Dynamics of dissolved organic matter (DOM) in a typical inland lake of the Three Gorges Reservoir area: Fluorescent properties and their implications for dissolved mercury species

Highlight

• Seasonal dynamic of DOM is controlled by autochthonous and allochthonous sources.

• Dissolved Hg co-varied with DOM properties, rather than DOC concentration.

• Simply correlation between DOC and Hg may overlook their intrinsic relationships.

• Besides DOC, DOM properties should be considered to evaluate its role in Hg fate.

Abstract

Dissolved organic matter (DOM) plays an important environmental and ecological role in inland aquatic systems, including lakes. In this study, using fluorescence analysis, we investigated the seasonal dynamics of DOM characteristics in Changshou Lake, which is a typical inland lake in the Three Gorges Reservoir (TGR) area. We also discuss the environmental implications of DOM for mercury (Hg) dynamics. Based on the origins of two end-members, the variations in DOM observed in this study in Changshou Lake suggest that hydrological processes (e.g., terrestrial inputs resulting from runoff and humic-like component residences) and biological activities (e.g., microbial and algae growth) are the two main principal components controlling the seasonal dynamics of DOM characteristics. Furthermore, the dynamics of dissolved Hg co-varied with variations in DOM properties, rather than with dissolved organic carbon (DOC) concentrations. This indicates that the previously reported simple correlations between DOC and Hg were not comprehensive and may lead to misunderstanding the interactions between DOM and Hg. Therefore, we recommend that when using DOM-Hg correlations to evaluate the role of DOM in the environmental fate of Hg, especially in field investigations of the spatial and temporal distribution of Hg, the properties of DOM must be taken into account.

Introduction

Dissolved organic matter (DOM), which is composed of heterogeneous structures and compositions, is ubiquitous in inland aquatic systems, such as lakes and reservoirs. Because of its active biogeochemical properties, DOM is one of the most important geochemical factors controlling the mobility of pollutants such as organic contaminants and trace heavy metals (Aiken et al., 2011). Additionally, it plays a substantial role in the global carbon cycle and can potentially impact climate change (Nelson and Siegel, 2013, Mopper et al., 2015). In recent decades, general public health and environmental pollution concerns about lake water quality have motivated the comprehensive evaluation of the molecular composition and sources of lake DOM (Henderson et al., 2009, Kothawala et al., 2014, Kellerman et al., 2014, Kellerman et al., 2015, Mazrui et al., 2016). Thus, understanding the quantities and complex qualities of DOM is essential to further constraining its environmental and ecological roles in lake systems. In particular, variations in the properties of DOM (e.g., aromaticity) pose significant challenges to processing drinking water because of the formation of disinfection byproducts (DBPs) (e.g., trihalomethanes (THMs) and haloacetic acids (HAAs)) (Weishaar et al., 2003) and evaluating the environmental risk of contaminants (Aiken et al., 2011, Hsu-Kim et al., 2013). Importantly, several recent studies of water quality management and sustaining aquatic ecosystems (Stanley et al., 2012) have illustrated that deciphering the characteristics of DOM is useful for assessing water quality and predicting potential pollution (Parr et al., 2015, Solomon et al., 2015, Mitrovic and Baldwin, 2016, Zhou et al., 2015a, Zhou et al., 2015b, Zhou et al., 2017). To accomplish this, it is necessary to characterize DOM and clarify its environmental implications derived from the compositional views of DOM.

Compared to the traditional quantification method, which relies on the bulk analysis of dissolved organic carbon (DOC) to obtain information about DOM biogeochemistry, within the last several decades, qualitative methods of determining the heterogeneous structures of DOM have become considerably more advanced (Leenheer and Croué, 2003). Of all the available characterization methods, spectrophotometric analysis has been rapidly developed and is the most widely used. This method is ideal because the optical-active moieties of DOM can reflect its compositional changes and links with biogeochemical reactivities. In particular, fluorescence spectroscopy provides an alternative to traditional (UV–Vis absorbance-based) approaches to characterizing DOM. Fluorescence spectroscopy is rapid, precise, and less expensive to conduct and is thus extensively used in temporal and spatial sampling programs to comprehensively trace DOM dynamics in a range of aquatic ecosystems, such as freshwater and marine environments (Coble, 1996, Coble, 2007, Fellman et al., 2010, Bergamaschi et al., 2012, Kothawala et al., 2014, Kellerman et al., 2014, Kellerman et al., 2015). Although florescent optical-active components account for a small fraction of DOM, the fluorescence characterization of DOM still provides reliable information about its composition and origins (Coble, 1996, Coble, 2007, Fellman et al., 2010, Zhang et al., 2011).

