Monomethylmercury concentrations on the eastern Texas-Louisiana shelf during the formation, peak, and disappearance of hypoxia

摘要

A study of monomethylmercury (MMHg) concentrations in the water and sediment ofthe hypoxic zone in the northeastern Gulf of Mexico was conducted on several cruisesbetween April 2004 and May 2005. Surface water MMHg concentrations were low andconstant throughout the sampling period. Bottom water concentrations displayed aseasonal trend: maximum MMHg concentrations were in June/July 2004, decreased to aminimum in October 2004, and in May 2005 concentrations had begun to increase.MMHg concentrations and MMHg as a percent of THg in surface sediment (0-2 cm)also followed this trend. Bottom water dissolved oxygen and temperature displayedinverse relationships with bottom water MMHg concentrations. This correlation betweendissolved oxygen and MMHg is typical for low-oxygen waters, but the relationshipbetween temperature and MMHg is relatively unique. A possible explanation is thatwarmer summer temperatures inhibited bacterial methylation. Stratification intensity(quantified as N2) was strongly correlated with bottom water MMHg concentrations,indicating either increased methylation at the pycnocline or that the pycnocline inhibitedvertical mixing, thus limiting MMHg to the bottom water. Benthic flux estimationsindicate that sediment release of MMHg could be a significant source of MMHg tobottom water. The presence of an oxygenated layer in the surface sediment could haveplayed a role in inhibiting MMHg flux during oxic conditions; a decrease in thethickness of this layer under hypoxic conditions likely allowed MMHg to diffuse into thebottom water. Dissolved oxygen seemed to play an important role in controllingsediment MMHg concentrations with highest methylation rates in sediment underhypoxic water. Overall, sites closest to the Mississippi River mouth displayed the highest MMHg concentrations. Further research will need to be done in this area to fullycharacterize the relationship between biogeochemical parameters and MMHgconcentrations.