Data from: Amino acid δ15N underestimation of cetacean trophic positions highlights poor understanding of isotopic fractionation in higher marine consumers

Compound specific stable isotope analysis (CSIA) of amino acids (AAs) has
been rapidly incorporated in ecological studies to resolve consumer
trophic position (TP). Differential 15N fractionation of ‘trophic’ AAs,
which undergo 15N enrichment with each trophic step, and ‘source’ AAs,
which undergo minimal trophic 15N enrichment and serve as a proxy for
primary producer δ15N values, allows for internal calibration of TP.
Recent studies, however, have shown the difference between source and
trophic AA δ15N values in higher marine consumers is less than predicted
from empirical studies of invertebrates and fish. To evaluate CSIA-AA for
estimating TP of cetaceans, we compared source and trophic AA δ15N 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. TP estimates (TPCSIA) using
several empirically-derived equations and trophic discrimination factors
(TDFs) were 1 to 2.5 trophic steps lower than stomach content-derived
estimates (TPSC) for all species. Although TPCSIA estimates using dual TDF
equations were in better agreement with TPSC 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)
δ15N values (δ15NGlu-Phe) did not follow expected TP order, indicating it
is not a reliable index of relative TP in these species. In contrast with
pioneering studies on invertebrates and fish, our data suggest trophic 15N
enrichment of Phe is not negligible and should be examined among the
potential mechanisms driving ‘compressed’ and variable δ15NGlu-Phe 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.

氨基酸（amino acids, AAs）的化合物特异性稳定同位素分析（compound-specific stable isotope analysis, CSIA）已快速应用于生态学研究，以解析消费者的营养级位置（trophic position, TP）。营养型氨基酸会随每一级营养级发生15N富集，而源型氨基酸的15N营养级富集程度极低，可作为初级生产者δ15N值的代用指标，二者的15N分馏差异可实现营养级位置的内校准。不过近期研究发现，海洋高等消费者的源型与营养型氨基酸δ15N值之差，低于无脊椎动物和鱼类实证研究的预测值。

为评估CSIA-AA在估算鲸类营养级位置中的适用性，我们对比了涵盖不同营养级范围的5个鲸类物种的多种组织（皮肤、鲸须及牙本质胶原蛋白）的源型与营养型氨基酸δ15N值：包括弓头鲸、白鲸、短吻真海豚、抹香鲸，以及食鱼型（FE）和食海洋哺乳动物型（MME）虎鲸生态型。采用多种经验推导公式及营养分馏因子（trophic discrimination factors, TDFs）得到的营养级位置估算值（TPCSIA），较所有物种的胃含物法营养级估算值（TPSC）低1至2.5个营养级。尽管采用双TDF公式得到的TPCSIA估算值，与弓头鲸、白鲸及食鱼型虎鲸的TPSC估算值吻合度更高，但我们的研究结果并不支持采用通用或现有双TDF公式来估算鲸类的营养级位置。

不过这种偏差并非仅由不准确的TDFs导致：消费者的谷氨酸（glutamic acid, Glu）与苯丙氨酸（phenylalanine, Phe）的δ15N值之差（δ15NGlu-Phe）并未遵循预期的营养级顺序，表明该指标并非这些物种相对营养级的可靠指数。与针对无脊椎动物和鱼类的开创性研究不同，我们的数据表明苯丙氨酸的15N营养级富集并非可忽略不计，且应在驱动高营养级下被压缩且可变的δ15NGlu-Phe值的潜在机制中加以考察。我们强调，在将CSIA-AA广泛应用于鲸类及其他海洋消费者的生态学研究前，需开展受控膳食实验，以明确驱动氨基酸特异性同位素分馏的机制。