Understanding Hg biogeochemical cycling in an Italian lagoon environment – A key to the coexistence of fish and clam farming and Hg contaminated sediments

The Marano and Grado Lagoon is the second largest wetland along the northern Adriatic coastline. Fishing, collection of clams and mussels, and fish and clam farming in the lagoon are important economically for the inhabitants. The Lagoon has experienced significant contamination by mercury from effluents from a chlor-alkali plant and during the last centuries, from the Idrija mercury mine. Factors controlling mercury cycling in lagoon environments are normally poorly defined, but critical to understanding the links between sources and higher trophic levels that are ultimately vectors of human exposure. Understanding the fate of the element when dredged sediments are used for saltmarshes reclamation could be, for instance, a future challenge in research. This presentation discusses results of a comprehensive investigation of mercury cycling in the Lagoon aimed at supplying the local and regional Administrators with information useful to minimize the risk of mercury contamination to humans. Mercury was investigated in several matrices (water, sediment, biota) and its mobility was tested along with its speciation in relation to biogeochemical processes occurring in this coastal environment, where bacterial communities have a primary role in converting mercury to its more toxic form, methylmercury. The large amount of mercury buried in the Lagoon sediments and the complexity of the lagoon ecosystem, suggest that in situ reclamation of the sediments may not be feasible. The presence of mercury in the lagoon environment must be then considered a permanent issue and the biogeochemical behaviour of this metal in the sediment-water system must be carefully monitored.