Tidal cycling of mercury and methylmercury between sediments and water column in the Venice Lagoon (Italy)

The sediment of Venice Lagoon regularly undergoes complex redistribution due to tidal forcing, which affects the cycling of contaminants such as mercury (Hg) between the sediment and the water column. We examined the distribution of total Hg (THg) and monomethylmercury (MMHg) in the water column, sediment and pore-water at two sites: VE1 (located in a depositional area adjacent to salt marshes) and VE2 corresponding to a moderately erosive, open area. We obtained instantaneous (using cores and micro-needle samplers) and time-integrated (using peepers) concentrations of the two mercury species in both dissolved and particulate forms. THg and MMHg concentrations were higher in the sediments at site VE1 (621.9 ± 213.7 ng g− 1 and 1.25 ± 0.63 ng g− 1 for THg and MMHg, respectively) than in those of the site VE2 (386.9 ± 92.7 ng g− 1 and 0.53 ± 0.30 ng g− 1). Hg concentrations in sediments were positively correlated with silts and organic matter content. Over two tidal cycles, the concentrations of THg and MMHg varied with the evolution of the tides. During the tidal flooding, both THg and MMHg peaked at the sediment–water interface and a moderate increase of dissolved MMHg was also observed in the water column. These fluctuations were observed during both tides and are suggestively related to advection of mercury species from surficial sediment pore-water to the water column and to desorption from suspended particles. The short-term increase in MMHg concentrations can result from in situ production, release from organic matter degradation, or from oxidative dissolution of redox-sensitive sulfide minerals and iron oxide reduction by micro-organisms; the two latter mechanisms being favored by redox oscillations in the surface sediment layers due to the tidal forcing. The decrease of both dissolved THg and MMHg concentrations at the sediment–water interface after high tide was attributed to a rapid adsorption onto particles. THg concentrations on suspended particles showed little variations during the tidal cycle with a minor peak at tide maximum, while MMHg concentrations on suspended particles slightly increased during ebb tide. MMHg concentrations on suspended particles were double than those in surface sediments, suggesting that tidal flushing may enhance dispersal of particle bound MMHg throughout the lagoon.