Mercury methylation in the sediments of a macrotidal estuary (Gironde Estuary, south-west France)

Diagenetic behaviour of mercury species in surface sediments of the highly turbid Gironde Estuary was investigated along 6 vertical profiles, including the water–sediment interface (SWI) in dredged and non-dredged zones. Sediment and pore water were collected for analyses of major redox parameters (e.g. dissolved Fe, Mn and sulphate), dissolved and particulate mono-methyl mercury (MeHg) and divalent mercury (Hg(II)). Concentrations of the dissolved Hg species were clearly (up to 70 times) higher in pore water than in the overlying water column, probably due to the dissolution or degradation of major Hg carrier phases such as Fe oxyhydroxides and particulate organic matter. The different profiles showed maximum MeHgD concentrations (up to 10 pmol L−1) and MeHgD/HgD ratios (up to 40%) at a few cm below the SWI, suggesting intense in situ methylation processes for this zone. Depth-related methylation potential was investigated by spiking the cores with 199Hg(II) and their subsequent incubation. Methylation of isotopically labelled 199Hg(II) was measurable in all cores, suggesting that methylation in the studied sediments was efficient in the uppermost ∼20 cm below the SWI. The comparison between sediments retrieved inside and outside the navigation channel showed higher methylation potential in dredged sediments and in intertidal sediment, suggesting stimulation of the bacterial activity either by inputs (e.g. fresh organic matter, sulphate) due to pore water renewal or by redox oscillation.The estimated MeHgD sediment-to-water flux was greater in non-dredged sediments, probably due to favourable anoxic conditions at the SWI. At the whole estuary scale, the estimated direct sediment to water transfer via passive diffusion was 1.1–2.7 mol yr−1 (210–55 g yr−1) for MeHgD and 3.8–4.0 mol yr−1 (750–790 g yr−1) for Hg(II)D, which is roughly equivalent to 15–50% and <3–12% of the respective annual fluvial gross inputs. The estimated dredging-related transfer of pore water Hg species to the water column appeared negligible (∼0.02 mol yr−1 (3.1 g yr−1) for MeHgD and ∼0.2 mol yr−1 (41 g yr−1) for Hg(II)D). Further work should aim at understanding the impact of suspending dredged anoxic sediments in the water column on the estuarine Hg cycle.