Multidimensional Assessment of Microplastic Pollution in Mangrove Wetlands: Driving Mechanisms, Carbon Contribution, and Ecological Risk

Microplastics (MPs) have emerged as pervasive contaminants in mangrove ecosystems, yet their driving factors, carbon contributions, and ecological risks remain poorly understood. This study systematically investigated MP pollution across six functional areas in two representative mangroves of Hainan, China. MPs were ubiquitous in both water and sediments, with higher abundances in fishing and aquaculture zones than in tourist protection and residential zones. Sediments in the low-tide zone acted as major MP sinks compared to those in the high-tide zone. PP was the main polymer type, and fibers dominated in water and fragments in sediments. The studied mangroves exhibited a moderate level of MP pollution on a global scale. Partial least-squares path modeling (PLS-PM) identified vegetation density, nutrient enrichment, and heavy metals as key drivers. However, its effects vary between sediment and water environments. Integrated ecological risk indices indicated generally low-to-middle risk with localized pollution hotspots in estuarine and aquaculture areas. MP mass concentrations averaged 5.69 mg/m³ in water and 4.73 mg/kg dw in sediments, contributing 0.0050-0.0147% to sedimentary organic carbon as an emerging inert carbon pool. In addition, it is estimated that the study area receives an additional annual MP load of 3457.08 kg and an MP carbon load of 2795.83 kg. These findings provide a comprehensive understanding of MP behavior and environmental effects in mangrove wetlands, offering a scientific foundation for targeted mitigation and long-term monitoring of coastal plastic pollution.