Transformations of mercury in the marine water column

Abstract

Methylation of mercury (Hg) in the marine water column has been hypothesized to serve as the primary source of the bioaccumulating chemical species monomethylmercury (MMHg) to marine food webs. Despite decades of research describing mercury methylation in anoxic sediments by anaerobic bacteria, mechanistic studies of water column methylation are severely limited. These essential studies have faced analytical challenges associated with quantifying femtomolar concentrations of the methylated Hg species dimethylmercury (DMHg) and MMHg in marine systems. In addition, the complex biogeochemical cycling of Hg in natural systems require consideration of gaseous, dissolved, and particulate species of Hg in order to probe potential controls on its ultimate transfer into marine food webs. The presented work provides a comprehensive study of Hg chemical speciation and transformations in Tropical Pacific waters. We developed an analytical method for MMHg determination from seawater that has the potential to ease measurements of MMHg distributions, as well as mechanistic studies of Hg species transformations. We used this method, in addition to previously established methods, to measure dissolved and particulate Hg species distributions and fluxes along a transect of the Pacific Ocean. Over significant gradients in oxygen utilization and primary productivity, we observed a region of methylated Hg species focused in the Equatorial Pacific that appeared spatially separated from higher concentrations in North Pacific Intermediate Waters. From the first full water column depth profiles of this region, we also observed the intrusion of elevated Hg into deep waters of the Equatorial and South Pacific Ocean. In addition we observed substantial potential rates of mercury methylation in subsurface and low oxygen waters along the Pacific transect as well as the Sargasso Sea using Hg isotope tracers. We observed dynamic production and decomposition of methylated Hg in low productivity waters, despite low ambient methylated Hg concentrations. From the addition of bulk organic matter as well as individual compounds important for methylation in anaerobic bacteria, we observe no simple limitation of Hg methylation in marine waters but highly dynamic conversion of Hg between methylated and inorganic species.