Temporal and Spatial Trends of Mercury Speciation in the Mediterranean Sea - 20 years of measurements

Abstract:

Despite the geological mercury anomaly in the Mediterranean Basin, the concentrations of Hg Hg and its compounds are generally lower than those found in the Atlantic and Pacific waters. Temporal and spatial trends of total mercury and its species in surface and deep waters (dissolved gaseous mercury – DGM, mono and di-methylmercuy - MMHg, and DMHg) obtained during last 20 years will be presented. Total mercury Hg levels in open waters are generally very low (0.81–2.33 pM, av. 1.46 pM) and remain constant over the years. Hg speciation data showed that DGM accounts for up to 50% while MMHg accounted to up to 15% of total Hg in water. Significant spatial and temporal differences were observed for mercury speciation. The relatively high concentrations and fractions of DGM and MMHg indicate the high reactivity of Hg in open marine waters. DGM was present in surface waters mainly as Hg0 as no DMHg was detected at the surface, while towards the bottom a noticeable, but relatively small fraction of DMHg was present. It was found that DGM species were lower in surface with an increasing trend towards the bottom, especially in the tectonically active areas, indicating its geotectonic origin. In surface waters, DGM exhibited higher values in summer. MMHg profiles are mostly related to Chl-a and oxygen concentrations. This underlines the role of planktonic production and regeneration in methylation/ demethylation processes. The low MMHg concentrations found in shelf edge or coastal sediments and water over sediment indicate that coastal or shelf sediments are not a significant MMHg source for adjacent open sea waters. Anthropogenic and natural point sources show local enrichments, while natural diffusive sources influence Hg speciation over larger areas. The results allowed estimation of temporal mass balances to be evaluated for the Mediterranean Sea. Comparability of the measured results was secured by strict quality control measures over the 20 years.