Dynamics of mercury species in surface sediments of a macrotidal estuarine–coastal system (Adour River, Bay of Biscay)

Estuarine and coastal surface sediments from the Adour estuary and adjacent coastal area (South Bay of Biscay) have been investigated to establish mercury species variability in a macrotidal system submitted to contrasted seasonal river discharges and downstream urbanisation. Surface sediments were sampled during three campaigns in the major sediment deposits of the Adour estuary and at selected discharge points. Concentrations of Hg2+ and MeHg+ in Adour estuarine sediments average 1600 and 1.3 pmol g 1, and range from !2 to 7300 pmol g 1 and from !0.5 to 8.0 pmol g 1, respectively. The concentrations of inorganic ionic mercury (Hg2+) and monomethylmercury (MeHg+) in the surface sediments display large seasonal variation, being one order of magnitude higher in June 2001 for Hg2+ and in October 2000 for MeHg+. The results show that Hg2+ can be accumulated in the estuarine sediments after seasonal river inputs and both remobilised and methylated during dry period. Mercury species concentrations in coastal sediments collected in June 2001 assessed the impact of estuarine inputs on the nearby coastal area. Except direct anthropogenic discharge points, the variability and transfer of Hg2+ and MeHg+ in surface sediments of the whole estuarineecoastal macrotidal system can be depicted by simple geochemical parameters of the bulk sediments, such as grain size distribution, organic carbon and total sulphur.