Nitrogen isotope fractionation in amino acids from harbor seals: implications for compound-specific trophic position calculations

Abstract

Compound-specific isotope analysis of individual amino acids (AA) is a rapidly growing tool in ecological studies to assess diet and trophic position (TP) in both modern and ancient foodwebs. We conducted the first controlled feeding study examining δ 15 N values in AAs in a marine mammal (harbor seal Phoca vitulina). The pattern of δ 15 N variation among AAs in seals was similar to that observed in other heterotrophs, although exceptions were found with proline and threonine. However, many δ 15 N changes with trophic transfer were very different than those reported for zooplankton and other lower TP marine consumers. In particular the measured trophic enrichment factor (TEF) now broadly used for TP estimation (TEF Glu-Phe) was much lower in harbor seals (~4.3 ‰) than the current commonly applied value (~7.5 ‰). Recently published data on wild marine birds (penguins) and elasmobranchs (stingrays) suggests that similar, low TEF values may also be characteristic of these taxa. Together, these data imply that marine mammals and other higher animals have different, but also diagnostic, changes in δ 15 N-AA with trophic transfer vs. organisms examined in previous feeding studies (e.g. zooplankton, bony fish and mollusks), possibly due to dietary protein content, trophic position, and/or form of nitrogen excretion (urea vs. ammonia). Therefore, we propose that for marine mammals, a multi-TEF calculation is required to account for variations of TEF between animals within a food web, and we demonstrate that this approach can predict accurate TP estimates for harbor seals. These results also have significant implication for the application of compound-specific isotope analysis of AAs on terrestrial ecology and trophic structure.

Spatial food‐web structure in the eastern tropical Pacific Ocean based on compound‐specific nitrogen isotope analysis of amino acids

“…Underestimated TPs based on CSIA‐AA have also been reported for penguins, two elasmobranchs, and the harbor seal (Lorrain et al ; Dale et al ; Germain et al ; Hoen et al ; McMahon et al ,b). TP estimates from our study support the contention of previous work that a TDF of 7.6‰, based on Chikaraishi et al (), is adequate only for species with TPs < 3.0 (Chikaraishi et al ; Hannides et al ; Dale et al ; Germain et al ; Bradley et al ). We empirically estimated TDF values using the methodology of Bradley et al () and found decreasing TDF values with increasing TPs.…”

“…Amino acids were grouped into two categories, trophic amino acids: alanine (Ala), aspartic acid (Asp), glutamic acid (Glu), leucine (Leu), and proline (Pro); and source amino acids: glycine (Gly), lysine (Lys), phenlyalanine (Phe), serine (Ser), and threonine (Thr). Since Thr values are extremely negative relative to other source AAs, we recognize that including Thr as a source AA may not be appropriate and recent studies are classifying Thr as a “metabolic” amino acid (Germain et al ; Bradley et al ).…”

Spatial segregation and similar trophic-level diet among eastern Canadian Arctic/north-west Atlantic killer whales inferred from bulk and compound specific isotopic analysis

“…This calculation was derived largely from experimental studies on invertebrates and fish, and recent studies have suggested that a TEF Glu-Phe of 7.6‰ results in lower than anticipated trophic position estimates of higher marine consumers (Lorrain et al, 2009;Dale et al, 2011), including marine mammals (Germain et al, 2013). Given equation (1) Best-fit generalized linear mixed effects models indicated GLG (age) was a significant predictor of d 15 N values (P , 0.005), but not d 13 C cor values (P .…”

Stable isotopes dissect aquatic food webs from the top to the bottom

“…This allowed them to estimate trophic levels for zooplankton and other consumers, and to derive the δ 15 N of basal resources. However, the trophic enrichment between glutamic acid and phenylalanine appears to be different for herbivory (plant-animal; 7.6 ‰) and carnivory (animalanimal; 4.3‰), complicating inferences based on compoundspecific δ 15 N analysis (Germain et al, 2013). Basal resources δ 15 N values are notoriously difficult to obtain because of high temporal variability due to intense recycling and assimilation of dissolved inorganic nitrogen by heterotrophic bacteria (Kirchman, 1994;Veuger et al, 2005).…”