Mercury (Hg) is a persistent pollutant, and its methylated form, methylmercury (MeHg), can bioaccumulate and biomagnify in aquatic food webs, posing significant risks to wildlife and human health. Coastal wetlands, which constitute approximately 27% of global wetland areas, are crucial for various ecological functions. However, the reducing conditions in coastal wetland sediments can convert inorganic Hg into highly toxic MeHg. Understanding the dual roles of coastal wetlands in ecological functions and MeHg production is essential for effective management. In our study, sediment cores were collected from Mai Po Nature Reserve in Hong Kong, China, along a transect from mangrove to pond. The cores were analyzed for total mercury (THg), MeHg, loss-on-ignition (LOI) as a proxy for organic matter (OM) content, and carbon to nitrogen (C/N) ratio. We found that THg (90.3-225.8 ng/g) showed a slightly negative relationship with MeHg, suggesting potential local point-source pollution. Elevated MeHg levels were found in surface sediments near mangrove trees, where lower C/N ratios indicated that mangrove trees provided a labile carbon source for methylating microbes. The percentage of THg as MeHg (%MeHg), a proxy for Hg methylation potential, was positively correlated with LOI (p<0.0001), indicating that sedimentary labile organic matter is a key factor in Hg methylation in mangrove sediments. We expanded the study to different vegetation covers in coastal wetlands, including mangroves and reeds, using emerging tools such as stable Hg isotopes and analyses of carbon, nitrogen, and phosphorus. Preliminary results from Mai Po showed the highest %MeHg in sediments within two estuarine ponds dominated by reeds (%MeHg: 1.09% and 0.86%). These ponds had the lowest dissolved oxygen levels (3.46 mg/L), likely due to reduced water aeration by reed stands, creating anaerobic conditions favorable for Hg methylation. MeHg levels were also relatively higher in ponds with buffalo activities, potentially due to the provision of digested OM for microbial methylators. Our study aims to identify the physicochemical conditions controlling inorganic Hg methylation under different vegetation covers. The results will elucidate the mechanisms of key biogeochemical processes and provide insights into the storage and transfer of THg and MeHg in wetlands. This study challenges the traditional view that coastal wetlands are solely beneficial for ecological functions and provides a scientific basis for managing wetlands to reduce Hg exposure to wildlife and humans.

methylmercury, mercury, coastal wetland, bioaccumulation, wildlife and human health risks