Ocean plastic floats in the water. Photo: The Ocean Conservancy
As many as 14-million metric tons of plastic pollution enter the ocean annually. Until now, our understanding of the plastic conundrum has been focused on the ocean’s surface. However, a new study published in Nature reveals how ocean plastic is distributed throughout all depths. Unsurprisingly, plastic has found its way from the surface to the bottom of the ocean.
According to the study, most previous plastic research had concentrated on the top 50 centimeters of the ocean. The new study combined data at various ocean depths from 1,885 stations collected between 2014 and 2024. It confirmed suspicions that microplastics are present in large quantities below the surface.
“Despite observational uncertainties, the substantial presence of subsurface microplastics in both nearshore and offshore waters underscores the ocean water column as a critical yet uncharacterized reservoir of plastics,” the study states.
More than 1,100 particles per cubic meter (m3) were observed at 100 to 270 meters of depth in an Atlantic sample, 16,600 particles per m3 were found at 2,000 meters in the North Pacific Subtropical Gyre, 25,200 particles per m3 at 2,500 meters depth in the Arctic, and 13,500 particles per m3 at 6,800 meters of depth in the Mariana Trench.
The analysis suggests that microplastics make up a not insignificant amount of the total particulate organic carbon (POC) – a term used to describe tiny bits of carbon-based particles, organic and inorganic, floating in the water. Microplastic prevalence in POC increases from 0.1 percent at 30 meters of depth to five percent at 2,000 meters.
Despite further confirmation that plastics accumulate in ocean gyres, experts point out that the plastic does not form “garbage patch” islands as is often portrayed.
“Plastic islands don’t exist. If we travel to the convergence zones of the subtropical gyres, where these famous islands are, you won’t see anything. You might see more bottles, bags, and other more buoyant plastics, but you won’t see accumulated masses of plastics,” says Patricia Villarrubia Gómez, an expert in plastic pollution at the Stockholm Resilience Centre in Sweden. “The situation is bad enough without the need for exaggeration.”
The study shows that microplastics gradually decrease with depth according to their size. Relatively large microplastics are found in higher abundance either floating on the ocean’s surface or sinking rapidly to the ocean floor. Smaller microplastics sink more slowly and thus are more evenly distributed across depths. Larger particles are also more significantly affected by ocean stratification (sharp changes in water density), which slows sinking and causes a congregation of plastic at certain depths.
Large microplastics were more common in samples taken at mid-latitudes, but elevated amounts were also observed at 1 to 60 meters of depth in the polar regions above 55 degrees north and 60 degrees south. The study says that this indicates that subsurface currents are transporting microplastics towards the poles, with the Atlantic Ocean being the largest contributor of such plastics.
While the study admits that this deeper understanding of the distribution of ocean plastic has knowledge gaps, it’s clear that the sub-surface plastic alters the marine ecosystems. Marine snow – organic and inorganic particles that sink from the surface to the deep ocean – sinks more slowly when plastics are included. That hinders and slows the ocean’s processes for capturing carbon and counterbalancing human-induced climate change.
