Pelagic Mercury Dynamics Drive Bioaccumulation in Arctic Fjord Seals

Mercury (Hg) bioaccumulation in Arctic marine predators reflects the biogeochemical pathways governing methyl-Hg (MMHg) production and transformation within fjord ecosystems. The relative importance of different Hg sources driving predator exposure remains poorly resolved. We measured total Hg (T-Hg), MMHg and inorganic Hg (iHg), Hg stable isotope ratios (δ²⁰²Hg, Δ²⁰⁰Hg, Δ¹⁹⁹Hg; as ‰), and bulk carbon, nitrogen and sulphur stable isotope ratios (δ¹³C, δ¹⁵N, δ³⁴S) in environmental matrices (marine sediments), prey (phyto-, zooplankton and fish fillet), and tissues (hair and whole blood) of three true seal species—harbour (Phoca vitulina, n = 14), ringed (Pusa hispida, n = 5), and bearded seals (Erignathus barbatus, n = 3)—from Kongsfjorden (79°N, 12°E, Svalbard). Bulk δ¹³C, δ¹⁵N, δ³⁴S values separated prey associated with pelagic and benthic food webs, with seals largely overlapping with pelagic prey (δ¹³C ≈ −20.0 to −20.5‰), indicating dominant reliance on pelagic trophic pathways. Values of Δ²⁰⁰Hg overlapped broadly among prey and predators, suggesting a common atmospheric Hg source, whereas δ²⁰²Hg and Δ¹⁹⁹Hg differed significantly among taxa (KW = 139; p < 0.001), separating seals, benthic- and pelagic-associated prey. Δ¹⁹⁹Hg values of all three seal species fell within the range of pelagic prey (Δ¹⁹⁹Hg ≈ 0.8–1.8‰), confirming pelagic food webs as the primary route of Hg exposure. Harbour and ringed seals exhibited Δ²⁰¹Hg–Δ¹⁹⁹Hg slope values between 0.89 and 0.98 in both hair and blood, consistent with inorganic Hg photoreduction in the water column (slope ± SE = 1.00 ± 0.02). Bearded seals showed a lower slope (0.48 ± 0.08 in whole blood), suggesting incorporation of a Hg pool influenced by benthic or non-photochemical processes. Seasonal comparison revealed limited differences between hair and blood in harbour and bearded seals, indicating relatively stable exposure across seasons. Ringed seals displayed significantly higher Δ¹⁹⁹Hg range in hair (1.6–2.4‰) than in blood (0.9–1.5‰), reflecting seasonal shifts in Hg sources between spring and summer. Seal exposure to MMHg mirrors water-column photochemical processing. As Arctic warming alters sea-ice cover, light regimes and Atlantic inflow, shifts in pelagic–benthic MMHg cycling may rapidly reshape contaminant transfer to top predators.