Methylmercury in the East China Sea: Distribution, Mass Budget, and Its Impact on the Northwestern Pacific Ocean

Methylmercury (MeHg) in coastal systems has drawn extensive concerns because of its biomagnification through food chain and great threats to human health. Although terrestrial regions bordering the East China Sea (ECS) rank among the “hot spots” for Hg emissions in the world, there is still a lack of knowledge on the dominant sinks/sources, and controlling factors for MeHg in this system, restricting the understanding of its long‐term risks. In this study, MeHg and relevant parameters in various media (i.e., seawater, sediment, overlying water above ∼10 cm of surface sediment, and porewater) of the ECS were measured, while MeHg mass budget in the ECS was established by calculating the in situ MeHg degradation/production and external MeHg output/input fluxes. The results showed that MeHg contents in ECS seawater and sediment were relatively higher compared with other coastal systems. The mass budget estimation indicated that the degradation in the seawater and net production in the sediment are the dominant MeHg sink and source in the ECS, a coastal region significantly influenced by a large river (the Yangtze River). Additionally, approximately 10 ± 7 tons of MeHg were exported from the ECS to the Northwestern Pacific Ocean (NWPO) per year, accounting for ∼10% of the MeHg inventory in upper 500 m seawater in the NWPO. This study highlights that in situ processes dominate MeHg cycling in coastal seas even for those significantly affected by large rivers, emphasizing the need for long‐term monitoring of Hg methylation/demethylation to assess Minamata Convention's effectiveness.