Résumé
Methylation rates of mercury have been determined in both surface sediments and the water column of a shallow coastal lagoon (Thau, France) using in situ incubation experiments. The experiments were conducted in order to evidence and evaluate the significance of such pathways on the fate of methylmercury (MeHg) as influenced by both benthic and pelagic dynamics. Isotopically labelled Hg species have been used as chemical tracers allowing the direct determination of specific methylation and demethylation yields. Each experimental method (cores experiments and water experiments) has been carefully evaluated in terms of sensitivity and reproducibility of the transformation rates and has been demonstrated as a powerful method to investigate transformation processes. Although mercury methylation in surface sediments is a major process, significant MeHg formation in the water column has been measured for the first time in a coastal environment. In spring conditions, methylation yields are found to be higher in the water column (6.3%) than in sediments (0.8–1.3%). Area integrated rates for the experimental site demonstrate, however, that MeHg is mostly produced in surface sediments with a formation rate of 12 nmol m−2 d−1 compared to 1.8 nmol m−2 d−1 in the water column. Biological characteristics of the incubated samples indicate that sediment and plankton microorganisms are involved in Hg methylation which is thus directly associated to the pelagic–benthic turnover occurring in the lagoon.