High Mercury levels: are Arctic seals “what” or “where” they eat?

In contrast to other regions of the world or even sympatric terrestrial species, Arctic marine predators continue to accumulate increasing levels of Mercury (Hg) in their tissues. Hg bioaccumulation in Arctic seals may be linked to their particular life style, or to their extreme physiological adaptations, such as a short period of lactation with very fatty milk. The present study aimed at assessing how dietary resources and hunting distribution influence Hg exposure in Arctic true seals, through the integration of isotopic tracers with Hg levels. Indeed stable Carbon, Nitrogen and Sulfur isotope ratios can be successfully used to study species’ ecology and indicate potential contamination sources.
For this reason hair was sampled from free-ranging hooded seal Cystophora cristata (Cc, n = 25) and harp seal Phoca groenlandicus (Pg, n = 36) in the pack ice of the Greenland Sea (near Jan Mayen). Stable isotope ratios were acquired via Isotope Ratio – Mass Spectrometry and used to model stable isotope niches (Standard Ellipses Areas; SEAs). Iterative Bayesian estimations were used to calculate the % of overlap between the ellipses. Total-Hg (T-Hg) concentrations were measured via Atomic absorption spectroscopy.
The Cc δ15N-δ13C SEA (3.02‰2) was larger than that of Pg (2.64‰2) in 69% of model runs and did not overlap (22%). This may reflect Cc wide migrations down to warmer sub-Arctic waters compared to Pg that have an exclusively Arctic distribution. Moreover, while Cc hunt for a variety of bentho-pelagic prey (e.g., halibut, redfish, cod and squid) during long dives down to 1000m, Pg feed mostly on pelagic schooling fish between 100 and 400m of depth. The Cc δ15N-δ34S SEA (21‰2) was also larger than that of Pg (16‰2) in 85% of model runs; but this time the ellipses overlapped considerably (52%). Indeed both species presented two distinct groups along the δ34S axis: the most 34S enriched group included adult individuals, while the 34S depleted one included juveniles. This may result from the shallower hunting behavior of juvenile seals and their reliance on ice food webs. Finally, the larger migratory patterns and deep feeding behavior of Cc seem to determine significantly higher levels of T-Hg levels in this species (3.2±3.6 µg g-1) with respect to Pg (1.7±0.9µg g-1; U = 322, P = 0.01), as a consequence of (1) the higher number of Hg sources in sub-Arctic waters and (2) Hg remobilization from the sea bottom and its uptake by benthic food webs.