Characteristics of dissolved organic matter (DOM) and relationship with dissolved mercury in Xiaoqing River-Laizhou Bay estuary, Bohai Sea, China

•In Xiaoqing River-Laizhou Bay estuary system, distinct DOM properties of two end-members were clearly identified.•Laizhou Bay estuary area was more inclined to be influenced by riverine rather than marine DOM sources.•Xiaoqing River-Laizhou Bay estuary system displayed no significant correlation between DOM and Hg.•Quasi-conservative behavior of DOM but non-conservative behavior of Hg might explain the decoupling relation of DOM vs. Hg.•Environmental role of DOM is hardly generalized due to influence of different biogeochemical processes on the given system.

Because of heterogeneous properties, dissolved organic matter (DOM) is known to control the environmental fate of a variety of organic pollutants and trace metals in aquatic systems. Here we report absorptive and fluorescence properties of DOM, in concurrence with concentrations of dissolved mercury (Hg), along the Xiaoqing River-Laizhou Bay estuary system located in the Bohai Sea of China. A mixing model consisting of the two end-members terrestrial and aquatic DOM demonstrated that terrestrial signatures decreased significantly from the river into the estuary. Quasi-conservative mixing behavior of DOM sources suggests that the variations in the average DOM composition were governed by physical processes (e.g., dilution) rather than by new production and/or degradation processes. In contrast to some previous studies of river-estuary systems, the Xiaoqing River-Laizhou Bay estuary system displayed a non-significant correlation between DOM and Hg quantities. Based on this and the variation of Hg concentration along the salinity gradient, we concluded that Hg showed a non-conservative mixing behavior of suggested end-member sources. Thus, rather than mixing, Hg concentration variations seemed to be controlled by biogeochemical processes.

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