11:00 AM - 11:15 AM
[MIS17-07] Numerical simulation of microplastic accumulation rates in Beppu Bay: Comparison among summers in 1993, 2004, and 2013
Keywords:microplastics, sinking process model, biofilm, residence time in the bay
Marine plastics have become a ubiquitous and indelible pollutant with a wide range of adverse environmental impacts depending on their sizes, polymer types, and categories (MacLeod et al., 2021). To assess the impacts of marine plastics on the marine environment, the recontraction of the long-term 3-D behaviors of plastics using numerical models is required. However, marine plastics budgets and their long-term trends have not yet been fully understood with a few exceptions in the upper layers of the North Atlantic and North Pacific because of a lack of long-term observation data.
Even microplastic particles (< 2 mm) of buoyant polyethylene and polypropylene have been found to sink into lower layers and accumulate in coastal to pelagic bottom sediments due to the buoyance reduction by biofouling, trapping with marine snow, and incorporation into sinking fecal pellets (van Sebille et al., 2020; Hinata et al., 2023). Sea bottoms with higher sediment grain deposition rates and limited biophysical perturbations could be a recording medium of the accumulation history of microplastics. Beppu Bay sediments meet these conditions (Kuwae et al., 2022; Hinata et al., 2023).
Hinata et al. (2023) revealed the sedimentation history of microplastic fragments in the deepest part of Beppu Bay (water depth: 73 m) from 1940 to 2015 based on multi-core analysis including 210Pb dating (Takahashi et al., 2017) and Chl-a sedimentation measurement (Tsugeki et al., 2017), and found a long-term linear increasing trend with an approximately 20-year variation with significant peaks around 1990 and 2014. The variation was highly and positively correlated with the Chl-a accumulation rate anomaly from its linear increasing trend.
Here we attempted to reproduce the accumulation rates of microplastics in the summers of 1993, 2004, and 2013 through numerical experiments using Princeton Ocean Model (POM). The model domain was the whole Seto Inland Sea with a horizontal grid size of 450 m and 10 sigma layers in the vertical. A biofilm (phytoplankton) formation model on the plastic surface (Kooi et al., 2017) and a fluid drag model by Dioguardi et al. (2018) were applied to calculate the sinking velocity of microplastic particles. A lower trophic level NPZD ecosystem model was used to calculate the concentration of Chl-a. The river mouth located in the bay was assumed to be the source of the microplastic particles with the inflow flux depending on the river discharge (Kudo et al., 2018). Wind waves, resuspension at the sea bottom, and the beaching-backwashing process were not considered in this preliminary study.
Numerical experiment shows the ratios between the number of released particles from the river mouth and that of accumulated on the bottom in the bay were 10% in 1993, 7% in 2004, and 13% in 2013, which have a close and positive correlation with measured accumulation rates from the multi-core analysis. Figure shows the microplastic particle distribution on the last day of July. Partilce color represents vertical location of microplastic particle. Effects of the residence time of microplastic particles and the Chl-a concentration in the bay on the ratio will be discussed in the presentation.
Even microplastic particles (< 2 mm) of buoyant polyethylene and polypropylene have been found to sink into lower layers and accumulate in coastal to pelagic bottom sediments due to the buoyance reduction by biofouling, trapping with marine snow, and incorporation into sinking fecal pellets (van Sebille et al., 2020; Hinata et al., 2023). Sea bottoms with higher sediment grain deposition rates and limited biophysical perturbations could be a recording medium of the accumulation history of microplastics. Beppu Bay sediments meet these conditions (Kuwae et al., 2022; Hinata et al., 2023).
Hinata et al. (2023) revealed the sedimentation history of microplastic fragments in the deepest part of Beppu Bay (water depth: 73 m) from 1940 to 2015 based on multi-core analysis including 210Pb dating (Takahashi et al., 2017) and Chl-a sedimentation measurement (Tsugeki et al., 2017), and found a long-term linear increasing trend with an approximately 20-year variation with significant peaks around 1990 and 2014. The variation was highly and positively correlated with the Chl-a accumulation rate anomaly from its linear increasing trend.
Here we attempted to reproduce the accumulation rates of microplastics in the summers of 1993, 2004, and 2013 through numerical experiments using Princeton Ocean Model (POM). The model domain was the whole Seto Inland Sea with a horizontal grid size of 450 m and 10 sigma layers in the vertical. A biofilm (phytoplankton) formation model on the plastic surface (Kooi et al., 2017) and a fluid drag model by Dioguardi et al. (2018) were applied to calculate the sinking velocity of microplastic particles. A lower trophic level NPZD ecosystem model was used to calculate the concentration of Chl-a. The river mouth located in the bay was assumed to be the source of the microplastic particles with the inflow flux depending on the river discharge (Kudo et al., 2018). Wind waves, resuspension at the sea bottom, and the beaching-backwashing process were not considered in this preliminary study.
Numerical experiment shows the ratios between the number of released particles from the river mouth and that of accumulated on the bottom in the bay were 10% in 1993, 7% in 2004, and 13% in 2013, which have a close and positive correlation with measured accumulation rates from the multi-core analysis. Figure shows the microplastic particle distribution on the last day of July. Partilce color represents vertical location of microplastic particle. Effects of the residence time of microplastic particles and the Chl-a concentration in the bay on the ratio will be discussed in the presentation.