Towards improvement to understand plastic dynamics in global rivers

Plastic pollution has been receiving considerable attention from scientists, policy makers and the public during the last few decades. Though some models succeeded to simulate transport and fate of plastic debris in freshwater systems (Meijer et al., 2021), a complete model is under development to elucidate the whole picture of plastic dynamics in continental scale. Previously, process-based eco-hydrology models, NICE (National Integrated Catchment-based Eco-hydrology)-BGC (BioGeochemical Cycle) (Nakayama, 2017), was applied to evaluate biogeochemical cycling in river basins ranging from local/regional to continental/global scales. Recently, the author linked NICE-BGC to plastic debris model that accounts for transport and fate of plastic debris (advection, dispersion, diffusion, settling, dissolution and biochemical degradation by light and temperature), and applied this new model to regional scale (Nakayama and Osako, 2023a) and global major rivers (Nakayama and Osako, 2023b). NICE-BGC was also improved to include the mass budget in water and bed sediment in order to show the seasonal variations of plastic fluxes. In this study, NICE-BGC was further extended to incorporate biofouling (with algae and phytoplankton) and heteroaggregation (with suspended particulate matter) to improve the accuracy of global plastic dynamics in global river basins and the amount of plastic flows from land into rivers and finally into the ocean. Model simulated size distribution of plastics in water and riverbed sediment of global major rivers and showed the difference of effect of biofouling and heteroaggregation in each river. In addition, the simulated result showed that flood events have a great impact on plastic mobilization and its high intra-annual variability mainly caused by settling, resuspension, and bedload transport (Hurley et al., 2018; van Emmerik et al., 2019). These results aid development of solutions and measures to reduce plastic input to the ocean, and help to quantify magnitude of plastic transport under climate change.   References Hurley, E., et al. 2018. Nature Geoscience, 11, 251-257, doi:10.1038/s41561-018-0080-1. Nakayama, T. 2017. Journal of Geophysical Research: Biogeosciences, 122, 966-988, doi:10.1002/2016JG003743. Nakayama, T., Osako, M. 2023a. Ecological Modelling, 476, 110243, doi:10.1016/j.ecolmodel.2022.110243. Nakayama, T., Osako, M. 2023b. Global and Planetary Change, 221, 104037, doi:10.1016/j.gloplacha.2023.104037. van Emmerik, T., et al. 2019. Scientific Reports, 9, 13549, doi:10.1038/s41598-019-50096-1.