Dispersion of Marine Plastic Debris in Southeast Asia: Contributions of Indonesian Riverine Inputs and Particle Buoyancy

The complex ocean circulation in Southeast Asia plays a significant role in the dispersion of marine plastic pollutants. Plastic pollution in South East Asian seas is critical environmental issues due to high coastal population and waste mismanagement, which contribute to degradation of marine ecosystem. A three-dimensional Lagrangian particle tracking model is used to analyze the advection mechanisms driving the dispersion and transfer of pollutants from major Indonesian river mouths into Indonesian Seas and broader Southeast Asian waters. To simulate the fate of marine pollution, particles with varying buoyancies were continuously released over four years with origins from 31 major rivers across Southeast Asia. The particle Lagrangian trajectories are calculated based on dynamic simulations at high horizontal resolution of 5 km currents from the SYMPHONIE model, integrating atmospheric, tidal and river forcing to represent coastal circulation dynamics accurately.

These Lagrangian simulations reveal that 5% of particles entered Indonesian waters, 22% exited from the open boundaries, 36.5% remained within, and 36.6% stayed outside Indonesia throughout the simulation period. This distribution indicates significant sub-basin connectivity of marine plastic debris, with more than one-fifth of the particles leaving Indonesian territorial waters, the impact of external oceanic forces on regional pollution dispersion. The vertical distribution of particles shows that 78.7% are concentrated in the upper 10 meters of the water column, primarily influenced by surface currents and wind-driven forces, enhancing their potential for coastal accumulation.

Further analysis demonstrates that particles with higher negative buoyancy tend to occupy shallower depths, while neutrally buoyant particles are more likely to sink to deeper layers. This vertical stratification highlights the critical role of buoyancy in determining the transport pathways and residence times of pollutants across different oceanic layers. These findings suggest that targeted pollution control efforts should focus on surface and deeper waters, adapting mitigation strategies to address the specific behavior of round particles based on their buoyancy characteristics and sizes. By advancing our understanding of pollutant dispersion and cross-boundary movement, this study provides essential insights for regional pollution management and collaborative efforts to mitigate marine pollution in Southeast Asia.

Keywords: plastic debris, particle dispersion, river, buoyancy effect, basin connectivity,  Indonesian Seas, Southeast Asia.