[M-IS09] Ocean Plastics, an earth science perspective

[MIS09-02] The estimation of removal-from-surface timescale of floating plastics in the North Pacific

*Haodong Xu^1,2, Yoshimasa Matsumura², Hideyuki Nakano³, Kazutaka Takahashi¹, Kentaro Miyazono¹, Shin-ichi Ito² (1.GSALS, The University of Tokyo, 2.AORI, The University of Tokyo, 3.MRI, Japan Meteorological Agency)

Keywords:The North Pacific, Floating plastics, Removal from the surface, Particle tracking model

The ubiquitous existence of plastics/microplastics in the ocean has been a serious social and environmental issue. Numerous publications have revealed and discussed the distribution, concentration of microplastics since 2000s (Moore et al., 2001). And in recent years, it is realized that without the knowledge of the sources, pathways to ocean, transport mechanism between compartments, the understanding of fates, pollution-alleviation strategies are hard to reach (Rochman, 2018). This research focuses on one of the most important processes and its timescale, the removal from the surface, which can well explain the budget imbalance between the plastics input amount and surface plastics observed amount, aka. the ‘missing plastics‘ or the ‘plastics paradox’ (Eriksen et al., 2014). Biological factors e.g. bio-fouling and bio-aggregation, are considered as important components of this process. Previous studies have evaluated the potential timescale. Isobe et al., (2019) estimated that 3 years can highly reproduce the observation results based on model calculation while Isobe et al., (2022) claimed that multiple combinations of removal process and fragmentation process can result the same effects. In addition, Lobelle et al., (2021) concluded that removal timescale is size-dependence ranging from less than 1 day to over 90 days and mentioned both physical processes and biological process affect the removal timescale. As a consequence, the removal process is still not explicit and neither is its timescale. Hence, this research aims to understand the removal process in order to know the fate of marine floating plastics.

Long-term larvae investigation samples from Fisheries Research and Education Agency (FRA) along with a particle tracking model that simulates plastics release and transport for 65 years are used to calculate the removal timescale. The flow field data is the reanalysis data from Meteorological Research Institute (MRI). The particle tracking model calculates the cumulative chlorophyll-a concentration experienced by particles, and assumes that the particles would be removed from the surface when the concentration exceeds the thresholds set for comparison. Sea surface plastic loads of both FRA data and model were calculated using Gaussian interpolation. The root-mean-square error (RMSE) was following computed to determine the optimal threshold.

As a result, 500 mg/cm³·day, equivalent to 5.2 years (average surface chlorophyll-a concentration is 0.265 mg/m³), has smallest RMSE. This study estimated the removal timescale with integration of 65-year observation date, which can be taken as the actual situation in the sea area. It is also robust to demonstrate the effectiveness of using chlorophyll a as an indicator of biofouling.