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

The role of the major biogeochemical processes in 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 ldrija 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), methyliriercury (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 cliamber in situ and (3) measurements of benthic fluxes during oxic-anoxic transition with a laboratory incubation experiment. The THg solid phase, ranging between 9.5 and 14.4 mu g g(-1), showed slight variability with depth and time. Conversely, MeHg contents were highest (up to 21.9 lig 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, S., Horvat, M., Faganeli, J., Brambati, A., 1999. Porewater distribution and benthic flux of mercury and methylluercury in the Gulf of Trieste (Northern Adriatic Sea). Estuar. Coast. Shelf Sci. 48, 415-428], although the lagoon sediments contained four-fold higher THg concentrations. Conversely, the THg diffusive fluxes in the lagoon (tip to 110 ng m(-2) day(-1)) were one- to two-fold higher than those previously estimated for tile Gulf of Trieste. The diurnal MeHg benthic fluxes were highest ill summer at both sites (41,000 and 33,000 ng m(-2) day(-1) at the fishfarm and in the open lagoon, respectively), thus indicating tile influence of temperature on microbial processes. The diurnal variations of dissolved THg and especially MeHg were positively correlated with 02 and inversely with DIC, suggesting all important influence of benthic photosynthetic activities on lagoon benthic Hg cycling, possibly through tile 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 tip to 15-fold higher in sediments overlain by O-2 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. The 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. (c) 2007 Elsevier Ltd. All rights reserved.