Tissue Turnover Rates and Isotopic Trophic Discrimination Factors in the Endothermic Teleost,Pacific Bluefin Tuna (Thunnus orientalis)

Stable isotope analysis (SIA) of highly migratory marine pelagic animals can improve understanding of their migratory patterns and trophic ecology. However, accurate interpretation of isotopic analyses relies on knowledge of isotope turnover rates and tissue-diet isotope discrimination factors. Laboratory-derived turnover rates and discrimination factors have been difficult to obtain due to the challenges of maintaining these species in captivity. We conducted a study to determine tissue- (white muscle and liver) and isotope- (nitrogen and carbon) specific turnover rates and trophic discrimination factors (TDFs) using archived tissues from captive Pacific bluefin tuna (PBFT), Thunnus orientalis, 1–2914 days after a diet shift in captivity. Half-life values for ¹⁵N turnover in white muscle and liver were 167 and 86 days, and for ¹³C were 255 and 162 days, respectively. TDFs for white muscle and liver were 1.9 and 1.1‰ for δ ¹⁵N and 1.8 and 1.2‰ for δ ¹³C, respectively. Our results demonstrate that turnover of ¹⁵N and ¹³C in bluefin tuna tissues is well described by a single compartment first-order kinetics model. We report variability in turnover rates between tissue types and their isotope dynamics, and hypothesize that metabolic processes play a large role in turnover of nitrogen and carbon in PBFT white muscle and liver tissues. ¹⁵N in white muscle tissue showed the most predictable change with diet over time, suggesting that white muscle δ ¹⁵N data may provide the most reliable inferences for diet and migration studies using stable isotopes in wild fish. These results allow more accurate interpretation of field data and dramatically improve our ability to use stable isotope data from wild tunas to better understand their migration patterns and trophic ecology.     

Trophic Enrichment Factors for Blood Serum in the European Badger (Meles meles)

Ecologists undertaking stable isotopic analyses of animal diets require trophic enrichment factors (TEFs) for the specific animal tissues that they are studying. Such basic data are available for a small number of species, so values from trophically or phylogenetically similar species are often substituted for missing values. By feeding a controlled diet to captive European badgers (Meles meles) we determined TEFs for carbon and nitrogen in blood serum. TEFs for nitrogen and carbon in blood serum were +3.0±0.4‰ and +0.4±0.1‰ respectively. The TEFs for serum in badgers are notably different from those published for the red fox (Vulpes vulpes). There is currently no data for TEFs in the serum of other mustelid species. Our data show that species sharing similar niches (red fox) do not provide adequate proxy values for TEFs of badgers. Our findings emphasise the importance of having species-specific data when undertaking trophic studies using stable isotope analysis.     

Trophic Discrimination Factors of Stable Carbon and Nitrogen Isotopes in Hair of Corn Fed Wild Boar

Stable isotope measurements are increasingly being used to gain insights into the nutritional ecology of many wildlife species and their role in ecosystem structure and function. Such studies require estimations of trophic discrimination factors (i.e. differences in the isotopic ratio between the consumer and its diet). Although trophic discrimination factors are tissue- and species- specific, researchers often rely on generalized, and fixed trophic discrimination factors that have not been experimentally derived. In this experimental study, captive wild boar (Sus scrofa) were fed a controlled diet of corn (Zea mays), a popular and increasingly dominant food source for wild boar in the Czech Republic and elsewhere in Europe, and trophic discrimination factors for stable carbon (Δ¹³C) and nitrogen (Δ¹⁵N) isotopes were determined from hair samples. The mean Δ¹³C and Δ¹⁵N in wild boar hair were –2.3 ‰ and +3.5 ‰, respectively. Also, in order to facilitate future derivations of isotopic measurements along wild boar hair, we calculated the average hair growth rate to be 1.1 mm d^-1. Our results serve as a baseline for interpreting isotopic patterns of free-ranging wild boar in current European agricultural landscapes. However, future research is needed in order to provide a broader understanding of the processes underlying the variation in trophic discrimination factors of carbon and nitrogen across of variety of diet types.     

