Hg species in the water column of Tyrrhenian Sea (Italy)

Abstract:

Mercury (Hg) speciation in the water column was successfully performed on samples from the Tyrrhenian Sea during the cruise Fenice2011. Primary objective was to increase sampling’s spatial resolution; thereby providing better insight into Hg biogeochemical cycling in environment with naturally higher Hg background. THg values ranged from 0.8 pM to 5.9 pM with an average of 2.5 ± 1.3 pM. Generally, the water column did not exhibit particular variation in THg values. The highest concentrations were measured in the coastal areas near Piombino industrial site and near geothermally active Ustica Island.DGM concentrations were on average 0.39 ± 0.23 pM and ranged from approximately 0.06 pM to 0.83 pM. DGM represented between 2 % and 96 % of THg, with higher proportions measured in deep sea water. This region is a source of Hg⁰ to the atmosphere, indicated by near depletion of DGM at surface. Waters became increasingly enriched in DGM below 250 m depth, with values ranging from 0.54 pM to 0.83 pM. The observed increase in deep waters was partly due to geothermal activity in the area. MeHg values ranged between 0.01 pM and 0.36 pM with an average of 0.097 ± 0.088 pM. The open sea stations had a slightly higher average value of 0.14 ± 0.10 pM. The surface was generally depleted in MeHg, which was enriched at depths showing oxygen consumption. That suggests spatial coupling with microbial carbon mineralization. Results with increased spatial resolution of sampling clearly show that besides transformations in the mixed layer, more attention should also be given to deeper water masses, particularly because of geothermal activity of the area. In light of global and regional changes of ocean and climate characteristics, minor variations in environmental conditions may change the ratios of Hg species. Toxic MeHg will accumulate and pose threat to human and wildlife populations, whereas DGM formation will only add Hg to the most effective global distribution system, the atmosphere.