Mercury levels in top predators as a valuable marker of environmental state and potential health risk to Arctic marine biota

Mercury (Hg) in Arctic biota is increasing in contrast with trends in the rest of the world. In top predators, tissue levels surpassed the established toxicity thresholds. New research has revealed how the Arctic Hg cycle has altered because of sea-surface temperature increase and sea-ice cover decline. True seals are Arctic top predators. As such, Hg level in their tissue may represent a valuable integrator for changes in Hg cycling in their food chain. Our objective was to assess how the health risk associated with Hg exposure has evolved in the last 20 years in response to environmental changes. We measured Total-Hg levels in liver of hooded seals Cystophora cristata (N = 10), harp seals Pagophilus groenlandicus (N = 13) and ringed seals Pusa hispida (N = 24) through Atomic Absorption Spectrometry (DMA-80 Milestone). We grouped seals in five toxicity risk classes and compared them to the results of Dietz et al. (2005), based upon effect threshold categories calculated for harp seals by Ronald et al. (1977). Overall, hooded seals presented the highest values, followed by ringed seals and harp seals (all p < 0.05). 100% of harp seals (all ages) were in the “no effect” category, as well as yearlings of hooded and harp seals. 22% of sub-adults and 45% of adult ringed seals belonged to the “low risk” category. Conversely, all hooded seals resulted at risk with 100% of sub-adults and 25% of adults in the “low risk” class, 25% of adults in the “high risk” class and 50% of adults in the “severe risk” class. Our classification resulted very different from the 2000s when only 20% of the hooded seals’ population was at “high risk” and 20% of ringed seals was at “low risk”. This shows that Arctic true seals are at increasing toxicity risk as a consequence of the undergoing environmental changes, with some species (hooded seal) being more affected than others (harp seal). The change in length of ice season may have determined a shift in prey diversity, modifying levels of Hg exposure to seals, while the decrease in ice cover altered rates of Hg methylation in the water column and consequently its bioavailability. Our future goal is to correlate ice-cover data with seals’ Hg levels in the last 100 years, to confirm the link between climate change and Hg accumulation in seals and potentially foresee future trends of toxicity risk in Arctic top predators.