Temperature Dependence of Carbon Isotope Fractionation in CAM Plants

The carbon isotope fractionation associated with nocturnal malic acid synthesis in Kalanchoë daigremontiana and Bryophyllum tubiflorum was calculated from the isotopic composition of carbon-4 of malic acid, after appropriate corrections. In the lowest temperature treatment (17°C nights, 23°C days), the isotope fractionation for both plants is −4‰ (that is, malate is enriched in ¹³C relative to the atmosphere). For K. daigremontiana, the isotope fractionation decreases with increasing temperature, becoming approximately 0‰ at 27°C/33°C. Detailed analysis of temperature effects on the isotope fractionation indicates that stomatal aperture decreases with increasing temperature and carboxylation capacity increases. For B. tubiflorum, the temperature dependence of the isotope fractionation is smaller and is principally attributed to the normal temperature dependences of the rates of diffusion and carboxylation steps. The small change in the isotopic composition of remaining malic acid in both species which is observed during deacidification indicates that malate release, rather than decarboxylation, is rate limiting in the deacidification process.     

Isotopic Differences between Forage Consumed by a Large Herbivore in Open,Closed, and Coastal Habitats: New Evidence from a Boreal Study System

Documenting habitat-related patterns in foraging behaviour at the individual level and over large temporal scales remains challenging for large herbivores. Stable isotope analysis could represent a valuable tool to quantify habitat-related foraging behaviour at the scale of individuals and over large temporal scales in forest dwelling large herbivores living in coastal environments, because the carbon (δ¹³C) or nitrogen (δ¹⁵N) isotopic signatures of forage can differ between open and closed habitats or between terrestrial and littoral forage, respectively. Here, we examined if we could detect isotopic differences between the different assemblages of forage taxa consumed by white-tailed deer that can be found in open, closed, supralittoral, and littoral habitats. We showed that δ¹³C of assemblages of forage taxa were 3.0‰ lower in closed than in open habitats, while δ¹⁵N were 2.0‰ and 7.4‰ higher in supralittoral and littoral habitats, respectively, than in terrestrial habitats. Stable isotope analysis may represent an additional technique for ecologists interested in quantifiying the consumption of terrestrial vs. marine autotrophs. Yet, given the relative isotopic proximity and the overlap between forage from open, closed, and supralittoral habitats, the next step would be to determine the potential to estimate their contribution to herbivore diet.     

Equations for Lipid Normalization of Carbon Stable Isotope Ratios in Aquatic Bird Eggs

Stable isotope ratios are biogeochemical tracers that can be used to determine the source of nutrients and contaminants in avian eggs. However, the interpretation of stable carbon ratios in lipid-rich eggs is complicated because ¹³C is depleted in lipids. Variation in ¹³C abundance can therefore be obscured by variation in percent lipids. Past attempts to establish an algebraic equation to correct carbon isotope ratios for lipid content in eggs have been unsuccessful, possibly because they relied partly on data from coastal or migratory species that may obtain egg lipids from different habitats than egg protein. We measured carbon, nitrogen and sulphur stable isotope ratios in 175 eggs from eight species of aquatic birds. Carbon, nitrogen and sulphur isotopes were enriched in lipid-extracted egg samples compared with non extracted egg samples. A logarithmic equation using the C∶N ratio and carbon isotope ratio from the non extracted egg tissue calculated 90% of the lipid-extracted carbon isotope ratios within ±0.5‰. Calculating separate equations for eggs laid by species in different habitats (pelagic, offshore and terrestrial-influenced) improved the fit. A logarithmic equation, rather than a linear equation as often used for muscle, was necessary to accurately correct for lipid content because the relatively high lipid content of eggs compared with muscle meant that a linear relationship did not accurately approximate the relationship between percent lipids and the C∶N ratio. Because lipid extraction alters sulphur and nitrogen isotope ratios (and cannot be corrected algebraically), we suggest that isotopic measurement on bulk tissue followed by algebraic lipid normalization of carbon stable isotope ratio is often a good solution for homogenated eggs, at least when it is not possible to complete separate chemical analyses for each isotope.     

