Optical Properties of Dissolved Organic Matter and Their Relation to Mercury Concentrations in Water and Biota Across a Remote Freshwater Drainage Basin

Abstract

Dissolved organic matter (DOM) includes an array of carbon-based compounds that vary in size and structure and have complex interactions with mercury (Hg) cycling in aquatic systems. While many studies have examined the relationship between dissolved organic carbon concentrations ([DOC]) and methyl Hg bioaccumulation, few studies have considered the effects of DOM composition (e.g., protein-content, aromaticity). The goal of this study was to explore the relationships between total and methyl [Hg] in water, invertebrates, and fish and optically derived measures of DOM composition from 47 lake and river sites across a boreal watershed. Results showed higher aqueous total [Hg] in systems with more aromatic DOM and higher [DOC], potentially due to enhanced transport from upstream or riparian areas. Methyl [Hg] in biota were all positively related to the amount of microbial-based DOM and, in some cases, to the proportions of labile and protein-like DOM. These results suggest that increased Hg bioaccumulation is related to the availability of labile DOM, potentially due to enhanced Hg methylation. DOM composition explained 68% and 54% more variability in [Hg] in surface waters and large-bodied fish, respectively, than [DOC] alone. These results show that optical measures of DOM characteristics are a valuable tool for understanding DOM-Hg biogeochemistry.

Global Meta‐Analysis on the Relationship Between Mercury and Dissolved Organic Carbon in Freshwater Environments

“…We found generally weaker relationships ( r ) and shallower slopes for MeHg–DOC compared to THg–DOC relationships, as expected based on current empirical evidence (Burns et al, ; Hall et al, ; Lescord et al, ). The most likely explanation is that biological processes responsible for the production of 1 mole of MeHg do not necessarily result in a 1:1 change in DOC concentration, hence reducing the coupling with DOC.…”

“…We propose that a better understanding of the molecular underpinnings of the DOC and Hg cycling in freshwaters may shed new light on Hg–DOC coupling. There is a growing interest in using DOC characterization techniques to improve our understanding of Hg dynamics in freshwaters, including SUVA 254 (and other wavelength), HPOA (Burns et al, ; Dittman et al, ), parallel factor analysis modeling of DOC fluorescence (Jiang et al, ; Lescord et al, ), semiquantitative analytical methods by a pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS) (Bravo et al, ), and fully quantitative liquid chromatography (LC)–MS or GC–MS analyses. Such analyses may help better quantifying Hg affinity with DOC under varying environmental conditions.…”

“…Indeed, Hg–DOC relationships have been repeatedly observed across space (Braaten et al, ; Luo et al, ; MacMillan et al, ) and time (Burns et al, ; de Wit et al, ; Gerson & Driscoll, ) scales. MeHg–DOC relationships have been found to be weaker than THg–DOC relationships in the literature (Burns et al, ; Hall et al, ; Lescord et al, ), which could be due to the origin and molecular composition of DOC (Bravo et al, , ) or to photodemethylation of MeHg in presence of DOC (Girard et al, ). Stronger relationships with Hg have been found for the aromatic fraction of DOC as measured by SUVA 254 or UV 254 (Burns et al, ; Riscassi & Scanlon, ) or by specific families of compounds measured by fluorescence and analyzed by parallel factor analysis (Jiang et al, ) than for bulk DOC concentrations.…”

“…DOC can have affinity with a wide range of trace metals including Hg (Mason, 2013). The origin (allochthonous vs. autochthonous) and composition (humic acid, fulvic acid, and humin) of DOC will impact the lability (labile vs. refractory), molecular weight, aromaticity, and amount of reduced sulfur binding sites (Bravo et al, 2017(Bravo et al, , 2018Jiang et al, 2018;Lescord et al, 2018;Skyllberg et al, 2003). The propensity of Hg to bind with DOC will be determined by the amount of reduced sulfur available for binding (commonly measured by hydrophobic organic acid fraction; HPOA) and aromaticity (commonly measured by specific UV absorbance at 254 nm; SUVA 254 or by HPOA; Bravo et al, 2017;Burns et al, 2013;Qian et al, 2002;Skyllberg et al, 2003;Xia et al, 1999).…”

“…The propensity of Hg to bind with DOC will be determined by the amount of reduced sulfur available for binding (commonly measured by hydrophobic organic acid fraction; HPOA) and aromaticity (commonly measured by specific UV absorbance at 254 nm; SUVA 254 or by HPOA; Bravo et al, 2017;Burns et al, 2013;Qian et al, 2002;Skyllberg et al, 2003;Xia et al, 1999). DOC structure will not only influence the binding and subsequent transport of Hg in watershed but also its bioavailability in aquatic organisms (Chiasson-Gould et al, 2014;French et al, 2014;Lescord et al, 2018;Tsui & Finlay, 2011). Hydrophobic fractions of DOC include humic and fulvic acids and have high reduced sulfur content (e.g., sulfides and thiols), which results in a preferred affinity with Hg (Qian et al, 2002;Skyllberg et al, 2003).…”