The production and distribution of elemental mercury in a coastal marine environment

The cycling of elemental mercury (Hg$\sp{\rm o}$) in natural waters is an extremely important facet of the global Hg biogeochemical cycle, due to its effects on the aqueous distribution, reactivity, and remobilization of the metal. While oceanic evasion is estimated to account for $\sim$30% of total atmospheric emission globally, little is known about the mechanisms of Hg$\sp{\rm o}$ production or its distribution in marine systems. The spatial/temporal distribution of Hg$\sp{\rm o}$ in seawater was examined for both coastal (Long Island Sound (LIS)) and open ocean (North, South, and Equatorial Atlantic Ocean) environments, and was coupled with laboratory investigations to determine its dependencies on environmental parameters.^ Experiments were conducted in distilled, deionized water in order to characterize Hg$\sp{\rm o}$ dynamics in a defined, ligand-free medium, and to establish baseline behavior relative to the complexities of seawater. Three or more mechanisms are proposed to be involved in the Hg-reduction process, including (1) the initial activity of a consumable pool of reducing agents, (2) complexation of Hg$\sp{+2}$, creating less reactive species, and (3) real-time, in situ production by thermal, biological or photochemical processes. Experiments suggest that the corresponding rate law for Hg$\sp{\rm o}$ production in DDI and seawater is second order, and first order with respect to (Hg$\sp{+2}$) and (Reducing agents). Incubation experiments show that similar Hg$\sp{\rm o}$ production mechanisms and dependencies exist between the coastal and open ocean, with a unique combination of chemical (organic material, ionic strength), physical (photochemistry, gas exchange), and biological (bacterial metabolism) factors ultimately determining its rate and distribution. Estimates of evasional fluxes from the Atlantic Ocean are comparable to those of LIS and the Equatorial Pacific, and indicate a distinct relationship to biological productivity.^ The significance of Hg$\sp{\rm o}$ production in LIS was confirmed, as evasion was estimated at $\sim$40% of total annual Hg inputs. Elevated Hg$\sp{\rm o}$ saturation and reactive Hg concentrations near the Connecticut River Estuary indicate that rivers are important sources of Hg substrate to coastal waters, and create regions of enhanced Hg$\sp{\rm o}$ production. These results suggest that Hg$\sp{\rm o}$ production and evasion are important components of the Hg cycle in other coastal regions throughout the world (e.g., urban centers and estuaries).