11:45 〜 12:00
[MIS02-11] 3D Simulation of Microplastic Dynamics Incorporating Upward Terminal Velocities Corresponding to Hydrodynamic Spatial and Temporal Changes in Hiroshima Bay
キーワード：Microplastic, Upward Terminal Velocity, Spatial Distribution, Temporal Distribution, Diffusion Coefficient
Plastics that flow into the ocean are affected by ultraviolet radiation and abrasion and are decomposed into fragments less than 5 mm in size, called microplastics (MP) (Andrady, 2011). The behavior of MP in marine environments is intricately related to their physical properties, such as particle density, size, and shape. In this study, we focused on the upward terminal velocity (UTV), which is determined by the density and size of the MP.
In Hiroshima Bay, a large number of polystyrene foam MPs originating from buoys used in oyster farming rafts have washed up on the shore, and pollution has reached the seabed (Sagawa et al., 2018). Polystyrene foam has a low density due to the presence of air bubbles; however, when the size is reduced and the bubbles are crushed, its density increases above that of seawater. In other words, even if the same polymer has a different density depending on its size, the UTV will also be different.
The purpose of this study was to establish three types of UTVs and clarify the effect of UTVs on the dynamics of MPs. We also considered the role of the vertical diffusion coefficient in influencing the distribution of MP in the bay. Here, the vertical diffusion coefficient represents the extent to which a substance, such as MP, spreads vertically through the water column. The hydrodynamic model used in this study is based on the Princeton Ocean Model (POM; Blumberg and Mellor, 1983), employing a sigma-coordinate system.
Based on the results of the Hiroshima Prefecture coastal drift survey and Kataoka et al. (2019), two sources of MPs were set in this study: coast and land (rivers). Calculations were performed for a total of six cases, with two types of sources and three UTV (0.001; 0 ; -0.001 m/s).
The results showed that particles with positive UTVs tended to accumulate in the ocean's surface layer, increasing the stock of beached MPs. Particles with 0 UTV were mixed in the seawater, increasing the overall stock of MPs. These particles are well distributed from the beach to the bottom sediment. In contrast, MPs with negative UTVs settled to the bottom layer, increasing their concentration in the ocean.
The vertical diffusion coefficient plays an important role in determining the vertical distribution of MPs. In particular, the vertical diffusion coefficient is high in winter, and MPs are uniformly dispersed. In addition, the difference in vertical diffusion coefficient between spring tides and neap tides affected the vertical distribution of MPs. Spatially, the vertical diffusion coefficient is consistently higher in narrow straits, contributing to the unique pattern of MP distribution in Hiroshima Bay.
Furthermore, we examined how MP distribution varies between different hydrodynamic regions. In stratified areas, where vertical mixing is limited, MPs tend to accumulate near the surface, leading to higher beaching flux. In contrast, in areas with a deeper mixed layer, MPs are more evenly distributed throughout the water column, potentially reducing beaching and increasing offshore transport. These variations suggest that local hydrodynamic conditions play a crucial role in determining the fate of MPs in coastal environments.
In Hiroshima Bay, a large number of polystyrene foam MPs originating from buoys used in oyster farming rafts have washed up on the shore, and pollution has reached the seabed (Sagawa et al., 2018). Polystyrene foam has a low density due to the presence of air bubbles; however, when the size is reduced and the bubbles are crushed, its density increases above that of seawater. In other words, even if the same polymer has a different density depending on its size, the UTV will also be different.
The purpose of this study was to establish three types of UTVs and clarify the effect of UTVs on the dynamics of MPs. We also considered the role of the vertical diffusion coefficient in influencing the distribution of MP in the bay. Here, the vertical diffusion coefficient represents the extent to which a substance, such as MP, spreads vertically through the water column. The hydrodynamic model used in this study is based on the Princeton Ocean Model (POM; Blumberg and Mellor, 1983), employing a sigma-coordinate system.
Based on the results of the Hiroshima Prefecture coastal drift survey and Kataoka et al. (2019), two sources of MPs were set in this study: coast and land (rivers). Calculations were performed for a total of six cases, with two types of sources and three UTV (0.001; 0 ; -0.001 m/s).
The results showed that particles with positive UTVs tended to accumulate in the ocean's surface layer, increasing the stock of beached MPs. Particles with 0 UTV were mixed in the seawater, increasing the overall stock of MPs. These particles are well distributed from the beach to the bottom sediment. In contrast, MPs with negative UTVs settled to the bottom layer, increasing their concentration in the ocean.
The vertical diffusion coefficient plays an important role in determining the vertical distribution of MPs. In particular, the vertical diffusion coefficient is high in winter, and MPs are uniformly dispersed. In addition, the difference in vertical diffusion coefficient between spring tides and neap tides affected the vertical distribution of MPs. Spatially, the vertical diffusion coefficient is consistently higher in narrow straits, contributing to the unique pattern of MP distribution in Hiroshima Bay.
Furthermore, we examined how MP distribution varies between different hydrodynamic regions. In stratified areas, where vertical mixing is limited, MPs tend to accumulate near the surface, leading to higher beaching flux. In contrast, in areas with a deeper mixed layer, MPs are more evenly distributed throughout the water column, potentially reducing beaching and increasing offshore transport. These variations suggest that local hydrodynamic conditions play a crucial role in determining the fate of MPs in coastal environments.