Our waste slowly degrades in the oceans and forms billions of microplastics, plastic fragments invisible to the naked eye. Resulting from the slow degradation of larger macroplastics, their formation involves numerous complex processes. However, understanding where and how these microplastics form in the sea is essential for assessing their impact on marine ecosystems and guiding strategies to combat this widespread pollution. A new study has estimated the rate of microplastic fragmentation in the Japan Sea for the first time.
by Laurie Henry
Cover photo : © Shutterstock.com/olenalavrova
Microplastics now invade all oceans, from the most densely populated coastal areas to the deepest abysses. Their massive presence is the result of a complex transformation cycle in which larger macroplastics – exposed to UV rays, currents and marine organisms – gradually fragment. While the precise dynamics of this degradation at sea remain largely unknown, a recent study conducted in the Japan Sea provides some key insights into the subject. This research makes it possible to estimate, for the first time, the rate of fragmentation in the marine environment.
Massive pollution from Asian rivers
The Japan Sea is at the centre of a veritable funnel of plastic pollution. Two of Asia’s largest rivers, the Yangtze and the Yellow River, are among the main sources of this contamination. Together, they dump huge amounts of plastic waste into the waters bordering China every year: 1.16 million tonnes for the Yangtze and 469,000 tonnes for the Yellow River. These figures, which are unmatched in the region, far exceed the direct contributions of other surrounding rivers, estimated at a maximum of 1,300 tonnes per year.
Once released into the sea, this waste is carried by ocean currents. The Tsushima Current, which enters the Japan Sea through the strait of the same name, plays a major role in moving this plastic northeast. This marine flow carries with it floating waste of all sizes: from visible macroplastics to invisible microplastics measuring less than 5 mm! This natural redistribution mechanism transforms coastal areas into veritable accumulation zones, making them an ideal setting for studying the fragmentation processes of plastics at sea.
An approach combining observations and numerical modelling
In order to better understand the processes involved in plastic fragmentation, a team of researchers led by Hiroki Takeda and Atsuhiko Isobe from Kyushu University deployed a specific method combining direct observations and numerical simulations.
An extensive sampling campaign was carried out along the Japanese coast between 2014 and 2021. More than 530 sampling stations were set up using neuston nets, designed to collect floating particles and capture fragments 0.3 mm in diameter and larger. Each sample was analysed in the laboratory: polymers were identified using infrared spectroscopy and the size of the fragments was measured using an optical microscope. This meticulous work, published in the journal Marine Pollution Bulletin, has enabled a precise map of microplastic particle sizes (MiPs) in the Japan Sea to be drawn up. It reveals a gradual decrease in size towards the north-east, suggesting continuous fragmentation of macroplastics (MaPs) as they drift.
In parallel with these direct measurements, a numerical particle tracking model was developed over five years. Complex simulations took into account ocean currents, Stokes drift* caused by waves, the effect of wind on floating debris, and the phenomena of stranding and return to sea. They also reproduce the conversion of MaPs (macroplastics) into MiPs (microplastics) and the gradual disappearance of the latter, either through fragmentation into very fine particles (too fine to be detected) or through sedimentation.
‘By comparing field data with modelling results, we were able to estimate the fragmentation time of plastics at sea, which is a major breakthrough,’ emphasises Atsuhiko Isobe.
Continuous fragmentation at sea
Until now, scientists believed that plastic fragmentation mainly occurred on beaches (where plastics are immobile and continuously exposed to UV rays and wave action) and that it was mitigated at sea due to currents and lower solar exposure.
However, this new study shows that the process of plastic fragmentation is much more active in the open sea. The researchers introduced a key parameter into their model to assess the proportion of fragmentation occurring in the water compared to that occurring on the coast. The simulations reveal that a fragmentation rate at sea equivalent to 20% of that observed on beaches best matches the data collected.
Time since the transformation of macroplastics into microplastics according to two scenarios: α=1 (fragmentation only on beaches: a and b) and α=0 (no fragmentation at sea: c and d). The maps show the results over 5 years and their averages by geographical area. The colours indicate the duration © Hiroki Takeda et al., 2024
This proportion is significant: at sea, plastics are exposed to prolonged ultraviolet radiation, wave action and interactions with marine organisms. Previous studies had already highlighted phenomena such as digestive fragmentation, whereby species such as krill and certain amphipods break down ingested plastics into smaller particles. ‘These biological and physical mechanisms partly explain the continuous fragmentation of microplastics during their drift,’ explains Hiroki Takeda.
Geographical distribution of the median size of microplastics (in mm)© Hiroki Takeda et al., 2024
The data also reveal a gradual decrease in the median size of MiPs, from approximately 1.77 mm in southern Kyushu to 1.25 mm near the Tsugaru Strait. This trend confirms the combined action of ocean transport and degradation at sea, slowly but surely reducing the size of floating plastics.
An unprecedented and quantified fragmentation rate
For the first time, a microplastic fragmentation rate in the marine environment has been estimated for this region of the Japan Sea. By analysing the relationship between the size of the MiPs collected and the modelled drift time, the researchers calculated an “apparent” size reduction rate of approximately 1.0 mm every 100 days. This fragmentation rate implies that a 5 mm microplastic could reach a size of less than 0.3 mm – the detection threshold of the sampling nets – in just 1.1 to 2.6 years.
Estimated time, based on modelled particles, for a macroplastic to become a microplastic (horizontal) and for this microplastic to disappear from the surface (vertical) © Hiroki Takeda et al., 2024
The authors specify that the total lifespan of floating microplastics, before they disappear from the surface through fragmentation or sedimentation, is estimated to be between 4.7 and 12.2 years. These estimates are consistent with the ages of microplastics reported by Okubo et al. (2023), which generally place them at less than 5 years. While the model highlights the dominant contribution of beaches, where fragmentation is four to five times faster than at sea, ocean degradation remains significant and must be incorporated into global plastic dispersion models. ‘These figures provide us with a quantitative basis for understanding the fate of plastics in the Japan Sea, an essential step in assessing ecological risks,’ insists Atsuhiko Isobe.
Shedding light on these dynamics will help refine predictions about the future distribution of plastics in the sea. It also highlights the urgent need to limit upstream inputs before this waste enters the complex ocean cycle revealed by this study. While this research focuses on the physical dynamics of plastics in the sea, it does not address the health impacts of microplastics, an issue that is currently the subject of extensive research in toxicology and public health.
* Stokes drift is the gradual movement of water and floating objects on the surface caused by the circular motion of waves. It causes plastic and marine debris to drift over long distances.
Source : Hiroki Takeda and Atsuhiko Isobe, “Quantification of ocean microplastic fragmentation processes in the Sea of Japan using a combination of field observations and numerical particle tracking model experiments”. Marine Pollution Bulletin, Volume 208, November 2024, 117032