New insights into sink-source dynamics of mangrove for microplastics: Quantitative evidences from field observation and resuspension simulation

Intertidal mangroves are major sinks for microplastics (MPs); however, the dynamics involved in their conversion to contamination sources remain unclear. This work explored the distribution patterns of MPs in an intertidal zone, estimated the retention rates of floating MPs in a mangrove ecosystem during spring tides, and quantified the resuspension features of sediment MPs based on field observations and simulation experiments. The mean abundance of sediment MPs (1202-12,131 n kg) tended to increase substantially from mudflats to the mangrove forest. Storm surge was identified the main driver of vertical MP displacement in sediments, whereas the compositional features of MPs (shape, color, and polymer type) were mainly determined by interception and radiation shielding effects of mangroves. During the spring tides, ∼40 % of floating MPs were intercepted by Acanthus ilicifolius under a normal hydrological regime, whereas typhoon-induced hydraulic disturbances resuspended MPs from sediments, leading to negative retention rates (< -44 %) by A. ilicifolius and Sonneratia apetala. Mangrove sediments displayed a greater resistance to MP resuspension, requiring higher critical shear stress (∼0.024 N m) for their release. At increasing shear stress levels (0.036-0.11 N m), more medium-sized (5002000 μm), fibrous, low-density MPs tended to be resuspended, allowing the mangrove forest to be converted into a secondary contamination source. This work provides quantitative evidence of the sink-source dynamics of MPs in mangroves, being expected to enhance a comprehensive understanding for environmental behaviors of MPs in intertidal ecosystems.