Dissolved gaseous mercury (DGM) in the Mediterranean Sea: Spatial and temporal trends

Dissolved gaseous mercury (DGM) was studied in surface and deep waters of the Mediterranean Sea over the last 15 years during several oceanographic cruises on board the Italian research vessels Urania and Minerva Uno, covering the Western and Eastern Mediterranean Basins as well as Adriatic Sea as its northernmost part. DGM was measured together with other mercury species (RHg - reactive Hg, THg - total Hg, MeHg - monomethyl Hg and DMeHg - dimethylmercury), and with water quality parameters in coastal and open sea deep water profiles. DGM represents a considerable portion of THg (on average 20%) in Mediterranean waters. Spatial and seasonal variations of measured DGM concentrations were observed in different identified water masses. DGM was the highest in the northern Adriatic, the most polluted part of the Mediterranean Sea as a consequence of Hg mining in Idrija and heavy industry in northern Italy, and near the Gulf of Lion. Generally, average DGM concentration was higher in the West and East Mediterranean Deep Waters (WMDW and EMDW) and Levantine Intermediate Water (LIW) than in overlaying Modified Atlantic Water (MAW); however, it was the highest in N Adriatic Surface waters (NAdSW) and consequently in outflowing Adriatic Deep Waters (AdDW). In deep water profiles the portion of DGM typically increased at depths with oxygen minimum and then towards the bottom, especially in areas with strong tectonic activity (Alboran Sea, Strait of Sicily, Tyrrhenian Sea), indicating its bacterial and/or geotectonic origin. During oceanographic cruises in 2011 and 2015 in the Tyrrhenian Sea, novel methods for continuous DGM determination in surface waters (Wangberg and Gardfeldt, 2011, Begu et al., 2016) were applied and compared to the standard method, and showed good agreement. (C) 2017 Elsevier B.V. All rights reserved.