Higher Mass-Independent Isotope Fractionation of Methylmercury in the Pelagic Food Web of Lake Baikal (Russia)

Abstract

Mercury undergoes several transformations that influence its stable isotope composition during a number of environmental and biological processes. Measurements of Hg isotopic mass-dependent (MDF) and mass-independent fractionation (MIF) in food webs may therefore help to identify major sources and processes leading to significant bioaccumulation of methylmercury (MeHg). In this work, δ(13)C, δ(15)N, concentration of Hg species (MeHg, inorganic Hg), and stable isotopic composition of Hg were determined at different trophic levels of the remote and pristine Lake Baikal ecosystem. Muscle of seals and different fish as well as amphipods, zooplankton, and phytoplankton were specifically investigated. MDF during trophic transfer of MeHg leading to enrichment of heavier isotopes in the predators was clearly established by δ(202)Hg measurements in the pelagic prey-predator system (carnivorous sculpins and top-predator seals). Despite the low concentrations of Hg in the ecosystem, the pelagic food web reveals very high MIF Δ(199)Hg (3.15-6.65‰) in comparison to coastal fish (0.26-1.65‰) and most previous studies in aquatic organisms. Trophic transfer does not influence MIF signature since similar Δ(199)Hg was observed in sculpins (4.59 ± 0.55‰) and seal muscles (4.62 ± 0.60‰). The MIF is suggested to be mainly controlled by specific physical and biogeochemical characteristics of the water column. The higher level of MIF in pelagic fish of Lake Baikal is mainly due to the bioaccumulation of residual MeHg that is efficiently turned over and photodemethylated in deep oligotrophic and stationary (i.e., long residence time) freshwater columns.

Investigation of Hg uptake and transport between paddy soil and rice seeds combining Hg isotopic composition and speciation

“…Regarding Hg isotopic composition, it was determined by cold-vapor MC-ICP-MS (Nu Plasma, Nu Instruments) as detailed elsewhere (Perrot et al, 2012). Analytical uncertainty was determined as the largest 2SD corresponding to the sample or to the measurement of the UM-Almadén secondary standard during each session.…”

Quantifying mercury isotope dynamics in captive Pacific bluefin tuna (Thunnus orientalis)

“…By combining MDF and MIF, previous studies have identified sources and gained insight into complex biogeochemical processes governing MMHg in many natural ecosystems (Gantner et al, 2009;Senn et al, 2010;Gehrke et al, 2011;Point et al, 2011;Day et al, 2012;Perrot et al, 2012;Tsui et al, 2012Tsui et al, , 2013Tsui et al, , 2014Blum et al, 2013;Demers et al, 2007;Kwon et al, 2014Kwon et al, , 2015.…”

“…Particularly in aquatic ecosystems, the analysis of Hg isotope ratios in fish tissues has become a useful tool for monitoring complex biological processes such as bioaccumulation and trophic transfer, and consequently for inferring bioaccumulative sources of MMHg (Gantner et al, 2009;Senn et al, 2010;Gehrke et al, 2011;Point et al, 2011;Perrot et al, 2012;Tsui et al, 2012Tsui et al, , 2013Tsui et al, , 2014Blum et al, 2013;Kwon et al, 2014Kwon et al, , 2015. For instance, fish feeding experiments documented the absence of d 202 Hg and D 199 Hg fractionation during bioaccumulation and trophic transfer of both IHg and MMHg to juvenile freshwater (i.e., yellow perch) and marine fish (i.e., amberjack) .…”

“…This absence indicated that the isotopic composition in fish reflects that of fish prey, which in turn reflects environmental sources of Hg accumulated from the base of the food web and retained in fish tissues. By coupling traditional isotope tracers (i.e., d 13 C and d 15 N), previous studies also characterized Hg isotope ratios in a diverse range of species of aquatic organisms in order to distinguish between bioaccumulative sources of MMHg in various marine compartments (i.e., coastal versus pelagic, sediment versus water-column feeder) (Senn et al, 2010;Perrot et al, 2012). The applicability of Hg isotope ratios in fish tissues as a natural ecological tracer can be further expanded with additional experiments that quantify the retrospective time frames represented by measured fish Hg isotope ratios, assess the influence(s) of metabolic processes on the fractionation of Hg isotopes, and couple internal behaviors of Hg isotopes with traditional isotope tracers.…”

Recent advances in speciation analysis of mercury, arsenic and selenium

“…Species-specific Hg isotope fractionation of MeHg has recently been observed in biological samples [138,139] and in chemical transformation processes such as methylation [140] and demethylation [138,141,142]. The fingerprint of mass-dependent and -independent fractionation of MeHg in natural environmental samples was suggested to be a good indicator of important processes in the global cycle of MeHg and will be helpful to understand and quantify these processes [138].…”