Labile Hg speciation in coastal, mine-impacted Gulf of Trieste (Northern Adriatic)

Mercury (Hg) is a toxic element, prone to microbial methylation to monomethyl mercury (MeHg) in anoxic marine sediments during sulphate and metal reduction processes. Bioaccumulation of Hg species results in biomagnification in marine food webs, often with detrimental effects to local ecosystems and increased exposure to humans. Gulf of Triste has been under continuous 500 year-long Hg stress due to Hg mining in Idrija, Slovenia. 1.5 tons of Hg are annually still transported into the Gulf, deposited onto the sediment or distributed further towards Adriatic, while actively involved in biogeotransformations.

To assess the lability, and by extension bioavailability, of Hg species, we applied for the first time Diffusive Gradients in Thin films (DGT) technique in the water column and sediment in the Gulf of Trieste. Our results showed that ratios of MeHg/THg in the water column and sediment porewaters ranged 10-75%, thus much higher than previously observed values. Comparison of our results with previously published labile Hg assessments (filtration and SnCl₂ reduction) revealed that labile THg values coincided more for water column and less for sediment porewater. On the contrary, labile MeHg values assessed with these techniques were more similar in porewater than seawater.

In sediment porewater, labile THg and MeHg concentrations varied with depth (1-7 ng/L and 0.3-2.3 ng/L for THg and MeHg, respectively). MeHg concentrations were higher at known depths of sulphate reduction. THg variations were probably due to fractionation between solid and dissolved phase. Compared to water column concentrations (1.13 ng/L and 0.78 ng/L for THg and MeHg, respectively), sediments appear to be a source of labile Hg species to the water column, facilitating complexation and transport with organic matter and/or uptake into the lower food web. Our results show that values of labile Hg species for water column and sediments in Gulf of Trieste do not agree entirely with previous research, highlighting the need for more investigation into Hg bioavailability in this representative Hg-laden environment.