Distribution, mass budgets and decoupled controls of total and dissolved mercury in the Yellow River Estuary, China

Estuaries are important interfaces for human mercury (Hg) exposure via marine fisheries, yet the contrasting controls on total Hg (THg) and more reactive dissolved Hg (DHg) remain poorly constrained. We investigated Hg distributions and controlling processes in the Yellow River Estuary (YRE), found that THg concentrations in the unfiltered seawater exhibited a clear seaward decrease, and mass-budget analysis showed that 88.5% of THg derived from riverine inputs, indicating strong riverine control. In contrast, DHg showed no spatial gradient and weak riverine influence, consistent with lower DHg (0.747 ± 0.456 ng L^-1) in the Yellow River water than the YRE levels (4.91 ± 3.63 ng L^-1). Instead, porewater Hg (45.9 ± 43.3 ng L^-1) was ∼5-fold higher than seawater DHg and positively correlated with overlying-water and seawater DHg. Moreover, porewater diffusion contributed 60% of DHg inputs and 54.4% of DHg inventory, demonstrating its critical role in sustaining DHg pools. A cross-estuarine synthesis also reveals a positive relationship between porewater Hg diffusion and seawater DHg. Collectively, THg is primarily governed by riverine loading, whereas DHg maybe largely sustained by internal recycling, implying that legacy sediments may continue to regulate DHg availability and ecological exposure despite declining external Hg inputs.