Spatiotemporal abundances and potential risks of microplastics in surface sediments of the inner Gulf of Thailand

Marine sediments are one of the final sinks for microplastics. However, research on microplastic pollution in the inner Gulf of Thailand (GoT) remains limited despite intense anthropogenic pressures in this region. The spatiotemporal distribution, sources, and ecological risks of microplastics in the inner GoT sediments were investigated. The abundance of microplastics ranged in 40-3180 items/kg dry weight, with significantly higher abundances during the dry season than the rainy season (p < 0.01). The majority of the microplastics were categorized as small-sized range of 0.03-0.3 mm (71.54%), with fibers (49.69%) and fragments (49.01%) being the dominant shapes. Polyethylene terephthalate (50.47%), polypropylene (18.09%), and polyethylene (14.94%) were the primary polymers identified. These findings suggest that secondary microplastics could represent a major source, such as cloth materials, fishing gear, and degraded plastic packaging. The prevalence of small-sized microplastics (< 0.3 mm) suggests significant degradation of microplastics. We hypothesize that this is driven by accelerated degradation from high temperatures and intense UV radiation in tropical environments, such as the inner GoT. Although the pollution load index (PLi) of the microplastics indicated a low pollution load (category II), their potential ecological risk index (PERi) reached a serious potential risk (category V), particularly in the Chao Phraya estuary. This discrepancy underscores that PERi is governed by polymer-specific hazard levels rather than microplastic abundance. This research enhances the understanding of microplastic fate and ecological risk in marine environments, offering a scientific foundation for evidence-based policies in the inner GoT.