2:15 PM - 2:30 PM
[MIS20-03] Numerical experiment on vertical motion of microplastics with biological processes
Keywords:Microplastic, Numerical model, Aggregation
Plastic waste has flowed out into the world’s ocean through various routes, and gradually changes to microplastics with sizes are smaller than 5 mm. The relatively large plastics are degraded due to ultraviolet radiation and hydrolysis and are thereafter fragmentized by physical stimuli received from waves and sand on beach. Very small microplastics are easily intruded into various marine organisms, which may be damaged by these plastic debris. To investigate the influences of microplastics on the natural environment, field observations have been conducted worldwide. Although these observations adopted surface net towing to sample microplastics with a density lower than seawater, previous studies suggested that some process actually occurred to transport microplastics vertically downward from the sea surface to deeper layers. One of them is the inclusion into aggregation of phytoplankton, however, there have been not yet quantitative studies showing how the transport by aggregates affects the distribution of microplastics. Therefore, the purpose of this study was to verify the effect of biological processes on the vertical distribution of microplastics by reproducing the horizontal and vertical transport of microplastics with a numerical model with both physical and biological processes. Specifically, the model results were compared with the vertical distribution of microplastics reported by Song et al., (2018), where they collected very small microplastics > 20 micrometers from the sea surface to 1 m above the ocean floor in coastal waters.
Before modeling to reproduce both physical and biological processes, we started with a transport model based only on the physical processes: wind speed, significant wave height and buoyancy of microplastics were given to the model reproducing the oceanic turbulence. From this model, the physical process was sufficient to reproduce the size distribution of microplastics with a particle size range 0.2 to 5 mm at a depth of 5 m from the sea surface as that expected by a theoretical model given by Kukulka et al., (2012). However, particles < 100 μm disappeared from the water column in the observed results of Song et al., (2018), while the disappearance was not revealed in the numerical model which reproduce the vertical distribution in line with a physical mixing. Therefore, it was suggested that this inconsistency with the physical processes is caused by biological processes.
In the model considering both physical and biological processes, we added the aggregation of phytoplankton as a biological process to the above physical model. In this model, the particles representing phytoplankton move by oceanic turbulence, and aggregate to a single particle if they exist within the aggregation radius: aggregation occurs within this distance. Meanwhile, particles representing microplastics will be involved in the aggregated particle when they exist within the aggregation radius, and when the particle size is less than or equal to the aggregates. This model was able to reproduce the vertical distribution of Song et al., (2018), in which microplastics were transported to the seafloor together with heavy aggregates generated by biological processes, and their particle number reached the maximum in the subsurface layer. This indicates that if phytoplankton is present to some extent, the vertical distribution of microplastics will not show the maximum at sea surface, thus biological processes are more important than physical processes to determine the vertical distribution of microplastics. To support this, it was also suggested that these aggregates may have a sinking rate sufficiently large for the vertical velocity typically in the actual ocean. It is noted that the phytoplankton concentration given to this model occurs during spring or autumn bloom and red tide in the actual oceans.
Before modeling to reproduce both physical and biological processes, we started with a transport model based only on the physical processes: wind speed, significant wave height and buoyancy of microplastics were given to the model reproducing the oceanic turbulence. From this model, the physical process was sufficient to reproduce the size distribution of microplastics with a particle size range 0.2 to 5 mm at a depth of 5 m from the sea surface as that expected by a theoretical model given by Kukulka et al., (2012). However, particles < 100 μm disappeared from the water column in the observed results of Song et al., (2018), while the disappearance was not revealed in the numerical model which reproduce the vertical distribution in line with a physical mixing. Therefore, it was suggested that this inconsistency with the physical processes is caused by biological processes.
In the model considering both physical and biological processes, we added the aggregation of phytoplankton as a biological process to the above physical model. In this model, the particles representing phytoplankton move by oceanic turbulence, and aggregate to a single particle if they exist within the aggregation radius: aggregation occurs within this distance. Meanwhile, particles representing microplastics will be involved in the aggregated particle when they exist within the aggregation radius, and when the particle size is less than or equal to the aggregates. This model was able to reproduce the vertical distribution of Song et al., (2018), in which microplastics were transported to the seafloor together with heavy aggregates generated by biological processes, and their particle number reached the maximum in the subsurface layer. This indicates that if phytoplankton is present to some extent, the vertical distribution of microplastics will not show the maximum at sea surface, thus biological processes are more important than physical processes to determine the vertical distribution of microplastics. To support this, it was also suggested that these aggregates may have a sinking rate sufficiently large for the vertical velocity typically in the actual ocean. It is noted that the phytoplankton concentration given to this model occurs during spring or autumn bloom and red tide in the actual oceans.