10:15 AM - 10:30 AM
[MIS02-06] Analysis of Long-term Microplastic Settling Flux Variability: A Sinking Model Approach
Keywords:Microplastics, sinking process, biofouling, long-term variations, one-dimensional particle sinking model
Plastic waste in the environment undergoes fragmentation due to heat and ultraviolet radiation, with particles smaller than 5mm classified as microplastics (MPs). It has been reported that plastic waste primarily enters marine environments through rivers from land, and after fragmentation, a significant portion sinks and accumulates on the seafloor. Recently, density increased due to biofilm formation has gained attention as one of the factors affecting this sinking process. Hinata et al. (2023) reported that MP settling flux in Beppu Bay seafloor exhibited both a long-term linear increase and a 20-year periodic fluctuation. A positive correlation was observed between the interannual variations of MP settling flux and chlorophyll-a settling flux, suggesting that environments favorable for phytoplankton growth may lead to increased MP settling flux. In this study, we used numerical modeling to reproduce the long-term interannual variations of MP settling flux in Beppu Bay and analyzed the factors controlling these variations.
For numerical simulations, we employed a one-dimensional vertical MP particle sinking model. This model is based on Kooi et al. (2017), which simulates the sinking process initiated when the overall particle density exceeds seawater density due to algal attachment on MP particle surfaces. Using the developed model, we calculated the settling time of MP particles under year-specific conditions of seawater temperature, nutrient concentrations, and chlorophyll-a concentrations during summer and autumn from 1960 to 2017. The number of MP particles reaching the seafloor each year was determined based on the settling time of MP particles, plastic input into Beppu Bay, and the residence time within Beppu Bay.
The developed model successfully reproduced the interannual variations in MP settling flux in Beppu Bay reported by Hinata et al. (2023). Through sensitivity analysis using the numerical model, we confirmed that nutrient concentrations and the residence time of seawater in the bay contributed to the 20-year periodic variations. Furthermore, the Pacific Decadal Oscillation (PDO) may be involved in the changes in nutrient concentrations and residence time in Beppu Bay, suggesting that global-scale circulation influences the local-scale settling of MP particles.
For numerical simulations, we employed a one-dimensional vertical MP particle sinking model. This model is based on Kooi et al. (2017), which simulates the sinking process initiated when the overall particle density exceeds seawater density due to algal attachment on MP particle surfaces. Using the developed model, we calculated the settling time of MP particles under year-specific conditions of seawater temperature, nutrient concentrations, and chlorophyll-a concentrations during summer and autumn from 1960 to 2017. The number of MP particles reaching the seafloor each year was determined based on the settling time of MP particles, plastic input into Beppu Bay, and the residence time within Beppu Bay.
The developed model successfully reproduced the interannual variations in MP settling flux in Beppu Bay reported by Hinata et al. (2023). Through sensitivity analysis using the numerical model, we confirmed that nutrient concentrations and the residence time of seawater in the bay contributed to the 20-year periodic variations. Furthermore, the Pacific Decadal Oscillation (PDO) may be involved in the changes in nutrient concentrations and residence time in Beppu Bay, suggesting that global-scale circulation influences the local-scale settling of MP particles.