Speciation of mercury in surface and deep-sea waters in the Mediterranean Sea

A summary of data recently obtained for mercury analysis and speciation (reactive Hg, total Hg and monomethylmercury (MMHg)) in filtered and non-filtered seawater samples, dissolved gaseous mercury (DGM) and dimethylmercury (DMHg) in open and coastal waters of the Mediterranean Sea is presented. The majority of the results were obtained during an oceanographic cruise aboard the research vessel Urania from 14 July to 9 August, 2000, as part of the MED-OCEANOR Project funded by the National Research Council of Italy. The results are compared with those obtained in contaminated coastal environments of the Adriatic (The Gulf of Trieste and Kastela Bay) and non-contaminated coastal waters of the eastern Adriatic coast obtained in 1998. Total mercury concentrations in surface ocean waters are relatively low with an average of 0.81 pM (0.49-1.91 pM). Reactive Hg represents a substantial part with an average of 57% of total Hg (15-97%). Most mercury in open ocean waters was present in the dissolved form (32-95%, av. 70%), which is mainly due to the low abundance of particulate matter, a phenomenon well known for the Mediterranean open ocean waters. On average the percentage of Hg as MMHg was about 20%, of which about 66% was present in the dissolved form. The percentage of DGM in the surface ocean waters represents about 9% of total Hg (2.5-24.5%) and may originate from photochemical, biologically mediated mechanisms or diffusion from deeper layer either due to biological and/or to tectonic activity which is typical of the Mediterranean region. The presence of DMHg was confirmed only in waters below 20 m (up to 12 fM), while in surface waters DMHg was below the limit of detection ( <0.1 fM). Surface concentrations of Hg in the eastern and western parts are comparable, except for DGM which shows significantly higher concentrations in the eastern part (mean value: 0.22 pM) as compared to the western Mediterranean (mean value: 0.09 pM). The distribution of Hg species with depth was only measured at two stations, and indicated variability comparing the eastern and the western leg of the Urania cruise. The depth profile pattern confirms the importance of dynamic processes at the surface layers, while deep water continuously supplies DGM to the surface layer. The importance of analytical quality control is also discussed in this paper. (C) 2003 Elsevier Science Ltd. All rights reserved.