Diet-tissue Discrimination Research Articles

Stable isotope discrimination (δ13C and δ15N) between soft tissues of the green sea turtle Chelonia mydas and its diet

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 308:271-278 (2006) - doi:10.3354/meps308271 Stable isotope discrimination (δ13C and δ15N) between soft tissues of the green sea turtle Chelonia mydas and its diet Jeffrey A. Seminoff1,*, T. Todd Jones2, Tomoharu Eguchi1, David R. Jones2, Peter H. Dutton1 1NOAA-National Marine Fisheries Service, Southwest Fisheries Science Center, 8604 La Jolla Shores Drive, La Jolla, California 92037, USA 2Department of Zoology, University of British Columbia, 6270 University Boulevard, Vancouver, British Columbia V6T 1Z4, Canada *Email: jeffrey.seminoff@noaa.gov ABSTRACT: The foraging ecology and movements of vertebrate species have been increasingly studied via stable isotope analyses of small quantities of body tissues. However, the theoretical and experimental basis of this method remains poorly validated for most taxa despite numerous studies using these techniques in natural systems. In this study, we measured stable carbon and stable nitrogen diet–tissue discrimination (Δdt) in whole blood, red blood cells, blood plasma, and epidermis of 8 captive green turtles Chelonia mydas maintained on a control diet (41% protein, 12% lipids, 4% fiber) for 619 d. During the course of the study, mean straight carapace length increased from 45.2 ± 1.2 to 53.7 ± 2.1 cm, whereas mean body mass increased from 11.7 ± 0.7 to 19.9 ± 2.2 kg. Both diet and tissue isotope values remained constant throughout the study, indicating that diet–tissue equilibrium had been achieved. Whereas Δdt13C ranged from –1.11‰ (red blood cells) to +0.17‰ (epidermis), Δdt15N ranged from +0.22‰ (red blood cells) to +2.92‰ (blood plasma). These results contrast with the widely accepted discrimination factors of 0 to 1‰ for δ13C and 3 to 5‰ for δ15N. Comprising the initial enrichment factors available for green turtles, these data will be useful in future interpretations of field isotopic values for this species. KEY WORDS: Carbon · Chelonidae · Ectotherm · Fractionation · Isotope enrichment · Nitrogen · Reptilia Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 308. Online publication date: February 16, 2006 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2006 Inter-Research.

Turnover rates of nitrogen stable isotopes in the salt marsh mummichog, Fundulus heteroclitus, following a laboratory diet switch

Nitrogen stable isotopes are frequently used in ecological studies to estimate trophic position and determine movement patterns. Knowledge of tissue-specific turnover and nitrogen discrimination for the study organisms is important for accurate interpretation of isotopic data. We measured delta15 N turnover in liver and muscle tissue in juvenile mummichogs, Fundulus heteroclitus, following a laboratory diet switch. Liver tissue turned over significantly faster than muscle tissue suggesting the potential for a multiple tissue stable isotope approach to study movement and trophic position over different time scales; metabolism contributed significantly to isotopic turnover for both liver and muscle. Nitrogen diet-tissue discrimination was estimated at between 0.0 and 1.2 per thousand for liver and -1.0 and 0.2 per thousand for muscle. This is the first experiment to demonstrate a significant variation in delta15 N turnover between liver and muscle tissues in a fish species.

Carbon, nitrogen, and sulfur diet–tissue discrimination in mouse tissues

Stable-isotope ratios are increasingly being used to examine ecological questions pertaining to dietary choices, physiological status, and animal migration. It has been shown that animal tissues reflect the isotopic signature present in food, altered by a small reproducible fractionation value. The average diet–tissue discrimination for δ13C and δ15N is approximately 1‰ and 3‰, respectively, although the degree of diet–tissue discrimination may be affected by a range of factors and vary between organisms and tissue types. Although the average δ34S is approximately zero, the sulfur diet–tissue discrimination values have only been determined for a few organisms. It is necessary to determine accurate diet–tissue discrimination values between tissue and dietary components to have confidence in any food-web study or determination of diet quality. In this paper, we determine carbon, nitrogen, and sulfur diet–tissue discrimination values for whole blood, liver, skeletal muscle, heart, brain, and fat (carbon only) from adult mice (Mus musculus L., 1758) equilibrated on three diets with unique isotopic signatures for carbohydrate carbon and for protein carbon, nitrogen, and sulfur. These data will aid researchers in choosing tissues to be used to examine food-web changes over time.

