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 dis- tribution 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 co res and micro-needle samplers) and time-integrated (using peepers) concentrations of the two mercury spe cies 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 wit h silts and organic matter content. Over two tidal cycles, the concentrations of THg and MMHg varied wit h the evolution of the tides. During the tidal fl ooding, both THg and MMHg peaked at the sediment - water interface and a moderate increase of dissolved MMHg was also ob- served inthe watercolumn. These fl uctuations were observed during both tides and are suggestively related to ad- vection of mercury species from sur fi cial sediment pore-water to the water column and to desorption from suspended particles. The short-term increase in MMHg con centrations can result from in situ production, release from organic matter degradation, or from oxidative dissolution of redox-sensitive sul fi de minerals and iron oxide reduction by micro-organisms; the two latter mechanisms being favored by redox oscillations in the surface sed- iment layers due to the tidal forcing. The decrease of b oth dissolved THg and MMHg concentrations at the sedi- ment - water interface after high tide was attributed to a rap id 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 sus- pended particles were double than those in surface sediments, suggesting that tidal fl ushing may enhance dispers- al of particle bound MMHg throughout the lagoon