Constructed wetlands (CWs), widely utilized for watershed water treatment, have the potential to intensify the production of methylmercury (MeHg) with high risks to wildlife and humans. However, MeHg production and its controlling factors in such artificial systems remain poorly understood, particularly the impacts of human interventions like subsurface flow CW (SSFCWs) and weirs. In this study, the hotspots and controlling mechanisms of MeHg production in a typical hybrid CW consisting of SSFCW and free water surface CW (FWSCW) were investigated. Despite 65.9% total mercury (THg) removal mainly via sedimentation, the CW increased MeHg by 209%, becoming a net MeHg source. The SSFCW was the hotspot of MeHg production, where anaerobic and eutrophic conditions could enhance Hg-methylator enrichment and methylation. Consequently, a high concentration of MeHg in water at 1.29 ng/L was exported to the subsequent FWSCW. Surprisingly, drop weirs increased MeHg levels by 1.6-2.1-fold under oxygenated conditions. The negative correlation between acid-volatile sulfide and sedimentary bioavailable Hg (p < 0.05), coupled with increased Hg-methylating microorganisms abundance, indicated that drop weirs enhance Hg bioavailability and methylation potential. These findings demonstrate that artificial configurations (e.g., drop weirs) can enhance net MeHg production and downstream release in hybrid CWs, posing ecological threats to receiving waters.
