Vertical distribution of mercury species at two sites in the Western Black Sea

The Black Sea is a unique natural laboratory for studying the biogeochemical cycling of mercury (Hg), a toxic metal present at trace levels in natural waters. The strong vertical density gradient in the Black Sea results in little ventilation of deep waters and therefore anoxic and sulfidic conditions at depth. In this way, the water column of the Black Sea resembles the geochemical transition that is observed in coastal sediments. This is of relevance to Hg studies, as coastal sediments are known to be locations of vigorous Hg methylation. Furthermore, the Black Sea supports a rich commercial fishery that could be at particular risk for accumulation of Hg as a result of its unique hydrography and anthropogenic inputs. The concentration of monomethylHg (CH3Hg+) was highest at the top of the suboxic zone in the Black Sea, where dissolved oxygen and sulfide were very low. This is circumstantially consistent with the hypothesis that it is the chemical form of Hg which determines bioavailability and therefore methylation rate and not the activity of sulfate reducing bacteria. Hg concentrations below the suboxic zone increased in tandem with increasing sulfide, and appear to be approximately consistent with cinnabar solubility control. These concentrations (ca. 12 pM) are among the highest reported for an open ocean environment. A preliminary mass balance for Hg in the Black Sea, including source terms such as precipitation, river and Mediterranean water inflow as well as the evasional sink and Bosphorus outflow indicates that a large additional input term is required that is greater than 68 kmol year− 1. Potential candidates include tectonic-related sources and river-derived pollution. Scaling the total inputs to the Black Sea by area of the basin indicates that this system is comparably impacted by Hg as an urbanized embayment such as Long Island Sound or Chesapeake Bay.