The impact of hypoxia on mercury methylation in bottom sediment of northern Gulf of Mexico

Abstract

Widespread concern has developed about high mercury content in fish in the Gulf of Mexico and adjacent estuaries and bays. Among the areas implicated as possible sources of the mercury that moves up the food chain from the methylmercury formed in sediments and anoxic waters is the seasonal hypoxic zone in the northern Gulf. This research was designed to determine if methylmercury formation is stimulated by the anaerobic sediment conditions accompanying the onset of summer hypoxia in the Gulf. Both field and laboratory studies were carried out. For the field study sediment samples were collected at three stations (i.e. C4, C6B and C8) along hypoxic transect C established by Nancy Rabalais at a monthly interval from April 05 to April 06 for total and methyl mercury analyses. In-situ concentrations of dissolved oxygen, temperature and salinity of bottom waters were measured using a hydro-lab. For the laboratory study the effects of anaerobic conditions and organic matter contents on methylation rate were investigated using sediment-water columns. The most probable number (MPN) of the sulfate reducing bacteria in sediments was enumerated to obtain insights into the microbial mechanism of mercury methylation. Seasonal variations in methylmercury concentrations were observed at three stations with the peak in spring 2005. This seasonal pattern might be attributed to the seasonal inputs of freshwater from the Mississippi river into the gulf. Annual average concentrations of methyl mercury were 0.31, 0.47 and 0.12 ug/kg at C4, C6B and C8, respectively. Annual averages of total mercury concentrations were 37.79, 41.06 and 11.02 ug/kg for C4, C6B and C8, respectively. Sediment texture may explain the spatial variations of methyl and total mercury among the stations. Statistical analyses indicate that the dissolved oxygen (P<0.0001), temperature (P=0.03) and sediment texture (P=0.001) significantly affected the methyl mercury concentration. The laboratory study indicates that organic matter was an important factor in controlling the rate of mercury methylation.