Dynamics of gaseous mercury exchanges at sea-air interface in differently contaminated coastal environments of the Northern Adriatic Sea

Gaseous elemental mercury (Hg0) formation in surface water and subsequent volatilisation to the atmosphere can contribute to limit Hg burden in aquatic environments. However, direct measurements of evasion are not frequent, mostly in shallow coastal marine areas subject to high Hg supplies from inland. The aim of this study was therefore to investigate the dynamics of Hg0 production and evasion in two selected coastal environments of the Northern Adriatic Sea. This area has been heavily contaminated by Hg due to historical Hg mining in Idrija (Slovenia) and to wastewater discharges of a decommissioned chlor-alkali plant. Incubations in field and a floating flux chamber were used to determine Hg0 concentrations and fluxes, respectively. To evaluate the effects of both Hg contamination degree and hydrodynamic conditions within this area, a confined heavily impacted fish farm in the Marano and Grado Lagoon (VN, Italy) and an open less contaminated environment in the Adriatic Sea (PR, Bay of Piran, Slovenia) were selected. Significantly higher Hg0 concentrations were observed at VN with comparable values throughout day and night, likely suggesting the occurrence of strong dark reduction processes in water and contaminated sediments. Surprisingly, comparable Hg0 fluxes were found at the two sites (range VN=7.43-41.17 ng m-2 h-1, PR=0-81.49 ng m-2 h-1) with higher emissions generally occurring in summer. Water stagnation and possible preferential re-oxidation likely limited Hg0 evasion at VN, resulting in relatively constant fluxes during sampling periods. Conversely, at PR, emissions generally decreased overnight but showed high peaks coinciding with increasing water turbulence. These findings indicate that static conditions may increase Hg residence time in aquatic environments. Increasing turbulence due to wave motion can help to mitigate the effects of Hg pollution on aquatic ecosystems through enhancing gaseous losses into the atmosphere and eventually limiting its availability for methylation.