Matthews, Cory;
Ruiz-Cooley, Iliana;
Pomerleau, Corinne;
Ferguson, Steven
—
2021-02-10
<ol>
<li>Compound specific stable isotope analysis (CSIA) of amino acids (AAs) has been rapidly incorporated in ecological studies to resolve consumer trophic position (TP). Differential <sup>15</sup>N fractionation of ‘trophic’ AAs, which undergo <sup>15</sup>N enrichment with each trophic step, and ‘source’ AAs, which undergo minimal trophic <sup>15</sup>N enrichment and serve as a proxy for primary producer δ<sup>15</sup>N values, allows for internal calibration of TP. Recent studies, however, have shown the difference between source and trophic AA δ<sup>15</sup>N values in higher marine consumers is less than predicted from empirical studies of invertebrates and fish.</li>
<li>To evaluate CSIA-AA for estimating TP of cetaceans, we compared source and trophic AA δ<sup>15</sup>N values of multiple tissues (skin, baleen, and dentine collagen) from five species representing a range of TPs: bowhead whales, beluga whales, short-beaked common dolphins, sperm whales, and fish-eating (FE) and marine mammal-eating (MME) killer whale ecotypes.</li>
<li>TP estimates (TP<sub>CSIA</sub>) using several empirically-derived equations and trophic discrimination factors (TDFs) were 1 to 2.5 trophic steps lower than stomach content-derived estimates (TP<sub>SC</sub>) for all species. Although TP<sub>CSIA</sub> estimates using dual TDF equations were in better agreement with TP<sub>SC</sub> estimates for bowhead whales, belugas, and FE killer whales, our data do not support the application of a universal or currently available dual TDFs to estimate cetacean TPs. Discrepancies were not simply due to inaccurate TDFs, however, because the difference between consumer glutamic acid (Glu) and phenylalanine (Phe) δ<sup>15</sup>N values (δ<sup>15</sup>N<sub>Glu-Phe</sub>) did not follow expected TP order, indicating it is not a reliable index of relative TP in these species.</li>
<li>In contrast with pioneering studies on invertebrates and fish, our data suggest trophic <sup>15</sup>N enrichment of Phe is not negligible and should be examined among the potential mechanisms driving ‘compressed’ and variable δ<sup>15</sup>N<sub>Glu-Phe</sub> values at high TPs. We emphasize the need for controlled diet studies to clearly understand mechanisms driving AA-specific isotopic fractionation before widespread application of CSIA-AA in ecological studies of cetaceans and other marine consumers.</li>
</ol>
