Abstract
Sea-ice declines in the European Arctic have led to substantial changes in marine food webs. To better understand the biological implications of these changes, we quantified the contributions of ice-associated and pelagic carbon sources to the diets of Arctic harp and ringed seals using compound-specific stable isotope ratios of fatty acids in specific primary producer biomarkers derived from sea-ice algae and phytoplankton. Comparison of fatty acid patterns between these 2 seal species indicated clear dietary separation, while the compound-specific stable isotope ratios of the same fatty acids showed partial overlap. These findings suggest that harp and ringed seals target different prey sources, yet their prey rely on ice and pelagic primary production in similar ways. From Bayesian stable isotope mixing models, we estimated that relative contributions of sympagic and pelagic carbon in seal blubber was an average of 69% and 31% for harp seals, and 72% and 28% for ringed seals, respectively. The similarity in the Bayesian estimations also indicates overlapping carbon sourcing by these 2 species. Our findings demonstrate that the seasonal ice-associated carbon pathway contributes substantially to the diets of both harp and ringed seals.
Highlights
Niche partitioning theory states that species coexisting in the same foraging space are expected to have different diets or resource use, effectively limiting competition (MacArthur 1958)
Of the 33 fatty acids found in all iPOM samples across stations, 3 dominated in mass contribution: 16:0, 16:1(n-7) and 20:5(n-3) (Fig. 3A, Table S1 in the Supplement at www.int-res.com/articles/suppl/ m675p181_supp.pdf)
In contrast to our expectations, the 2 seal species were strongly separated in all fatty acid patterns, indicating a significant degree of dietary niche separation
Summary
Niche partitioning theory states that species coexisting in the same foraging space are expected to have different diets or resource use, effectively limiting competition (MacArthur 1958). In the European Arctic, harp (Pagophilus groenlandicus) and ringed (Pusa hispida) seals are found in similar habitats for at least part of the year, along the sea-ice edge and in coastal Svalbard waters during spring and summer (Hamilton et al 2021). The overall reduction or loss of sea-ice primary production and its cumulative effects on food webs are currently unknown, though harp seals are showing declining body condition that is thought to be due to competition with other predators, including the large standing stock of Atlantic cod Gadus morhua, which has shifted northward in the Barents Sea region (Stenson et al 2020)
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