Stable Mercury Isotopes Research Project

Evaluating sources and bioaccumulation of Hg within aquatic food webs using stable mercury isotopes

(B.A.Bergquist)

Position: 1 PhD

Background: Mercury is a globally distributed pollutant that bioaccumulates in aquatic food webs leading to dangerous exposure to humans and wildlife. Despite decades of research, many knowledge gaps hinder our understanding of both the modern and past Hg cycle and make it challenging to predict how changes in emissions, climate and land use will affect the Hg. In aquatic systems, Hg is transformed into methylmercury (MeHg), the form of Hg that bioaccumulates in aquatic food webs and poses a health risk for humans and wildlife via fish consumption. Methylation in the environment can happen in many environments and the links between Hg sources, methylation and MeHg in biota are not easy to trace. Linking Hg sources to MeHg in biota is challenging for several reasons. First, Hg from sources can be globally distributed and often deposited and re-emitted several times. Second, the aquatic ecosystems vulnerable to high MeHg in biota are systems where there are conditions favorable to methylation and can be far from point sources. Third, aquatic ecosystems can have several sources of MeHg from the watershed and also in situ methylation within water bodies. Mercury isotopes are a powerful addition to the tools used to understand bioaccumulation, especially in aquatic ecosystems where there are multiple pools of MeHg. The success of Hg isotopes is because the different pools of MeHg often have very different Hg MIF signatures, both the extent and the slope between the MIF of different isotopes. Also, the MIF signatures are not altered during trophic transfer and elimination and are preserved in the food web.

Objectives and Methodology: The long-term goal of this project is to develop Hg isotopes as a tool for tracing sources of Hg to biota in a variety of aquatic ecosystems. This will start with the sub-Arctic estuarine system of Lake Melville in Labrador, and other field areas will be identified as the project develops. Understanding MeHg bioaccumulation in Lake Melville is vital as the ecosystem is a source of fish for the indigenous people in the area. New hydroelectric developments are proposed and the area will also be impacted by climate change, which will effect Hg cycling and methylation. A recent study, using direct measurements and modelling, suggested that the largest source of MeHg in the Lake Melville estuary was in stratified oxic surface water where terrestrial organic matter inputs stimulated methylation and stratification restricted plankton to the same waters where methylation was high. Because much of the Arctic shelf has large freshwater inputs, this could be a common process in Arctic coastal waters. Mercury isotopes will be used to prove whether the oxic surface pool of MeHg is a major source to biota. We will also sample at multiple trophic levels and include several high trophic level species in order to identify what species have foraging behaviours that make them sensitive to the oxic surface pool of MeHg.

In addition to field applications, The PhD will perform Hg and MeHg photoreduction experiments in seawater with marine organic matter. Many successful studies have used Hg isotopes to understand Hg bioaccumulation in marine systems, but the conclusions are tied to results from experiments performed with freshwater matrices augmented with terrestrial organic matter and pure organic ligands. There are no published experiments in seawater, and these experiments are greatly needed in order to confirm many of the current interpretations (including our own) and to make full use of Hg isotope signatures in marine biota.

Relevant References:

Bergquist BA & Blum JD (2009) The odds and evens of mercury isotopes: applications of mass-dependent and mass-independent isotope fractionation. Elements 5(6):353-357.

Blum JD, Popp BN, Drazen JC, Anela CC, & Johnson MW (2013) Methylmercury production below the mixed layer in the North Pacific Ocean. Nat. Geosci. 6(10):879-884.

Gehrke GE, Blum JD, Slotton DG, & Greenfield BK (2011) Mercury isotopes link mercury in San Francisco Bay forage fish to surface sediments. Environ. Sci. Technol. 45(4):1264-1270.

Point D, et al. (2011) Methylmercury photodegradation influenced by sea-ice cover in Arctic marine ecosystems. Nat. Geosci. 4(3):188-194.

Schartup AT, et al. (2015) Freshwater discharges frive high levels of methymercury in Arctic marine biota. Proc. Natl. Acad. Sci. USA 112(38):11789-11794.

Zheng W (PDF), Xie Z, & Bergquist BA (2015) Mercury stable isotopes in ornithogenic deposits as tracers of historical cycling of mercury in Ross Sea, Antarctica. Environ. Sci. Technol. 49(13):7623-7632.

Description: One or more graduate positions are available to study trace metals in the Arctic focusing on lead and iron isotopes with Professor B.A. Bergquist.