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

Abstract

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.

Introduction

The Venice Lagoon is a complex and dynamic system where notable exchanges of materials and energy between the main land (continent) and the Adriatic Sea occur (Ravera, 2000). The lagoon is shallow (average water depth 1.2 m) and consists of intertidal or submerged mudflats, salt marshes and man-made navigation canals (Molinaroli et al., 2009, Pranovi et al., 2004). The sedimentary dynamic strongly depends on the tidal flushing, leading to highly heterogeneous deposition and erosion rates between canals and salt-marshes areas. Sediment dynamics is also influenced by human activities related to construction, tourism and clam harvesting activities (Degetto et al., 2005).

Venice Lagoon has been recognized as a mercury (Hg) contaminated area (Bloom et al., 2004a, Pavoni et al., 1992). The two chlor-alkali plants of the industrial area of Porto Marghera were identified as the main contributors to the contamination of the lagoon (Bellucci et al., 2002, Bloom et al., 2004b, Zonta et al., 2007). Until recently, antifouling paints used near the town of Chioggia (in the southern part of the lagoon) were another local source of mercury (Berto et al., 2006).

Many authors described the Venice Lagoon as a ‘biological reactor’, because of considerable nutrient input, shallow and seasonally warm water, and extensive wetlands. Bloom et al. (2004b) described it as a “methyl Hg incubator”, where even small inputs of inorganic Hg could result in high levels of monomethylmercury (MMHg) production; the most toxic and bioaccumulating species of Hg (Cheng et al., 2009, Sirot et al., 2008, Wheatley and Wheatley, 2000). However, only few studies performed in the Venice Lagoon did consider the influence of tides on the production and release of MMHg from sediments to the water column.

In a study conducted in the Canale SS Apostoli (City of Venice), Bloom et al. (2004b) showed that MMHg concentrations in unfiltered water varied only slightly during a tidal cycle, while total Hg (THg) was inversely correlated with tide height. In addition, the dynamics of total suspended solids (TSS) associated with tidal flushing was suggested as the vehicle for Hg dispersal from the sources to the whole of Venice's Lagoon (Berto et al., 2006) and may play an important role in the adsorption of dissolved Hg species produced in the surficial sediments (Bloom et al., 2004b).

In the present study, we considered the influence of the tidal flushing on the short term THg and MMHg concentration changes in the water column and sediment pore water. First, we determined the THg and MMHg concentrations in sediments and pore water in two different areas typical of the sedimentary dynamic of the lagoon, (i.e., a remote site adjacent to salt marshes and open site close to the Burano canal bordering a marshy area). We also used various tools to get instantaneous and time integrated information on the dissolved and particulate THg and MMHg distribution in the water column and the sediment pore waters. Second, we followed temporal variations in particulate and dissolved THg and MMHg concentrations in water column and at the sediment–water interface during two tidal cycles to evaluate the exchanges between the surficial sediments and the water column for both Hg species.