Atmospherically deposited elemental mercury drives evasion of mercury from the ocean and freshwaters

The sources and mechanisms driving fluxes of dissolved gaseous mercury (DGM) in aquatic ecosystems represent a critical yet poorly constrained component of the global mercury (Hg) cycle. Current models assume that DGM is primarily formed through the reduction of Hg^II in water, largely supplied by atmospheric Hg^II deposition. Here we quantify the Δ²⁰⁰Hg signatures of DGM across marine (median: 0.02‰) and freshwater (0.02‰) ecosystems, intermediate between water dissolved Hg^II and atmospheric Hg⁰. This indicates that DGM in natural waters is not derived solely from Hg^II reduction as previous assumed but also from atmospheric input of Hg⁰ during air-sea gas exchange. A Δ²⁰⁰Hg-based mixing model reveals that ∼40% and 54% of DGM in seawater and freshwater, respectively, is derived directly from atmospheric Hg⁰ input. Combining these findings with an existing oceanic Hg budget, we show that re-emission of previously deposited atmospheric Hg⁰ accounts for ∼70% of gross oceanic Hg⁰ evasion. Consequently, we demonstrate that existing models have systematically underestimated gross atmospheric Hg⁰ deposition while overestimating net oceanic Hg⁰ emissions.