Inorganic sulfur and mercury speciation in the water level fluctuation zone of the Three Gorges Reservoir, China: The role of inorganic reduced sulfur on mercury methylation☆
Graphical abstract
Introduction
The Three Gorges Reservoir (TGR) on the upper reaches of the Yangtze River, China, is one of the largest hydroelectric reservoirs in the world. At its highest water level (175 m a.s.l.), the reservoir has a total water surface area of 1080 km2 and a storage capacity of 39.3 billion m3 (Bao et al., 2015). The TGR was first impounded to 135 m a.s.l. in 2003, and then to the designed highest water level in late 2010 (Bao et al., 2015). Since then the water level has been regulated to about 175 m a.s.l. in winter and 145 m a.s.l. in summer for the purposes of power generation, flood protection and navigation. This regulation results in a massive water-level fluctuation zone (WLFZ) with a total area of 306.3 km2 that undergoes annual flooding and drying alternation cycle (Bao et al., 2015). The fluctuating redox conditions (Wang and Zhang, 2013), dynamic exchanges across the water-soil interface (Ye et al., 2011), and vegetation decomposition (New and Xie, 2008) and revegetation (Ye et al., 2015) in the WLFZ make it a sensitive area for the biogeochemical cycling of redox-sensitive elements, nutrients and chemical contaminants. Guo (2014) found that the reduction of Fe(III) in the WLFZ of the TGR can promote the release of phosphorus into the water body, potentially contributing to eutrophication. The redox chemistry involving humic substances may affect the cycling and toxicity of trace metals and organic contaminants in the WLFZ (Zheng et al., 2012, Yuan et al., 2014). Of particular interest is the coupling between sulfur and mercury cycling in the WLFZ, as microbially mediated reduction of sulfate or iron is known to promote the methylation of inorganic divalent mercury (Hg(II)i) (Choi and Bartha, 1993, Fleming et al., 2006). Moreover, the processes including sulfate reduction in sulfur cycling have often been overlooked in terrestrial systems due to much lower sulfur concentration than in marine systems (Ng et al., 2017).
As a global pollutant, the toxicity of Hg is greatly enhanced following methylation, as the resulting methylmercury (MeHg) can biomagnify in aquatic ecosystems and is a neurotoxin that can cross the blood-brain barrier in humans (Clarkson, 1997). It has been well-documented that newly built reservoirs are hotspots for MeHg production, as microbial decomposition of newly flooded organic matter fuels Hg(II)i-methylation (Lucotte et al., 1999, St. Louis et al., 2004, Schartup et al., 2015). As a result, fish Hg contents in reservoirs increase dramatically in the first 5–10 years after the impoundment and decline thereafter (St. Louis et al., 2004, Bodaly et al., 2007, Willacker et al., 2016). These newly-built reservoir characteristics are found in the TGR because of the seasonal water level fluctuations (Zhao et al., 2015). Indeed, the production and accumulation of MeHg in the WLFZ of the TGR have been confirmed by field monitoring (Zhang et al., 2014) and laboratory incubation (Liu et al., 2017). Wang and Zhang (2013) pointed out that Hg biogeochemistry in the TGR may shift from source-driven to process-driven due to dramatic changes in biogeochemical processes caused by the large-scale damming, which is similar to the case of the Arctic Ocean where the changes were due primarily to climate warming (Wang et al., 2010).
