Mercury (Hg) mobility at the sediment-water interface was investigated during a laboratory
incubation experiment on highly contaminated sediments (up to 23 μg g-1) of the Gulf of Trieste. Undisturbed
sediment was collected in front of the Isonzo River mouth, which inflows Hg-rich suspended material
originating from the Idrija (NW Slovenia) mining district. Since hypoxic and anoxic conditions at the bottom
are frequently observed, a redox oscillation was simulated in the laboratory at in situ temperature, using a
dark flux chamber. Temporal variations of several parameters were monitored simultaneously: dissolved Hg
and methylmercury (MeHg), O2, NH4
+, NO3
-+NO2
-, PO4
3-, H2S, dissolved Fe and Mn, dissolved inorganic and
organic carbon (DIC and DOC). Benthic fluxes of Hg and MeHg were higher under anoxic conditions while
re-oxygenation caused concentrations of MeHg and Hg to rapidly drop, probably due to re-adsorption onto
Fe/Mn oxyhydroxides and enhanced demethylation. Hence, during anoxic events, sediments of the Gulf of
Trieste may be considered as an important source of dissolved Hg species for the water column. However,
re-oxygenation of the bottom compartment mitigates Hg and MeHg release from the sediment, thus acting as
a natural “defence” from possible interaction between the metal and the aquatic organisms.
