Benthic fluxes of mercury species in a lagoon environment (Grado lagoon, Northern Adriatic Sea, Italy)

The role of the major biogeochemical processes in the mercury (Hg) cycling at the sediment-water interface was investigated in the Grado lagoon (northern Adriatic Sea). This wetland system has been extensively contaminated from the Idrija Hg Mine (Slovenia) through the Isonzo River suspended load carried by tidal fluxes. Three approaches were used to study the sediment-water exchange of total Hg (THg), methylmercury (MeHg), reactive Hg (RHg) and dissolved gaseous Hg (DGHg): (1) estimation of diffusive fluxes from porewater and overlying water concentrations, (2) measurements of benthic fluxes using a deployed light benthic chamber in situ and (3) measurements of benthic fluxes during oxic-anoxic transition during a laboratory incubation experiment. The THg solid-phase, ranging between 9.5 and 14.4 µg g-1, showed slight variability with depth and time. Conversely, MeHg contents were the highest (up to 21.9 ng g-1) at the surface; they tended to decrease to nearly zero concentration with depth, thus suggesting that MeHg production and accumulation occur predominantly just below the sediment-water interface. Porewater MeHg concentrations (0.9-7.9 ng L-1, 0.15-15 % of THg) varied seasonally; higher contents were observed in the warmer period. The MeHg diffusive fluxes (up to 17 ng m-2 day-1) were similar to those in the nearby Gulf of Trieste (Covelli et al., 1999), although the lagoon sediments contained 4 folds higher THg concentrations. Conversely, the THg diffusive fluxes in the lagoon (up to 110 ng m-2 day-1) were once-twice higher than those previously estimated for the Gulf of Trieste. The diurnal MeHg benthic fluxes were the highest in summer at both sites (41000 and 33000 ng m-2 day-1 at the fishfarm and in the open lagoon, respectively), thus indicating the influence of temperature on microbial processes. The diurnal variations of dissolved THg and especially MeHg were positively correlated with O2 and inversely with DIC, suggesting an important influence of benthic photosynthetic activities on lagoon benthic Hg cycling, possibly through the production of organic matter promptly available for methylation. The results from the dark chamber incubated in the laboratory showed that the regeneration of dissolved THg was slightly affected by the oxic-anoxic transition. Conversely, the benthic flux of MeHg was up to 15 folds higher in sediments overlain by O2 depleted waters. In the anoxic phase, the MeHg fluxes proceeded in parallel with Fe fluxes and the methylated form reached approximately 100% of dissolved THg. The MeHg is mostly released into overlying water (mean recycling efficiency of 89%) until the occurrence of sulphide inhibition, due to scavenging of the available Hg substrate for methylation. Our results suggest that sediments in the Grado lagoon, especially during anoxic events, should be considered as a primary source of MeHg for the water column.