Sulfur isotopes (δ³⁴S) in Arctic marine mammals: Indicators of benthic vs. pelagic foraging

Consumer tissue stable carbon isotope compositions (δ¹³C) are well-established indicators of benthic and pelagic foraging in marine ecosystems. Stable sulfur isotope compositions (δ³⁴S) arealso potentially useful in this regard but have not been widely utilized outside of estuaries and salt marsh ecosystems. To test the ability of δ¹³C and δ³⁴S to reflect benthic and pelagic foraging, we analyzed the stable carbon, nitrogen (δ¹⁵N), and sulfur isotope compositions of bone collagen from walrus (an obligate benthic feeder) and ringed seal (a mixed benthic/pelagic feeder) sampled from across the North American Arctic. Both had relatively low δ³⁴S values compared to those typically observed in marine consumers. These data suggest an important role for benthic microalgae in coastal marine food webs in the Arctic. At all of the nine sites where both taxa could be sampled, walrus had lower δ³⁴S values than ringed seals, suggesting that this measurement is a useful indicator of benthic and pelagic foraging in nearshore Arctic environments. Contrary to expectations, there were no consistent differences in δ¹³C between walrus and ringed seal at any of these sites, suggesting that this measurement may not always be best interpreted in light of benthic vs. pelagic foraging, particularly when comparisons are made across trophic levels. When the foraging ecology of a consumer is unknown, our data suggest that δ³⁴S may be a more sensitive indicator of the relative importance of benthic and pelagic prey in the diet than δ¹³C.