Dissolved gaseous Hg and mercury speciation in the Mediterranean Sea: temporal and spatial trends over 15 years of observations

Mercury speciation and its distribution in surface and deep waters of the Mediterranean Sea were studied during several oceanographic cruises on board the Italian research vessels Urania and Minerva since summer 2000 as part of different EU (Mercyms, GMOS) and national research projects, covering regions from Strait of Gibraltar on the west, and Levantine Sea on the east, as well as Adriatic Sea with the Gulf of Trieste as Mediterranean most northern part. The study includes deep water profiles of dissolved gaseous Hg (DGM), reactive Hg (RHg), total (THg), monomethyl Hg (MeHg) and dimethyl Hg (DMeHg), and with some water quality parameters in coastal and open sea deep water profiles.

Concentrations of measured Hg species were characterized by seasonal and spatial variations. Relatively high portions of DGM indicate high reactivity of Hg in open marine waters. DGM was present in surface waters mainly as Hg⁰ as no DMeHg was detected, while towards the bottom a noticeable, but relatively small portion of DMeHg (<5 %) is present in most of the Mediterranean. In Adriatic DMeHg was present in higher fraction and was detected in surface waters, but in very low concentrations. DGM generally increases with depth, suggesting a source of volatile Hg in deeper waters. Average DGM concentration was the highest in deep water masses (WMDW and EMDW). From certain profiles it is possible to observe that increase of DGM corresponds to a decrease in dissolved oxygen levels, suggesting that DGM is produced by biologically mediated processes in the oxygen minimum zone. The observed decrease towards the surface is a result of the balance between production and loss processes on boundary between water and atmosphere.

Special attention was given to quality control steps by comparing data on board the ship and collaboration in Brooks Rand and GEOTRACES intercalibration exercises by different laboratories using different analytical protocols on samples collected on board.