[M-IS19] Ocean Plastics, an earth science perspective

[MIS19-03] A Study of Vertical Convection Affecting the Accumulation of Marine Plastics at Sea

*Chiba Shuka^1,8, Yukihiro Takahashi^2,8, Nobuyasu Naruse^3,8, rimpei katada^4,8, Katsuhama Naoya^5,8, Ken Hirata^6,8, Fujita Shigeru^7,8 (1.Sapporo Nihon University High School, 2.Department of Cosmosciences, Graduate School of Science, Hokkaido University, 3.Faculty of Medicine, Shiga University of Medical Science, 4.School of Agriculture, Meiji University, 5.Graduate school of Agriculture, Forestry and Fisheries, Kagoshima University, 6.School of Science, Hokkaido University, 7.Department of Veterinary Medicine, College of Bioresource Sciences, Nihon University, 8.Super Scientist Program Plus)

Keywords:Marine Plastics, Velocity of Vertical Convection , Accumulation Mechanism

It is estimated that 8 million tons of plastic are released into the ocean annually, 98% of which are said to be missing. Those plastics can become entangled in living organisms, degrade due to ultraviolet light and cold temperatures, and be accidentally swallowed by living organisms, and can also cause environmental pollution due to the release of chemical substances. Although we are beginning to gather knowledge on the accumulation of marine plastics in areas close to land, such as coasts and inlets, which can be observed by drones and aircraft, research on the aggregation and movement mechanisms of plastics discharged into the ocean has only just begun. However, research on the disaggregation and transport mechanisms of plastics discharged into the sea has only just begun. There is insufficient knowledge to proceed with these studies and to formulate efficient recovery and spill prevention measures. In the previous works, related studies have included tracking marine debris using satellites and simulating the accumulation of marine plastics based on their trajectories and ocean currents. However, those plastics are more than tens of centimeters in size, and considering that the majority of plastics spilled at sea are on a micro scale, understanding the accumulation mechanism of small plastics is important for establishing an effective recovery method. In addition, theoretically, it is considered that trash agglomerates at the convergence of ocean currents and at the sea surface where vertical downward convection is occurring. However, there is no comprehensive knowledge of the vertical convection velocities required for the accumulation of plastic debris depending on the material, density, and size of the debris, and it would be significant to experimentally clarify the vertical convection velocities required for the accumulation under each of these conditions.

In this study, we aimed to experimentally clarify the flow velocity of vertical convection that affects the accumulation of marine plastics using actual plastics. We used cylindrical polyethylene beads with a specific gravity of 0.96 g/cm3 (W0.26 cm×H0.28 cm) as the plastic floating on the water surface to imitate marine microplastics. These beads were colored blue, and their movements were recorded by a video camera to show their accumulation on the water surface. Regardless of the number of submerged motors, the beads accumulated in the area where the vertical downward flow occurred, and the velocity of the vertical convection was about 0.39 km/h. Similar experiments were conducted on plastics of different materials, densities, and sizes, and knowledge was obtained about the vertical convection velocity required for accumulation.