Mobilization and Transformation of Mercury Across a Dammed Boreal River Are Linked to Carbon Processing and Hydrology

Abstract

Reservoirs are known to accelerate the mobilization and cycling of mercury and carbon as a result of flooding of terrestrial organic matter, which can lead to environmental concerns at local and broader spatial scales. We explored the covariation of mercury (Hg) and carbon (C) functional pools in natural and recently dammed portions of the aquatic network of the Romaine River watershed in Northern Quebec, Canada, to understand how the fate of these elements varies across systems with contrasting hydrology and environmental conditions. We found that total Hg (THg) concentrations in surface waters were relatively constant along the network, whereas both the concentrations and proportions of MeHg tended to increase in reservoirs compared to surrounding nonflooded systems, and along the cascade of reservoirs. Whereas THg was related to total and terrestrial pools of dissolved organic carbon (DOC), MeHg was weakly related to DOC but strongly linked to surface concentrations of CO 2 , as well as to concentrations of iron and manganese. The latter are proxies of cumulative organic matter processing within the network, presumably in anoxic portions of shallow bays, deep reservoir waters, and river sediments, as well as in prior seasons (e.g., under ice). Our results suggest that these deep boreal reservoirs acted more as transformation sites for Hg that was already present than as mobilizers of new Hg, and that under ice metabolism plays a role in MeHg production in these systems as we found strong dichotomies in MeHg patterns between spring and summer.

Understanding Food Web Mercury Accumulation Through Trophic Transfer and Carbon Processing along a River Affected by Recent Run-of-river Dams

“…Relationships between [MeHg] and δ 13 C have been observed for lake organisms, , with pelagic organisms having higher [MeHg] and lower δ 13 C compared to littoral organisms. ,− Few studies have related organism [MeHg] to δ 13 C in streams or rivers. One reason is that the δ 13 C range is generally larger in lakes than in rivers because of the higher methanogenesis and CO 2 recycling in lakes and related methylation. ,, We found only one study in lotic environments that observed δ 13 C to be a better predictor than δ 15 N for determining Hg accumulation in stream macroinvertebrates . The authors linked this observation to, at least in part, scrapers and filterers foraging in sun-exposed sites characterized by a higher production of periphytic biofilm (low δ 13 C and high Hg) as compared with invertebrates in shaded sites that forage on land detritus (high δ 13 C and low Hg).…”

“… , At DOC concentrations higher than 10 mg/L, a negative relationship with MeHg concentrations in water and organisms has sometimes been observed, in part related to the lower Hg bioavailability if bound to DOC. − The bacterial respiration has also been shown to respectively plateau or decline at the highest DOC and TP ranges from several lakes (>10 mg/L and >11 μg/L, respectively) . We suggest to more often include the measurement of CO 2 concentrations in future Hg studies to better explain the DOC vs MeHg relationships …”

“…46 We suggest to more often include the measurement of CO 2 concentrations in future Hg studies to better explain the DOC vs MeHg relationships. 50 Hg concentrations in soils, water, and sediments are strongly correlated with the amount of organic matter in these matrices. 51,52 Here, bacterial decomposition of allochthonous and autochthonous organic carbon having a δ 13 C of −28‰ 53 led to the production of 13 C-depleted CO 2 of a similar δ 13 C signature.…”

Contrasting seasons and land uses alter riverine dissolved organic matter composition

“…The model was based on 1078 samples originating from various Québec aquatic ecosystems from temperate and boreal climates. The model validated 5 distinct components (De Bonville et al, 2020; Supplementary Fig. 1), described in Table 1 as the excitation and emission fluorescence peaks.…”

Mercury Export From Freshwater to Estuary: Carbocentric Science Elucidates the Fate of a Toxic Compound in Aquatic Boreal Environments

“…Overall, in freshwater systems, the concentration of mercury compounds increases concomitantly with the increasing concentration of OM (Wetzel, 2001). Specifically, MeHg and/or total mercury (T-Hg) concentrations have been shown to be positively related to DOC (dissolved organic carbon), and more precisely to the concentration of aromatic molecules in the OM pool (Watras et al, 1998;Braaten et al, 2014;Lavoie et al, 2019;De Bonville et al, 2020), with the latter often considered to be of allochthonous origin (Lapierre and Del Giorgio, 2014). In fact, Hg has a high affinity for molecules containing reduced sulfur such as thiols, and the availability of these molecules in the system may control Hg-DOC associations and co-transport (Skyllberg et al, 2003;Ravichandran, 2004;French et al, 2014).…”