Understanding sources of methylmercury in songbirds with stable mercury isotopes: Challenges and future directions

Abstract

Mercury (Hg) stable isotope analysis is an emerging technique that has contributed to a better understanding of many aspects of the biogeochemical cycling of Hg in the environment. However, no study has yet evaluated its usefulness in elucidating the sources of methylmercury (MeHg) in songbird species, a common organism for biomonitoring of Hg in forested ecosystems. In the present pilot study, we examined stable mercury isotope ratios in blood of 4 species of songbirds and the invertebrates they are likely foraging on in multiple habitats in a small watershed of mixed forest and wetlands in Acadia National Park in Maine (USA). We found distinct isotopic signatures of MeHg in invertebrates (both mass-dependent fractionation [as d 202 Hg] and mass-independent fractionation [as D 199 Hg]) among 3 interconnected aquatic habitats. It appears that the Hg isotopic compositions in bird blood cannot be fully accounted for by the isotopic compositions of MeHg in lower trophic levels in each of the habitats examined. Furthermore, the bird blood isotope results cannot be simply explained by an isotopic offset as a result of metabolic fractionation of d 202 Hg (e.g., internal demethylation). Our results suggest that many of the birds sampled obtain MeHg from sources outside the habitat they were captured in. Our findings also indicate that massindependent fractionation is a more reliable and conservative tracer than mass-dependent fractionation for identifying sources of MeHg in bird blood. The results demonstrate the feasibility of Hg isotope studies of songbirds but suggest that larger numbers of samples and an expanded geographic area of study may be required for conclusive interpretation. Environ Toxicol Chem 2018;37:166-174. C 2017 SETAC

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

“…To our knowledge, there have only been a few reports in the literature of negative odd-MIF of MeHg found in biota. One recent study reported slightly negative odd-MIF (Δ 199 Hg ranging from −0.28 to −0.20‰) in several soil-dwelling invertebrate species from a wetland habitat in Acadia National Park in Maine, U.S.A. (Tsui et al 2018). Even though our study river system receives extensive sunlight due to the wide channel width, aqueous MeHg is highly "shielded" from photodegradation since UV radiation is highly attenuated by the high levels of DOM in blackwater rivers (Morris et al 1995) and the associated forested wetland habitats.…”

Controls of Methylmercury Bioaccumulation in Forest Floor Food Webs

“…Individual samples were extracted for MeHg using 4.6 M nitric acid at 60 °C for 12 h, 33 and the remaining acid digest was then completely oxidized by KMnO 4 and K 2 S 2 O 8 for subsequent THg analysis. 34 Sample MeHg or THg was quantified by cold vapor atomic fluorescence spectroscopy (Brooks Rand; for both MeHg and THg) or cold vapor atomic absorption spectroscopy (Nippon Instruments Corporation; for THg analysis on samples collected in 2011 and 2012). All acid digestions for THg and MeHg analyses were accompanied by standard reference materials (SRMs; NIST-1515 Apple Leaves, NRCC TORT-2 lobster hepatopancreas, NRCC DORM-3 and DORM-4 fish protein) and reagent blanks.…”

“…Individual samples were extracted for MeHg using 4.6 M nitric acid at 60 °C for 12 h, and the remaining acid digest was then completely oxidized by KMnO 4 and K 2 S 2 O 8 for subsequent THg analysis . Sample MeHg or THg was quantified by cold vapor atomic fluorescence spectroscopy (Brooks Rand; for both MeHg and THg) or cold vapor atomic absorption spectroscopy (Nippon Instruments Corporation; for THg analysis on samples collected in 2011 and 2012).…”

The impact of mercury on North American songbirds: effects, trends, and predictive factors

“…1 Watershed Hg sensitivity connects landscape features related to Hg input and transport, methylation potential, and food web transfer dependent on a number of factors including: (1) objective (e.g., to identify spatiotemporal trends or assess conservation concern), (2) ecological attributes such as landscapes occupied (e.g., breeding, migration and non-breeding), and geographical distribution across its annual life cycle, and (3) potential ability for bioaccumulation over the individual's lifetime. The choice of indicator biota is actively being assessed now to fulfill the long-term obligations of a global multilateral treaty, the Minamata Convention on Mercury (Evers and Sunderland 2019), and to connect biotic dietary uptake of MeHg with differing source types of Hg (using Hg isotopes; Tsui et al 2018). The findings of this special issue significantly further that goal.…”