Natural Iron Fertilization Moderates Hydrothermal Mercury Inputs from Arc Volcanoes

Methylmercury is a bioaccumulative neurotoxin that poses severe risks to marine ecosystems and human health worldwide. Hydrothermal systems and submarine volcanoes are natural sources of mercury, yet the magnitude of emissions, their transport, and their impact on marine ecosystems remain poorly understood. Quantifying natural mercury fluxes is essential to understanding anthropogenic perturbations and guiding effective reduction strategies. We investigate hydrothermal mercury inputs at the Tonga volcanic arc and their impact on the local ecosystem. Our results show that hydrothermal and volcanic activity in the Tonga Arc increases mercury concentrations in seawater. Comprehensive surveys identified mercury-rich plumes (up to 22.7 pmol L^–1) associated with high mercury fluxes (4763 pmol m^–2 day^–1) reaching productive surface waters, resulting in an estimated total flux of 4.23 t y^–1 for the entire Tonga Arc. Despite these significant inputs, mercury concentrations in phytoplankton remain unexpectedly low. We demonstrate that phytoplankton blooms, stimulated by natural iron fertilization from hydrothermal sources, dilute mercury at the cellular level, reducing the impact of hydrothermal mercury. Additionally, we provide a revised global estimate of hydrothermal mercury inputs with a maximum of 120 t y^–1, which is considerably lower than atmospheric and riverine inputs to the ocean.