Distribution of monomethylmercury and mercury in surface sediments of the Tagus Estuary (Portugal)

摘要

Mercury is a global pollutant that cycles between air, water, sediment, soil and organisms (Moreno et al., 2005). Although anthropogenic Hg emissions have been reduced by half in the last decades (Pacyna et al., 2001), ongoing contamination is still a worldwide problem. Depending on the Hg discharge and the ecosystem dynamics, Hg in sediments may be present in concentrated hotspots or dispersed over large areas (Hinton and Veiga, 2001; Moreno et al., 2005). The conversion of inorganic Hg into monomethylmercury (MMHg), a strong neurotoxin, is a critical step in its pathway and toxicity. The MMHg concentrations result from the balance between methylation and demethylation processes, which are not completely understood (Mason and Benoit, 2003). The most important factors influencing biological methylation are the availability of inorganic mercury and the nature of the microbial community (Mauro et al., 1999), although other influences include physical and chemical parameters such as temperature, pH, salinity, organic carbon and redox potential (Gil-mour and Henry, 1991; Barkay et al., 1997; Mason and Benoit, 2003). Despite the paucity of information concerning sources, in situ production, biogeochemistry and bioaccumulation of MMHg in marine organisms, sediments appear as potentially significant sources to food webs in the coastal zone (Covelli et al., 1999; Gill et al., 1999; Mason et al., 1999; Langer et al., 2001; Hammerschmidt et al., 2004) and possibly to the open ocean via hydrological or biological transport.