| Summary: | Arctic predators like true seals have been experiencing a strong increase in Hg levels in their tissues in the last 150 years. This is in contrast with other terrestrial animals or other parts of the world. Dietary Hg accumulates and biomagnifies in marine food webs in its most toxic form, monomethylmercury (MMHg). Hg seven stables isotopes (196Hg, 198Hg, 199Hg, 200Hg, 201Hg, 202Hg and 204Hg) undergo both mass-dependent and mass-independent fractionation (MDF and MIF, respectively) as a result of abiotic and biotic reactions occurring in the environment and within the organism. For this reason, they are a promising tool for tracing Hg cycling in the natural environment. The interpretation of Hg isotopic data remain however a challenge, especially when taking into account species such as Arctic true seals, which have highly complex life styles and metabolism. In 2012, six pups of hooded seals Cystophora cristata were captured on the ice edge of the Greenland Sea and kept in captivity for 2 years. During this period, they were fed on a constant diet made of Norwegian herring Clupea harengus and vitamin complements. This allowed us to study Hg kinetic in an Arctic top predator without the influence of age, distribution and diet specialization. The main objective was to select the tissue in which the information about Hg pathways would be conserved, leading to the optimal tracing of Hg sources along the food web. Total Hg (THg) concentrations were determined on a Milestone direct mercury analyzer, while MeHg and IHg concentrations were determined by isotope dilution-gas chromatography-inductively coupled plasma-mass spectrometer (ID-GC-ICP-MS) following microwave-assisted extraction and aqueous phase derivatization. Mercury isotopic composition analysis was performed using cold vapor generation (CVG) with multicollector-inductively coupled plasma-mass spectrometer (MC-ICP-MS, Nu Instruments). The analysis was conducted in seal muscle, liver, hair, and kidney, plus muscle of herring. Hg speciation changed ...
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