Isotopic Discrimination between Food and Blood and Feathers of Captive Penguins: Implications for Dietary Studies in the Wild

Using measurements of naturally occurring stable isotopes to reconstruct diets or source of feeding requires quantifying isotopic discrimination factors or the relationships between isotope ratios in food and in consumer tissues. Diet-tissue discrimination factors of carbon ((13)C/(12)C, or delta (13)C) and nitrogen ((15)N/(14)N, or delta (15)N) isotopes in whole blood and feathers, representing noninvasive sampling techniques, were examined using three species of captive penguins (king Aptenodytes patagonicus, gentoo Pygoscelis papua, and rockhopper Eudyptes chrysocome penguins) fed known diets. King and rockhopper penguins raised on a constant diet of herring and capelin, respectively, had tissues enriched in (15)N compared to fish, with discrimination factors being higher in feathers than in blood. These data, together with previous works, allowed us to calculate average discrimination factors for (15)N between whole lipid-free prey and blood and feathers of piscivorous birds; they amount to +2.7 per thousand and +4.2 per thousand, respectively. Both fish species were segregated by their delta (13)C and delta (15)N values, and importantly, lipid-free fish muscle tissue was consistently depleted in (13)C and enriched in (15)N compared to whole lipid-free fish. This finding has important implications because previous studies usually base dietary reconstructions on muscle of prey rather than on whole prey items consumed by the predator. We tested the effect of these differences using mass balance calculations to the quantification of food sources of gentoo penguins that had a mixed diet. Modeling indicated correct estimates when using the isotopic signature of whole fish (muscle) and the discrimination factors between whole fish (muscle) and penguin blood. Conversely, the use of isotopic signatures of muscle together with discrimination factors between whole fish and blood (or the reverse) leads to spurious estimates in food proportions. Consequently, great care must be taken in the choice of isotopic discrimination factors to apply to wild species for which no controlled experiments on captive individuals have been done. Finally, our results also indicate that there is no need to remove lipids before isotopic analysis of avian blood.

Effects of diet change on carbon and nitrogen stable-isotope ratios in blood cells and plasma of the long-nosed bandicoot (Perameles nasuta)

Carbon (C) and nitrogen (N) stable isotopes offer a powerful tool for assessing the extent of tissue assimilation of dietary components. However, the method relies on knowledge of diet–tissue isotopic discrimination and how quickly diet shifts become apparent in various tissues. In the present study, blood plasma and blood cells, tissues that are easily obtained under field conditions, were used to validate the stable isotope method over a period of 4–5 weeks using captive long-nosed bandicoots (Perameles nasuta). Diet–tissue discrimination effects appeared to be small. For C, derived diet–tissue isotopic discriminations were 1.4‰ for blood plasma and –0.2‰ for blood cells. For N the values were 2.8‰ and 2.1‰, respectively, and were independent of the nitrogen content of the food. C and N turnover measurements in the blood plasma and cells of the bandicoots indicated that blood plasma provides dietary information integrated over a period of ~3 weeks, whereas blood cells give an impression of the assimilated diet over a period of as much as half a year. These turnover rates were low compared with the little information available for birds and eutherian mammals, and probably relate to the typically low metabolic rate of marsupials.

Effects of elemental composition on the incorporation of dietary nitrogen and carbon isotopic signatures in an omnivorous songbird

The use of stable isotopes to infer diet requires quantifying the relationship between diet and tissues and, in particular, knowing of how quickly isotopes turnover in different tissues and how isotopic concentrations of different food components change (discriminate) when incorporated into consumer tissues. We used feeding trials with wild-caught yellow-rumped warblers (Dendroica coronata) to determine delta15N and delta13C turnover rates for blood, delta15N and delta13C diet-tissue discrimination factors, and diet-tissue relationships for blood and feathers. After 3 weeks on a common diet, 36 warblers were assigned to one of four diets differing in the relative proportion of fruit and insects. Plasma half-life estimates ranged from 0.4 to 0.7 days for delta13C and from 0.5 to 1.7 days for delta15N . Half-life did not differ among diets. Whole blood half-life for delta13C ranged from 3.9 to 6.1 days. Yellow-rumped warbler tissues were enriched relative to diet by 1.7-3.6% for nitrogen isotopes and by -1.2 to 4.3% for carbon isotopes, depending on tissue and diet. Consistent with previous studies, feathers were the most enriched and whole blood and plasma were the least enriched or, in the case of carbon, slightly depleted relative to diet. In general, tissues were more enriched relative to diet for birds on diets with high percentages of insects. For all tissues, carbon and nitrogen isotope discrimination factors increased with carbon and nitrogen concentrations of diets. The isotopic signature of plasma increased linearly with the sum of the isotopic signature of the diet and the discrimination factor. Because the isotopic signature of tissues depends on both elemental concentration and isotopic signature of the diet, attempts to reconstruct diet from stable isotope signatures require use of mixing models that incorporate elemental concentration.

Factors that influence assimilation rates and fractionation of nitrogen and carbon stable isotopes in avian blood and feathers.

By switching great skuas Catharacta skua from one isotopically distinct diet to another, we measured diet-tissue discrimination factors and tested the assumption that dietary nitrogen and carbon isotope signatures are incorporated into blood and feathers at similar rates. We also examined the effects of metabolic rate and looked for evidence of isotopic routing. We found that blood delta(15)N and delta(13)C signatures altered after the diet switch at similar rates (14.4 d and 15.7 d, respectively). Qualitative analyses imply that the same was true with feathers. Mass balance calculations suggest that only a small amount of lipid is likely to be involved in the synthesis of blood and feathers. Differences in diet-tissue discrimination factors before and after the diet switch may mean that toward the end of the experiment, some of the nutrients for blood synthesis had been coming from stores. Repeated measures mixed models provided evidence that increases in metabolic rate might accelerate fractional turnover rates in blood. There is a need for more laboratory-based experimental isotope studies in order to address further questions that this study has raised.