Importance of a tidal flat-saltmarsh system as a source-sink of mercury in a contaminated coastal lagoon environment (northern Adriatic Sea)

Saltmarshes are important constituents of marine-coastal transitional environments that provide several services to the ecosystem. Due to anthropogenic activities, several contaminants are increasingly introduced into these environments where are disperse and can accumulate in the saltmarsh sediments. The Marano and Grado Lagoon (northern Adriatic Sea) is affected by Hg contamination, which mainly coming from historical regional mining activities (Idrija, western Slovenia) and, subordinately, from more recent industrial input due to a chlor-alkali plant. Several studies have also demonstrated the Hg contamination of saltmarsh sediments and halophyte vegetation that cover them. Moreover, saltmarshes not only trap contaminated sediments, but can act as a secondary source of contamination. This work aims at determining the potential role of the tidal flat-saltmarsh (TF-S) system as a sink or secondary source of Hg in this coastal lagoon in relation to the chemical-physical processes leading their remobilisation. The main objective was to understand the role played by periodic flow of tide in a TF-S pilot site in terms of transport, accumulation and release of Hg. Tidal flows and water chemistry were measured at the mouth of a principal tidal creek which collect the waters of a dense channel network draining a 5.5-ha tidal flat-saltmarsh system. Tidal fluxes were estimated by combining discrete hourly tidal flow measurements with weighted measurements of particulate (PHg) and dissolved (DHg) mercury obtained by water samples. The highest values of DHg and PHg were recorded during ebb tide and the quantitative fluxes estimated for all parameters highlighted a tendency for metal to be exported from the TF-S system due to the tidal flows in ebb tide conditions. The results obtained for the PHg fluxes, in particular, are in agreement with those observed on a macro-scale at one of the lagoon tidal inlets considering an annual mass-balance of PHg performed via several water column sampling campaigns. A simple estimation provides a negative sedimentary budget for the TF-S system, which loses PHg towards the main lagoon channel during a tidal semi-cycle thus confirming other evidence of serious morphological deterioration of this critical coastal environment.