In an article published on August 7, 2023, in the journal Nature Geoscience, Mikael L. A. Kaandorp and co-authors from Utrecht University, the Netherlands, and Forschungszentrum Jülich, Germany, developed a three-dimensional model for the global transport of buoyant marine plastics. This model allows the scientists to estimate the total amount of plastic litter floating on the ocean surface, the annual inputs, the residence times, and the size distribution of this litter.
In their model, the authors considered plastic concentrations measured on surface waters (14,977 measurements), on beaches (7,114), and in the deep ocean (120) considering data between 1980 and 2020. In contrast to their previous mass budget study, Kaandorp et al. increased the complexity of their current model by considering “different processes affecting marine plastic transport: sinking via biofouling, beaching, turbulent vertical mixing, and fragmentation.” They only included plastics that are buoyant when produced while neglecting the non-buoyant ones which account for 23-40% of the marine plastic mass.
Kaandorp and colleagues’ model showed that the world’s oceans contained 3,200 kilotonnes of initially buoyant plastic in 2020. The vast majority of the items, 90-98%, were larger than 25 mm while only the remaining 2-10% were between 5 and 25 mm and thus in the microplastics range (5 mm). Nearly two-thirds of this plastic mass floats on the ocean’s surface, another third is located deeper in the water column (initially buoyant plastics can sink due to biofilms growing on them), and only below 2% on beaches.
The researchers further calculated that 500 kilotonnes enter the marine environment per year via the coastlines (39-42%), fishing activities (45-48%), and from rivers (12-13%). This means that ocean plastics increase by 4% annually. Accordingly, the buoyant plastic mass would double within the next 20 years if no mitigation efforts are implemented. While their estimates on the total amount of buoyant marine plastic litter largely exceed previous estimates, the calculated input is much lower than those calculated by others. “The decreased input and increased standing stock suggest that there is no ‘missing sink’ for marine plastic pollution, which has been the focus of many recent papers,” the authors concluded. Moreover, this would indicate that the “residence time of plastics in the marine environment is much higher than previously estimated.” According to the scientists’ scenario over two years 10% of the plastic mass is removed from the oceans.
Kaandorp and co-authors warn that the “projected exponentially increasing input and long persistence of marine plastics means a likely increasing negative impact of marine plastic pollution on ecosystems in the future.”
Reference
Kaandorp, M. L. A. (2023). “Global mass of buoyant marine plastics dominated by large long-lived debris.” Nature Geoscience. DOI: 10.1038/s41561-023-01216-0
