In this post, I wish to highlight some key findings and points made in Pattiaratchi et al.’s (2022) article entitled ‘Plastics in the Indian Ocean – sources, transport, distribution, and impacts’, which focuses on the flows of buoyant plastic.

The context of this article is that marine plastic pollution is currently not well understood, with regards to the environmental impact and what actually happens to plastic debris in the ocean.  Out of all the plastic entering the ocean annually, only an estimated 1% floats on the ocean surface, such as in the gargantuan subtropical garbage patches worldwide (Figure 1). As such, there is a “fundamental gap in the understanding of the fate of ocean plastic and suggests that there are unknown sinks of marine plastic debris” (p. 2). These other sinks may include biological sinks (plastic ingested by marine biota), sea ice cover, coastlines where ocean plastic may make landfall, or the bottom of the ocean where plastics may settle.

Figure 1: Map of the world’s garbage patches, formed by gyres i.e. ocean currents. Accumulated marine debris are transported together in these large patches via the currents. (Source: Filho, Hunt & Kovaleva, 2021)

Pattiaratchi et al. emphasise how the Indian Ocean is subject to plastic pollution from 5 of 10 of the world’s top ocean plastic polluters (Thailand, Malaysia, Indonesia, India and Bangladesh), and 2 large and heavily polluted rivers (the Ganges and Indus Rivers). Pattiaratchi et al. found that it is mainly rivers that transport plastic into the ocean. Consequently, the Indian Ocean is heavily polluted with global plastic waste and is a means by which the buoyant plastics becomes spatially distributed. Figure 2 shows this spatiality:

Figure 2: “Schematic showing major plastic sources (green arrows), plastic transport pathways (blue arrows), and major beaching locations of plastics (red regions) in the Indian Ocean”. (Source: Pattiaratchi et al., 2022, p. 20)

The paper also explains that ocean surface currents and wind drive the accumulation and flows of buoyant marine debris. The geography of the Indian Ocean is such that a garbage patch accumulates in only the southern Indian Ocean (indicated by the oval in Figure 2), albeit not a well-defined one. This is because the northern Indian Ocean is bounded by the Indian and African landmasses and blocked from connections with the other ocean basins. On the other hand, the southern Indian Ocean can connect to the Pacific Ocean and the South Atlantic Ocean, and has a wind-driven subtropical gyre, thereby facilitating debris exchanges between the basins.

That said, buoyant plastics in the northern Indian Ocean are transported via the monsoon winds which are strong there. The reversal of monsoon winds each season, along with various other ocean currents, contribute to the movement of marine debris between the Arabian Sea and the Bay of Bengal (explained in greater detail in the paper). Importantly, these seasonal dynamics mean that buoyant plastics can travel between the northern and southern parts of the Indian Ocean, particularly during the Northeast monsoon season and September-October inter-monsoon period. Drifting plastics are not confined to the hemispheres of their sources.

Pattiaratchi et al. illustrate their points about the transport, distribution and harmful impact of ocean plastic through the 2021 X-Press Pearl nurdle spill in Sri Lanka, which I covered in a previous post. They explain how as the monsoon currents reverse, the countless nurdles coating Sri Lankan beaches will make landfall on the coasts of countries bordering the northern Indian Ocean. This extends the harm done to ecosystems and brings chaos to coastal communities reliant on fishing and tourism in countries like India, Maldives and Somalia i.e. the red regions in Figure 2.

Nurdle spill from the X-Press Pearl (Source: Pattiaratchi et al., 2022)

 

As more and more focus is put on plastic pollution, research like that of this paper is essential to gain a better understanding of the consequences and to address the current scarcity of data on the fate of plastics in the ocean.

 

Reference:

Filho, W.L., Hunt, J., & Kovaleva, M. (2021). Garbage Patches and Their Environmental Implications in a Plastisphere. Journal of Marine Science and Engineering, 9(11), 1289. MDPI AG. Retrieved from http://dx.doi.org/10.3390/jmse9111289

Pattiaratchi, C., Mirjam van der Mheen, Schlundt, C., Narayanaswamy, B. E., Sura, A., Hajbane, S., . . . Wijeratne, S. (2022). Plastics in the Indian Ocean – sources, transport, distribution, and impacts. Ocean Science, 18(1), 1-28. doi:http://dx.doi.org.libproxy1.nus.edu.sg/10.5194/os-18-1-2022