Intrapopulation variability shaping isotope discrimination and turnover: experimental evidence in arctic foxes

Background

Tissue-specific stable isotope signatures can provide insights into the trophic ecology of consumers and their roles in food webs. Two parameters are central for making valid inferences based on stable isotopes, isotopic discrimination (difference in isotopic ratio between consumer and its diet) and turnover time (renewal process of molecules in a given tissue usually measured when half of the tissue composition has changed). We investigated simultaneously the effects of age, sex, and diet types on the variation of discrimination and half-life in nitrogen and carbon stable isotopes (δ¹⁵N and δ¹³C, respectively) in five tissues (blood cells, plasma, muscle, liver, nail, and hair) of a top predator, the arctic fox Vulpes lagopus.

Methodology/Principal Findings

We fed 40 farmed foxes (equal numbers of adults and yearlings of both sexes) with diet capturing the range of resources used by their wild counterparts. We found that, for a single species, six tissues, and three diet types, the range of discrimination values can be almost as large as what is known at the scale of the whole mammalian or avian class. Discrimination varied depending on sex, age, tissue, and diet types, ranging from 0.3‰ to 5.3‰ (mean  = 2.6‰) for δ¹⁵N and from 0.2‰ to 2.9‰ (mean  = 0.9‰) for δ¹³C. We also found an impact of population structure on δ¹⁵N half-life in blood cells. Varying across individuals, δ¹⁵N half-life in plasma (6 to 10 days) was also shorter than for δ¹³C (14 to 22 days), though δ¹⁵N and δ¹³C half-lives are usually considered as equal.

Conclusion/Significance

Overall, our multi-factorial experiment revealed that at least six levels of isotopic variations could co-occur in the same population. Our experimental analysis provides a framework for quantifying multiple sources of variation in isotopic discrimination and half-life that needs to be taken into account when designing and analysing ecological field studies.     

Isotopic turnover rate and fractionation in multiple tissues of red rock lobster (Jasus edwardsii) and blue cod (Parapercis colias): Consequences for ecological studies

This study experimentally determined the turnover rates of δ¹³C and δ¹⁵N as a function of growth and metabolism and isotopic fractionation for different tissues in captive populations of red rock lobster (Jasus edwardsii) and blue cod (Parapercis colias). Isotopic turnover was estimated using the model of Hesslein et al. [Hesslein, R., Hallard, K., Ramlal, P., 1993. Replacement of sulfur, carbon, and nitrogen in tissue of growing broad whitefish (Coregonus nasus) in response to a change in diet traced by δ³⁴S, δ¹³C, and δ¹⁵N. Can. J. Fish. Aquat. Sci. 50, 2071–2076.]. Isotopic fractionations relative to diet differed among tissues and isotopes. Lobster muscle was more enriched than hemolymph and blue cod fin tissue was more enriched than blood for δ¹³C and δ¹⁵N. The metabolic component of turnover accounted for > 90% of the total isotopic turnover in lobster tissues and 30%–60% in blue cod tissues. Lobster muscle (half-life 147 d) and hemolymph (half-life 117 d) turnover rates were not significantly different but were faster than turnover rates of blue cod tissues. Whole blood, blood plasma fraction, and the blood cellular fraction had similar turnover rates; the whole blood half-life was 240 d for blue cod. Measuring turnover in larger, slower growing animals allowed for a more precise estimate of the metabolic component of isotopic turnover than in fast growing animals in which change is predominantly the result of dilution through growth. The differences in fractionation values among tissues observed here demonstrate that using generic trophic fractionation values would introduce error into diet reconstruction or migration studies. We demonstrate that a modified version of Hesslein et al.'s [Hesslein, R., Hallard, K., Ramlal, P., 1993. Replacement of sulfur, carbon, and nitrogen in tissue of growing broad whitefish (Coregonus nasus) in response to a change in diet traced by δ³⁴S, δ¹³C, and δ¹⁵N. Can. J. Fish. Aquat. Sci. 50, 2071–2076.] turnover model could be used to estimate the temporal component of migration.     

