The dual role of coastal mangroves: Sinks and sources of microplastics in rapidly urbanizing areas

Mangrove ecosystems are vital for coastal protection, biodiversity, and pollution interception, yet their interactions with microplastics in rapidly urbanizing regions remain underexplored. This study investigated the microplastic dynamics in the Maozhou River and Dasha River, along with the coastal Xiwan Mangrove Park in the Pearl River Estuary, the second largest estuary in China. Samples were collected from mangrove and surrounding areas, identifying microplastics using Fourier-transform infrared spectroscopy (FTIR) and Laser Direct Infrared (LDIR) techniques. Microplastic concentrations ranged from 245.8 to 1562.4 n/m³ in water and 374.3 to 7475.3 n/kg in sediments. The Maozhou River exhibited consistent microplastic levels across varying hydrological conditions, while the Dasha River and Xiwan Mangrove showed greater sensitivity to water flow changes influenced by urban land use. During high-flow periods, urban river microplastic concentrations decreased due to dilution, whereas mangrove areas experienced elevated levels in water from urban runoff, upstream retention, and sediment resuspension, suggesting a potential for outward release. Weaker water dynamics led to increased microplastic accumulation in mangrove sediments. The distribution of microplastic types was influenced by multiple urban pollution sources, with synthetic rubbers linked to urban transportation comprising over 50 % of some samples, peaking at 79 %. These findings underscore the dual role of mangroves as microplastic sinks and potential sources, highlighting the significant impact of hydrological conditions on their function. This study offers new insights into microplastic pollution in urban mangrove ecosystems and emphasizes the urgent need for improved management strategies in coastal areas facing rapid urbanization.