Sources de mercure et transformations dans l'estuaire du Sinnamary, Guyane française

[Departement_IRSTEA]MA [TR1_IRSTEA]QSA / DYNAQInternational audienceThe distribution, partition and speciation of mercury (Hg) were studied along the redox gradient of an anthropically perturbed tropical estuary, the Sinnamary Estuary in French Guiana. This system is a partially mixed estuary characterized by an anoxic freshwater end-member, while the marine end-member consists of the Amazon Plume. The set up of an artificial oxygenation system in the anoxic freshwater end-member generates sharp gradients of major chemical species (iron, sulfides, etc.) coupled to intense organic matter (OM) turnover. The coexistence of oxygenated waters and dissolved sulfides in an organic rich environment depicts the USE (Upper Sinnamary Estuary: part of Sinnamary Estuary under the tidal influence but upstream the salt intrusion) as a potential site for Hg methylation. The concentrations of all mercurial species (HgT) in the unfiltered samples (HgTUNF), in the dissolved (HgTD) and particulate (HgTP) phases of the USE average 11 ± 3, 6 ± 2 and 5 ± 3 (i.e. 600 ± 200 pmol g-1) pmol L-1 respectively. Average concentrations of monomethylmercury (MMHg) in the unfiltered (MMHgUNF), dissolved (MMHgD) and particulate (MMHgP) phases were 3.8 ± 0.6, 2.0 ± 0.9 and 1.8 ± 1.2 (i.e. 220 ± 130 pmol g-1) pmol L-1 respectively. Water oxygenation and sulfides concentrations emerged to play a critical role in controlling MMHg levels. Besides, iron cycling, acid-base mechanisms, and redox-dependent processes were involved in the MMHg partitioning between phases. Overall, the USE constitutes a biogeochemical reactor that gathers partitioning and methylation processes. The permanent MMHg inputs from the anoxic freshwater end-member combined to the intense endogenous Hg methylation ensures high MMHg levels in both dissolved and particulate phases. To illustrate, the USE exports 60 % more MMHgUNF than it does import: 5.5 versus 3.5 kg y-1