The Three Gorges Dam, which is located on the Yangtze River, is the largest artificial hydraulic dam in the world. It has played an important role in the economic development of China since it was completed in 2012. The Three Gorges Reservoir (TGR) areas were formed by the construction of this dam, and they span an area of approximately 79,000 km². Overall, although regional economic development has benefited from this project, the TGR areas have aroused series environmental concerns, especially in terms of the water quality of inland lakes from the TGR areas (Wang and Zhang, 2013, Lam, 2015, Floehr et al., 2015, Liu et al., 2017). Until now, studies of DOM characterization and its environmental implications in the TGR areas have been rare. Thus, to address these concerns, it is essential to investigate the variability in the quantity and quality of DOM to improve our understanding of the relevant environmental biogeochemical mechanisms of contaminants in the TGR areas. In addition to characterizing the composition and sources of DOM in inland lakes, it is also necessary to improve our understanding of the causal relationships between DOM and contaminants such as mercury (Hg).

Currently, mercury is identified as a global pollutant by the United Nations Environment Programme (UNEP), and awareness of environmental Hg contamination has risen widely due to the risks it poses to human health and the environment. Methylmercury (MeHg), which is mainly formed by the biotic methylation of inorganic Hg, is the form of Hg that is most notorious for poisoning aquatic organisms and humans (Hsu-Kim et al., 2013). In particular, lakes that include reservoirs are usually recognized as “Hg hotspots” (Chen et al., 2012, Fu et al., 2010, Fu et al., 2013a, Fu et al., 2013b, Feng et al., 2017). In aquatic environments, DOM regulates Hg dynamics (including the production of MeHg) through numerous mechanisms, including ligand binding (Ravichandran, 2004, Skyllberg et al., 2006) and redox processes (Gu et al., 2011, Jiang et al., 2014, Jiang et al., 2015). Previous studies have reported a dual role (i.e., enhancing vs. inhibiting) of DOM in both the production and biological uptake of MeHg, which is associated with the effects of DOM on the speciation and bioavailability of Hg (Graham et al., 2012, Graham et al., 2013, Zhang et al., 2012, Zhang et al., 2014, Hsu-Kim et al., 2013, Jonsson et al., 2012, Jonsson et al., 2014, Jonsson et al., 2017, Mazrui et al., 2016). Thus, untangling the complicated associations between DOM and Hg is necessary. Despite the recognition of the critical functional role that DOM plays in the environment, DOM characterization has mainly been conducted in lab-scale experiments to investigate its mechanisms (Aiken et al., 2011, Mazrui et al., 2016). In contrast, such characterization of DOM is still rarely routinely incorporated into field observations or in monitoring works to track contaminant dynamics. The DOC or simple DOM quality parameters derived from the UV–Vis absorption spectrum (e.g., SUVA₂₅₄) are often used to explain the relationship between DOM and Hg (Mierle and Ingram, 1991, Driscoll et al., 1995, Hurley et al., 1998, Dittman et al., 2009, Bergamaschi et al., 2012). Strongly positive correlations between Hg and DOC concentrations are often observed in field monitoring, suggesting that Hg mobility in aquatic systems is associated with DOM (Han et al., 2006, Ci et al., 2011, Chen et al., 2012, Fu et al., 2010, Fu et al., 2013a, Fu et al., 2013b). However, across aquatic systems with diverse geochemical processes and sources of DOM, the correlation between Hg and DOC is not always consistent or significant. The heterogeneities of DOM, including variations in its quantity or quality, may explain the reported inconsistencies in the relationships between DOM and Hg. Therefore, the lack of DOM compositional information may lead researchers to underestimate or neglect the importance of DOM due to the lack of a significant correlation between DOC and Hg.

Hence, given this background information, we selected a typical inland reservoir-type lake in the TGR areas, Changshou Lake, in which to characterize the properties and sources of DOM using fluorescence spectroscopy and to discuss the relationship between DOM and Hg in the field. The objectives of this study were two-fold: (1) Characterization of DOM in a Lake in the TGR Areas - to elucidate the seasonal variations in DOM and characterize the properties and origins of DOM using fluorescence analysis; (2) Environmental Implication of DOM Properties - to link the fluorescent properties of DOM with Hg dynamics and further validate whether optical indices (e.g., fluorescent peaks, the humification index or the biological index) can reasonably explain and predict the dissolved Hg variations in this study area in addition to DOC. As a preliminary characterization of aquatic DOM in the lakes of the TGR areas, this study represents an initial but necessary step to improve our insights into the crucial role that DOM plays in determining the fate of contaminants (e.g., Hg), which may be beneficial for further water quality management in this region.