Concentration and isotopic composition of mercury in a blackwater river affected by extreme flooding events

Abstract

Torrential rain and extreme flooding caused by Atlantic hurricanes mobilize a large pool of organic matter (OM) from coastal forested watersheds in the southeastern United States. However, the mobilization of toxic metals such as mercury (Hg) that are associated with this vast pool of OM are rarely measured. This study aims to assess the variations of total Hg (THg) and methylmercury (MeHg) levels and the isotopic compositions of Hg in a blackwater river (Waccamaw River, SC, U.S.A.) during two recent extreme flooding events induced by Hurricane Joaquin (October 2015) and Hurricane Matthew (October 2016). We show that extreme flooding considerably increased filtered THg and MeHg concentrations associated with aromatic dissolved organic matter. During a 2‐month sampling window each year (October–November), we estimate that about 27% (2015) and 78% (2016) of the average amount of Hg deposited atmospherically during these 2 months was exported via the river. The isotopic composition of Hg in the river waters was changed only slightly by the substantial inputs of runoff from surrounding landscapes, in which mass‐dependent fractionation (as δ202Hg) decreased from −1.47 to −1.67‰ and mass‐independent fractionation (as ∆199Hg) decreased from −0.15 to −0.37‰. The slight variations in Hg isotopic composition during such extreme flooding events imply that sources of Hg in the river are nearly unchanged even under the very high wet deposition of Hg derived from the intensive rainfall. The majority of Hg exported by the river (74–85%) is estimated to have been derived from dry deposition to the watersheds. An increase in frequency and intensity of Atlantic hurricanes is expected in the next few decades due to further warming of ocean surface waters. We predict that increased hurricanes will mobilize more dry‐deposited Hg and in situ produced MeHg from these coastal watersheds where MeHg can be extensively bioaccumulated and biomagnified in the downstream aquatic food webs.

Internal Dynamics and Metabolism of Mercury in Biota: A Review of Insights from Mercury Stable Isotopes

“…This can be attributed to the difficulty in collecting enough biomass and amount of Hg to perform high-precision Hg isotope analysis. 147 Further, the highly variable fraction of MeHg present as total Hg (%MeHg between 0.7 and 100% in forest floor food webs 146 ) makes it difficult to interpret the Hg isotope data of total Hg because the isotope ratios of iHg and MeHg could be drastically different even in a single region or ecosystem. For example, in a study on stream and forest food webs in a northern California watershed, Tsui et al (2012) 42 demonstrated that %MeHg is significantly correlated with δ 202 Hg and Δ 199 Hg in each food web.…”

“…Similar to phytoplankton, measurements of Hg isotope ratios and the fractionation occurring in invertebrates are not widely studied in comparison to animal consumers of higher trophic levels. This can be attributed to the difficulty in collecting enough biomass and amount of Hg to perform high-precision Hg isotope analysis . Further, the highly variable fraction of MeHg present as total Hg (%MeHg between 0.7 and 100% in forest floor food webs) makes it difficult to interpret the Hg isotope data of total Hg because the isotope ratios of iHg and MeHg could be drastically different even in a single region or ecosystem.…”

Challenges in quantifying and characterizing dissolved organic carbon: Sampling, isolation, storage, and analysis

“…In addition, DOC can form extremely strong complexes with metals, not only enhancing metal solubility and mobility but also affecting its bioavailability and toxicity (Aiken et al., 2011; Baken et al., 2011). For example, strong correlations between total mercury and DOC concentrations were reported worldwide, including in blackwater streams in the southeastern United States (Tsui et al., 2020), runoff after wildfires in California (Ku et al., 2018), and other ecosystems (Ravichandran, 2004). Uptake of metals by aquatic organisms can also depend on the characteristics of DOC (Baken et al., 2011).…”

Using Carbon, Nitrogen, and Mercury Isotope Values to Distinguish Mercury Sources to Alaskan Lake Trout

“…Carbon stable isotope values (δ 13 C) change very little through trophic linkages and act as indices of foraging behavior and fish habitat . Stable isotope values of nitrogen (δ 15 N) undergo predictable trophic enrichment from prey to predators, allowing us to estimate trophic position. , While information regarding MeHg bioaccumulation and biomagnification can be inferred from δ 13 C and δ 15 N analyses, the connection of Hg sources through dietary pathways is best assessed by including Hg stable isotope analyses. ,,, …”

“…12,13 While information regarding MeHg bioaccumulation and biomagnification can be inferred from δ 13 C and δ 15 N analyses, the connection of Hg sources through dietary pathways is best assessed by including Hg stable isotope analyses. 1,12,14,15 Combining δ 13 C and δ 15 N with Hg stable isotope values facilitates determination of Hg sources and energy pathways to fish, but less commonly tested is the connection between fish habitat use and Hg sources in ecosystems where heterogeneity of Hg sources exists. 12 Mass-dependent fractionation (MDF) of Hg isotopes (represented as δ 202 Hg) enables inferences about Hg sources 16−18 and reaction processes.…”