in the deep sea.

The Big Unknown: The problem is, we still don’t fully understand what happens to microplastics beneath the surface. Where do they go? How do they behave? And what long-term impacts might they have from the surface to the deep sea? Without a global picture, we don’t yet know the best way to tackle this growing crisis.

4. Results

Mapping Microplastics: A Hidden Reservoir in the Ocean
To better understand how microplastics are distributed in the ocean’s interior, we analyzed data from 1,885 sampling stations collected between 2014 and 2024 (ref.4). Our findings reveal that microplastics are widespread throughout the water column, with a median concentration of 205 particles per cubic meter (Fig. 3). This suggests that the ocean’s interior is an important, yet underexplored, reservoir of plastic debris.

Microplastics Sink Faster in Coastal Waters
Microplastic concentrations decline with depth, but this drop is far steeper nearshore than offshore (Fig. 3). In coastal waters, abundances can decrease by up to a thousandfold, likely due to high biological and mineral productivity, which accelerates sinking.

Size matters: Small microplastics (1–100 µm) gradually decrease with depth, indicating a more even spread and longer residence time in the water column (Fig.3B). In contrast, large microplastics (100–5000 µm) tend to accumulate in the density gradient interfaces, where they become trapped (Fig. 4A).

Mid-Gyre Plastic Hotspots Extend Below the Surface
The floating plastic patches in the middle of ocean gyres—massive, slow-moving currents—are well known, but we found that these accumulation zones extend into deeper waters, primarily within the top 100 meters (Fig. 4B). However, most of the plastics here are large microplastics (100–5000 µm), further reinforcing how size affects vertical movement.

Plastic is Biasing Carbon Dating
Since 99% of plastics come from fossil carbon, they contain no radiocarbon (¹⁴C). Our study shows that microplastics make up 1.1% of total particulate organic carbon (POC) on average, reaching up to 5% at 2,000 meters (Fig. 5). This contamination can artificially age deep-ocean carbon samples by ~420 years, complicating carbon cycle studies, ocean circulation models, and climate reconstructions.

5. Future Perspectives

Our study establishes a global benchmark but highlights the substantial uncertainties arising from the lack of standardization. This challenges progress in understanding microplastic distribution and its environmental impacts. Our work emphasizes the urgent need for standardized methodologies, high-resolution observations, finer-scale process-based research, and enhanced international coordination. These efforts are crucial for developing comprehensive, long-term monitoring and more accurate model projections, ultimately improving our understanding of microplastic dispersion, fate, and impacts, and informing effective policy and management strategies.

Together with international collaborators, Dr. Zhao at JAMSTEC is leading the analysis of water-column microplastic samples from the global ocean using standardized methodologies. Our goal is to generate a high-quality, comparative dataset that refines the mapping of microplastics in the ocean’s interior and supports Earth system models in assessing their long-term impacts.

Fig.1. Particle size spectrum of particulate materials in seawater. Microplastics are now commonly defined as plastic particles ranging from 1 μm and 5 mm. Plastic particles smaller than 1 μm, which are difficulted to be individually identified from environmental samples, are classified as “nanoplastics”, with a lower size limit of 1 nm. In contrast, “nanoparticles” refer to particles with size between 1 nm and 100 nm. Historically, a threshold of 200 nm (0.2 μm) has been used as an operational distinction between ‘dissolved’ and ‘particulate’ pools in oceanic research.

Fig.2. The map (A) displays the distribution of observation stations studying microplastics in the global ocean. The bar plot (B) highlights the differences in research efforts focused on microplastics floating at the sea surface versus those found beneath it.

Fig.3. Depth profiles of microplastic abundances (particles m^-3) observed in nearshore (A) and offshore (B) waters with the log–log regression fits (dashed lines) between MP abundances and water depth of individual studies. The exponent (⁠ξ), also referred to as the slope, provides information on the rate of abundance change along the depth gradients.

Fig.4 Latitudinal trends in the large microplastics (A), residing within the top 60 m and those floating microplastics at the sea surface. Relationship between microplastic abundance within pycnocline layers to those above the pycnocline layers as function of microplastics size category: small MP (gold triangles) and larger MP (brown dots). Dots locate above the ratio of 1:1 (the dashed line), indicating an accumulation of microplastics.

Fig.5. Relationship between ratios (%) of microplastic-carbon (C) to particulate organic carbon and water depth in the North Pacific (blue diamonds) and North Atlantic (purple diamonds) Subtropical Gyres.