Marine plastic pollution is a global crisis, with 9 to 14 million metric tons of plastic entering the ocean every year. Tiny fragments called microplastics鈥攔anging from 1 micron to 5 millimeters鈥攎ake up the vast majority of plastic pieces found and pose serious risks to ocean health.

Most research has focused on surface waters, usually sampling just the top 15 to 50 centimeters using net tows. However, microplastics come in many forms with different properties, influencing how they move and interact with their surroundings.

A researcher from Florida Atlantic University is among an international team of scientists who have moved beyond just "scratching the surface," marking a turning point in our understanding of how microplastics move through and impact the global ocean.

For the first time, scientists have mapped microplastic distribution from the surface to the deep sea on a global scale鈥攔evealing not only where plastics accumulate, but how they infiltrate critical ocean systems. For the study, researchers synthesized depth-profile data from 1,885 stations collected between 2014 and 2024 to map microplastic distribution patterns by size and polymer type, while also evaluating potential transport mechanisms.

Results, in Nature, reveal that microplastics are not just surface pollutants鈥攖hey're deeply embedded in the ocean's structure. Ranging from a few to thousands of particles per cubic meter, their size determines how they move. Smaller microplastics (1 to 100 micrometers) spread more evenly and penetrate deeper, while larger ones (100 to 5,000 micrometers) concentrate near the surface, especially within the top 100 meters of gyres. Gyres act like massive, slow-moving whirlpools that trap and concentrate floating debris鈥攅specially plastic.

Strikingly, microplastics are becoming a measurable part of the ocean's carbon cycle, making up just 0.1% of carbon particles at 30 meters but rising to 5% at 2,000 meters. This suggests that microplastics are not only persistent pollutants but may also be altering key biogeochemical processes in the deep sea.

"Microplastics are not just floating at the surface鈥攖hey're deeply embedded throughout the ocean, from coastal waters to the open sea," said Tracy Mincer, Ph.D., co-author and an associate professor of biology and biochemistry in FAU's Harriet L. Wilkes Honors College.

Researchers identified more than 56 types of plastic polymers in their synthesized microplastic dataset. While buoyant plastics dominate overall, denser microplastics are more prevalent offshore鈥攍ikely because they fragment more readily. Dense polymers become brittle and break down faster, particularly after prolonged exposure to environmental weathering. These small, persistent particles鈥攐ften originating from fishing gear and containers like polyester bottles鈥攃an remain in the ocean for decades.

Polypropylene, commonly found in items like yogurt containers and rope, photodegrades more quickly than polyethylene, which is used in plastic bags and water bottles. This may account for its lower abundance in offshore waters. Nonetheless, significant uncertainties remain in subsurface microplastic data due to inconsistent sampling techniques and limited coverage, highlighting the need for specialized equipment and greater collaboration to improve data reliability.

The ocean's water column鈥攖he largest habitat on Earth鈥攑lays a crucial role in global carbon cycling, supporting half of the planet's primary production and absorbing human-made CO₂. As microplastics move through this vast space, they interact with natural particles and processes, potentially affecting how the ocean functions.

"Our findings suggest 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."