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Climate-mediated changes in the phenology of Arctic sea ice and primary production may alter benthic food webs that sustain populations of Pacific walruses (Odobenus rosmarus divergens) and bearded seals (Erignathus barbatus). Interspecific resource competition could place an additional strain on ice-associated marine mammals already facing loss of sea ice habitat. Using fatty acid (FA) profiles, FA trophic markers, and FA stable carbon isotope analyses, we found that walruses and bearded seals partitioned food resources in 2009-2011. Interspecific differences in FA profiles were largely driven by variation in non-methylene FAs, which are markers of benthic invertebrate prey taxa, indicating varying consumption of specific benthic prey. We used Bayesian multi-source FA stable isotope mixing models to estimate the proportional contributions of particulate organic matter (POM) from sympagic (ice algal), pelagic, and benthic sources to these apex predators. Proportional contributions of FAs to walruses and bearded seals from benthic POM sources were high [44 (17-67)% and 62 (38-83)%, respectively] relative to other sources of POM. Walruses also obtained considerable contributions of FAs from pelagic POM sources [51 (32-73)%]. Comparison of δC values of algal FAs from walruses and bearded seals to those from benthic prey from different feeding groups from the Chukchi and Bering seas revealed that different trophic pathways sustained walruses and bearded seals. Our findings suggest that (1) resource partitioning may mitigate interspecific competition, and (2) climate change impacts on Arctic food webs may elicit species-specific responses in these high trophic level consumers.

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<p><strong>Abstract.</strong> Microphytobenthos (MPB) tends to be omitted as a possible carbon source to higher trophic level consumers in high latitude marine food web models that use stable isotopes. Here, we used previously published relationships relating the concentration of aqueous carbon dioxide ([CO₂]aq), the stable carbon isotopic composition of dissolved inorganic carbon (DIC) (<i>δ</i>¹³C_DIC), and algal growth rates (<i>μ</i>) to estimate the stable carbon isotope composition of MPB-derived total organic carbon (TOC) (<i>δ</i>¹³C_<i>p</i>) and fatty acid (FA) biomarkers (<i>δ</i>¹³C_FA). We measured [CO₂]aq and <i>δ</i>¹³C_DIC values from bottom water at sampling locations in the Beaufort and Chukchi Seas (<i>n</i> = 18), which ranged from 17 to 72 mmol kg^&amp;ndash;1 and −0.1 to 1.4 &amp;permil; (0.8 ± 0.4&amp;permil;, mean ±1 s.d.), respectively. We combined these field measurements with a set of stable carbon isotopic fractionation factors reflecting differences in algal taxonomy and physiology to determine <i>δ</i>¹³C_<i>p</i> and <i>δ</i>¹³C_FA values. The<i>δ</i>¹³C_<i>p</i> and <i>δ</i>¹³C_FA values for a mixed eukaryotic algal community were estimated to be −23.6 ± 0.4&amp;permil; and −30.6 ± 0.4&amp;permil;, respectively. These values were similar to our estimates for <i>Phaeodactylum tricornutum</i> (<i>δ</i>¹³C_<i>p</i> = −23.9 ± 0.4&amp;permil;, <i>δ</i>¹³C_FA = −30.9 ± 0.4&amp;permil;), a pennate diatom likely to be a dominant MPB taxon. Taxon-specific differences were observed between a centric diatom (<i>Porosira glacialis</i>, <i>δ</i>¹³C_<i>p</i> = −20.0 ± 1.6&amp;permil;), a marine haptophyte (<i>Emiliana huxleyi</i>, <i>δ</i>¹³C_<i>p</i> = −22.7 ± 0.5&amp;permil;), and a cyanobacterium (<i>Synechococcus</i> sp., <i>δ</i>¹³C_<i>p</i> = −16.2 ± 0.4&amp;permil;) at <i>μ</i> = 0.1 d⁻¹. <i>δ</i>¹³C_<i>p</i> and <i>δ</i>¹³C_FA values increased by &amp;simeq; 2.5&amp;permil; for the mixed algal consortium and for <i>P. tricornutum</i> when growth rates were increased from 0.1 to 1.4 d⁻¹. We compared our estimates of <i>δ</i>¹³C_<i>p</i> and <i>δ</i>¹³C_FA values for MPB with previous measurements of <i>δ</i>¹³C_TOC and <i>δ</i>¹³C_FA values for other carbon sources in the Arctic, including ice-derived, terrestrial, and pelagic organic matter. We found that MPB values were significantly distinct from terrestrial and ice-derived carbon sources. However, MPB values overlapped with pelagic sources, which may result in MPB being overlooked as a significant source of carbon in the marine food web.</p>