The redox chemistry of sulfur within the WLFZ of TGR could affect Hg cycling and Hg(II)i-methylation in several ways. Sulfate-reducing bacteria (SRB) (Choi and Bartha, 1993), iron-reducing bacteria (IRB) (Kerin et al., 2006) and methanogens (Wood et al., 1968) are three commonly known Hg(II)i methylators. However, the reaction product (sulfide) of SRB can complex with Hg(II)i, which may affect the bioavailability of inorganic Hg to SRB and hence the production of MeHg (Benoit et al., 1999, Benoit et al., 2001). Intermediate products generated from sulfate reduction and sulfide oxidation, such as thiosulfate and polysulfides, may also change the bioavailability of Hg (Jay et al., 2002, Wang et al., 2012). Furthermore, FeS and FeS2 may fix the dissolved Hg(II)i through surface adsorption or co-precipitation of HgS(s) (Wolfenden et al., 2005, Gong et al., 2016). The roles of inorganic sulfur cycling in Hg dynamics have been reported in reservoir sediments (Bravo et al., 2014), estuary sediments (Schartup et al., 2014), coastal marine sediments (Pierre et al., 2014), lakes (Small and Hintelmann, 2014, Bravo et al., 2015), forest catchments (Skyllberg et al., 2003), wetland soil (Skyllberg, 2008), paddy soil (Rothenberg and Feng, 2012, Zhao et al., 2016) and peatlands (Coleman Wasik et al., 2012). These sites are either submerged in water all the time, or undergoing natural, usually small scale, fluctuations in the water level. As the WLFZ of the TGR undergoes frequent and large-scale drying and flooding rotations, how the dynamic inorganic sulfur cycling affects Hg distribution and Hg(II)i-methylation remains unknown. The objective of this work was thus to examine the distribution of inorganic sulfur species, total Hg and MeHg through successive field monitoring within the WLFZ of the TGR, and investigate the relationship between Hg(II)i-methylation and inorganic sulfur cycling.
Section snippets
Study area
Two study sites within the WLFZ of the TGR were selected for this study: one is along the main stream of the Yangtze River at Shibaozhai (Site SB, 30°25′29.02″N, 108°10′54.97″E) in Zhongxian County, and the other is along one of the largest tributaries at the north shore of the Yangtze River in Kaixian County (Site KX, 31°07′58.20″N, 108°29′50.10″E) (Fig. 1). Both sites are located at the central region of the TGR, far away from anthropogenic influences. The WLFZ area of these two counties
Distribution and variations of inorganic sulfur species
The contents of AVS, S(0), CRS, WSS and total sulfur in the water line samples are shown in Fig. 2. It can be seen that all the reduced inorganic sulfur (RIS) species (AVS, S(0) and CRS) are strongly influenced by water level variations. At both sites, AVS and S(0) contents are very low, often below the detection limit, during the flood-up season (from September to December), but increase rapidly when the water level begins to draw-down from January. With few exceptions, AVS and S(0) contents
Transformation of inorganic sulfur species within the WLFZ
Different from the intertidal zone, riparian zone and riparian ecotone that are only affected by natural hydrological processes, the WLFZ of the TGR undergoes large-scale, annual water level alternations due to hydroelectric regulation. Therefore, the redox reactions caused by water level changes act as the major driving force for the variations of RIS species (AVS, CRS, S(0)) within the WLFZ. The AVS in the sediment/soil measures primarily metastable iron and manganese monosulfides (FeS, MnS),
Conclusions
In this study, a year-long monitoring of inorganic sulfur species and mercury was conducted at two typical sites in the WLFZ of the TGR. The transformation of different inorganic sulfur species was driven by water level fluctuation induced redox rotation, rather than the sources of inorganic sulfur. Both THg and MeHg contents were increased within the WLFZ, especially in sediment during the flooding season. Flooding caused sedimentation is the main source of THg to the WLFZ, and THg may be
Acknowledgement
This work was supported by National Basic Research Program of China (973 Program) (No. 2013CB430004) and National Natural Science Foundation of China (No. 41403079, 41373113). Liu J. was funded in part by the China Scholarship Council (No. 201606990037). The authors thank Liang L., Sun S., Qian S., Qin C. Q., Lu S., and Wang W. for their assistance in sample collection and pretreatment. The authors also thank the helpful comments of the three reviewers on the manuscript.
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This paper has been recommended for acceptance by Dr. Harmon Sarah Michele.