Watershed runoff as a dominant pathway for mercury loading into a karst lake as unraveled by a novel mercury isotope and flux balance model.

Identification of sources, transport pathways, and transformation mechanisms of mercury (Hg) is fundamental to our understanding of the biogeochemical cycling of Hg in lake ecosystems. In this study, we conducted a comprehensive investigation on the isotopic compositions of Hg in water columns, sediments, atmospheric deposition endmembers, watershed river, and runoff in Hongfeng Lake (HFL) ecosystem in southwestern China. We observed significant difference in δ202Hg between dissolved Hg (DHg) and particulate Hg (PHg) in lake, river, and runoff samples, with lighter isotopes preferentially adsorbed onto particulate matter. ∆199Hg was also significantly higher for DHg than PHg in lake and river waters, primarily due to the higher susceptibility of DHg to photochemical reduction processes. ∆199Hg and ∆200Hg of Hg in sediments were similar to those of PHg in water column, but significantly different from those of DHg, reflecting the dominant role of sedimentation of PHg from water column in the Hg accumulation in sediments. The isotopic compositions of Hg in lake water and sediments lay between those of atmospheric deposition and watershed runoff end-members. Using a Hg isotope mass balance model, we estimated that runoff input accounted for 62.4 % of total Hg input into the HFL ecosystem, followed by riverine input (14.5 %), particulate-bound Hg (PBM) dry deposition (11.0 %), Hg(II) wet deposition (5.5 %), Hg(0) dry deposition (4.2 %), and gaseous oxidized Hg (GOM) dry deposition (2.4 %). These results highlight the key role of watershed runoff as a source of Hg in lake ecosystems and improve our understanding of the biogeochemical cycling of Hg such ecosystems.