Does anoxia affect mercury cycling at the sediment-water interface in the Gulf of Trieste (northern Adriatic Sea)? Incubation experiments using benthic flux chambers

Coastal areas in the northernmost part of the Adriatic Sea (Gulf of Trieste and the adjacent Grado Lagoon) are characterized by high levels of mercury (Hg), both in sediments and in the water column, mainly originating from the suspended material inflowing through the Isonzo/Soča River system, draining the Idrija (NW Slovenia) mining district, into the Gulf of Trieste. Hypoxic and anoxic conditions at the sediment-water interface (SWI) are frequently observed in the Gulf of Trieste and in the lagoon, due to strong late summer water stratification and high organic matter input. Hg mobility at the SWI was investigated at three sampling points located in the Gulf of Trieste (AA1, CZ) and in the Grado Lagoon (BAR). Experiments were conducted under laboratory conditions at in situ temperature, using a dark flux chamber simulating an oxic-anoxic transition. Temporal variations of dissolved Hg and methylmercury (MeHg) as well as O2, NH4+, NO3-+ NO2-, PO43-, H2S, dissolved Fe and Mn, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) were monitored simultaneously. Benthic Hg fluxes were higher under anoxic conditions than in the oxic phase of the experiment. MeHg release was less noticeable (low or absent) in the oxic phase, probably due to similar methylation and demethylation rates, but high in the anoxic phase of the experiment. The MeHg flux was linked to sulphate reduction and dissolution of Fe (and Mn) oxyhydroxides, and formation of sulphides. Reoxygenation was studied at sampling point CZ, where concentrations of MeHg and Hg dropped rapidly probably due to re-adsorption onto Fe (Mn) oxyhydroxides and enhanced demethylation. Sediments, especially during anoxic events, should be, hence, considered as a primary source of MeHg for the water column in the northern Adriatic coastal areas.