🌊 What happens to plastic pollution in the oceans?

The ocean surface retains a stubborn trace of our plastic waste. Even if we were to stop all pollution today, these residues would persist for decades, or even more than a century. This worrying reality emerges from a recent British study that models the fate of polymers in the marine environment.

Researchers from Queen Mary University of London have just published a simulation revealing the slow journey of plastics. Their work, published in Philosophical Transactions of the Royal Society A, forms the final part of a series dedicated to the long-term fate of microplastics. They describe how floating debris gradually fragments before sinking into the depths.

The slow degradation process of polymers

The model focuses on the behavior of a 10-millimeter (0.4-inch) fragment of polyethylene, representative of common pollution. Its degradation begins immediately under the combined action of UV radiation, salinity, and the mechanical action of waves. Fragmentation causes a gradual transformation of the material, which loses about 0.45% of its mass each month. This slow erosion generates, in a cascade effect, an increasing number of microparticles.

These newly formed microplastics then enter a complex cycle of vertical transport. Their small size allows them to aggregate with "marine snow," the continuous rain of organic particles that descends towards the ocean floor. However, this process is neither linear nor definitive, as the aggregates can break apart, releasing the polymer fragments again, which then rise back towards the surface.

The research demonstrates that this transformation is the limiting factor in the natural elimination of plastics. Even after 30 years, nearly two-thirds of the original material persists as microplastics. Dr. Nan Wu, the lead author of the study, emphasizes that this slow rate of degradation maintains a constant source of pollution, even if discharges were to stop immediately.

The role of marine snow

Marine snow functions as a natural conveyor belt, transporting organic matter to deep-sea ecosystems. The published work shows that it preferentially captures microplastics smaller than 75 micrometers. This physical interaction depends on the size and density of the fragments, creating a sophisticated sorting mechanism between the different ocean layers.

This transport undergoes multiple interruptions and resumptions. Marine snow aggregates reform and disintegrate according to ocean conditions, causing microplastics to move back and forth between the surface and the depths. This dynamic explains why only a fraction of the polymers ultimately reaches the marine sediments after such a long process.

On a century timescale, nearly 90% of the initial plastic mass ends up trapped in the sediments of the seafloor. Professor Kate Spencer, the project supervisor, stresses the intergenerational nature of this pollution. The persistence of the remaining 10% at the surface maintains an active reservoir for the decades to come.

The modeling also suggests a risk of alteration to the ocean's biological pump. The continuous increase in microplastic concentrations could, in the long term, disrupt marine biogeochemical cycles and the role of the oceans as a carbon sink.