Critical factors affecting mercury biogeochemical cycle in fish farm contaminated sediments (Grado Lagoon, Adriatic Sea)

The Marano and Grado Lagoon has been subjected to mercury (Hg) input from two anthropogenic sources. The first long-term source was the Isonzo River, which has been the largest contributor of this metal into the northern Adriatic Sea since the 16th century due to Hg mining in Idrija, western Slovenia. The second was a chlor-alkali plant located in the Aussa-Corno River system flowing into the Lagoon. As a consequence, Hg content in sediments was recognized to be up to three orders of magnitude higher than the natural background level. Since local fish farming is extensively practiced, there is a great concern about the risk posed by potential Hg harmful effects on the trophic chain and consumers. Previous investigations showed Hg bioaccumulation in commercial fish species (e.g. Sparus aurata, Dicentrarchus labrax). Due to this, a three-years research project has been recently planned in order to understand critical factors affecting Hg cycling in a fish farm and, possibly, to provide useful advices to mitigate Hg bioaccumulation. In this work, a preliminary evaluation of the concentration of Hg in different matrices (sediments, suspended particulate matter and waters) was performed. Moreover, the input/output of Hg associated with tidal flows in a tidal cycle through the sluice gates, which is the only way of water exchange between the Lagoon and the fish farm, was calculated. High Hg concentrations in short core profiles of bottom sediments of shallow ponds (up to 22.3 ± 2.29 mg kg-1), in suspended particulate matter (8.21 mg kg-1) and in the dissolved phase of the water column (201 ± 12.1 ng L-1) characterized the fish farm. From these first evidences, it seems that Hg enters into the fish farm through the sluice gates during the two daily loading phases (259 and 154 ng L-1, at flood tide) showing concentrations higher than those observed when waters flow out from the fish farm at ebb tide (14.1 and 31.8 ng L-1). Conversely, higher concentrations in ebb tide (76 ng L-1) than in flood tide conditions (7 ng L-1) were observed during the whole tidal cycle. Further investigations are to be planned in the next future in order to elucidate how the water circulation, sedimentary inputs and trophic state influence the Hg biogeochemical processes at the water-sediment interface in this complex and heavily modified ecosystem.