Because mercury (Hg) undergoes significant biogeochemical processes along the estuarine-coastal continuum, the objective of this work was to investigate the distribution and reactivity of methylmercury (MeHg), inorganic mercury (Hg(II)) and gaseous Hg (DGM) in plume waters of the Adour River estuary (Bay of Biscay). Vertical profiles, spatial and tidal variability of Hg species concentrations were evaluated during two campaigns (April 2007 and May 2010) characterized by significant plume extents over the coastal zone. Incubations with isotopically enriched tracers were performed on bulk and filtered waters under sunlight or dark conditions to investigate processes involved in Hg methylation, demethylation and reduction rates. Total Hg(II) concentrations were more dispersed in April 2007 (5.2±4.9 pM) than in May 2010 (2.5±1.1 pM) while total MeHg concentrations were similar for both seasons and averaged 0.13±0.07 and 0.18±0.11 pM, respectively. DGM concentrations were also similar between the two campaigns, averaging 0.26±0.10 and 0.20±0.09 pM, respectively. Methylation yields remained low within the estuarine plume (<0.01-0.4% day(-1)) while MeHg was efficiently demethylated via both biotic and abiotic pathways (2.3-55.3% day(-1)), mainly photo-induced. Hg reduction was also effective in these waters (0.3-43.5% day(-1)) and was occurring in both light and dark conditions. The results suggest that the plume is overall a sink for MeHg with integrated net demethylation rates, ranging from 2.0-3.7g (Hg) d(-1), in the same range than the estimated MeHg inputs from the estuary (respectively, 0.9 and 3.5g (Hg) d(-1)). The large evasion of DGM from the plume waters to the atmosphere (8.8-26.9g (Hg) d(-1)) may also limit HgT inputs to coastal waters (33-69g (Hg) d(-1)). These processes are thus considered to be most significant in controlling the fate of Hg transferred from the river to the coastal zone.