Spatiotemporal microplastic occurrence study of Setiu Wetland, South China Sea

This study reports the distribution of microplastics (MPs) in surface water and estuarine sediments in South and North Setiu Wetland in the South China Sea. Sampling was conducted bimonthly for one year from November 2016 to November 2017, including the northeast and southwest monsoons. Water surface and sediment samples were collected from six different sampling stations (STs). Samples were sorted based on physical analysis (optical observation) and selected particles were further analyzed by chemical characterizations. The findings of this study indicate that a total of 0.36 items/L and 5.97 items/g particles of MPs were found from characterizations surface water and dry sediment, respectively. Among the selected stations included in this research, ST3 (1.375 ± 0.347 items/L) and ST2 (14.250 ± 4.343 items/g) were individually identified as high potential MP sinking areas, exacerbated during the northeast and southwest monsoons. Transparent, film, and filament MP types were consistently found across all stations. Microplastic filaments revealed a functional group of polypropylenes based on the main peak spectrum at 2893-2955 cm^-1 (CH alkyl stretching), 1458 cm^-1 (CH₂ bending), and 1381 cm^-1 (CH₃ bending). Microplastic materials were thermally decomposed by pyrolysis-gas chromatography-mass spectrometry (Pyr-GC/MS) and identified as cyclohexane and cyclohexene derivatives, as well as precursors of polymer blends. The distribution of MPs in both matrices varied according to different seasons. These findings provide useful baseline information on the distribution of MPs from the estuarine area in Malaysia and South China Sea waters.