Stable isotopes reveal seasonal dietary responses to agroforestry in a venomous mammal,the Hispaniolan solenodon (Solenodon paradoxus)

While trends in tropical deforestation are alarming, conservation biologists are increasingly recognizing the potential for species survival in human‐modified landscapes. Identifying the factors underlying such persistence, however, requires basic ecological knowledge of a species’ resource use. Here, we generate such data to guide conservation of an understudied venomous mammal, the Hispaniolan solenodon (Solenodon paradoxus), that occupies a mosaic landscape of agriculture and forest fragments in the western Dominican Republic. Using feces collected in both wet and dry seasons, we found significant differences in the stable isotope values of carbon (δ¹³C) between pasture (−24.63 ± 2.31‰, Las Mercedes) and agroforestry (−28.07 ± 2.10‰, Mencia). Solenodon populations in agricultural areas occupied wider isotopic niche spaces, which may be explained by more diverse resource within these patches or individuals combining resources across habitats. We detected elevated δ¹⁵N values in the dry season of pasture areas (8.22 ± 2.30‰) as compared to the wet season (5.26 ± 2.44‰) and overall narrower isotopic niche widths in the dry season, suggestive of the impacts of aridity on foraging behavior. Our work highlights the importance of considering a more nuanced view of variations in ‘modified’ or “agricultural” landscapes as compared with strictly protected national parks. We suggest that seasonal differences in foraging should be considered as they intersect with landscape modification by landowners for maintaining resources for focal consumers. This work adds to a growing body of literature highlighting that fecal stable isotopes are a non‐invasive and cost‐effective monitoring tool that is particularly well‐suited for cryptic small mammal species, ensuring actionable and evidenced‐based conservation practices in the tropic''s rapidly changing landscapes.     

Isotope Fractionation in Photosynthetic Bacteria during Carbon Dioxide Assimilation

The δ _PDB¹³C values have been determined for the cellular constituents and metabolic intermediates of autotrophically grown Chromatium vinosum. The isotopic composition of the HCO₃^- in the medium and the carbon isotopic composition of the bacterial cells change with the growth of the culture. The δ _PDB¹³C value of the HCO₃^- in the media changes from an initial value of −6.6‰ to +8.1‰ after 10 days of bacterial growth and the δ _PDB¹³C value of the bacterial cells change from −37.5‰ to −29.2‰ in the same period. The amount of carbon isotope fractionation during the synthesis of hexoses by the photoassimilation of CO₂ has a range of −15.5‰ at time zero to −22.0‰ after 10 days. This range of fractionation compares to the range of carbon isotope fractionation for the synthesis of sugars from CO₂ by ribulose 1,5-diphosphate carboxylase and the Calvin cycle.     

Distinguishing Nitrous Oxide Production from Nitrification and Denitrification on the Basis of Isotopomer Abundances

The intramolecular distribution of nitrogen isotopes in N₂O is an emerging tool for defining the relative importance of microbial sources of this greenhouse gas. The application of intramolecular isotopic distributions to evaluate the origins of N₂O, however, requires a foundation in laboratory experiments in which individual production pathways can be isolated. Here we evaluate the site preferences of N₂O produced during hydroxylamine oxidation by ammonia oxidizers and by a methanotroph, ammonia oxidation by a nitrifier, nitrite reduction during nitrifier denitrification, and nitrate and nitrite reduction by denitrifiers. The site preferences produced during hydroxylamine oxidation were 33.5 ± 1.2‰, 32.5 ± 0.6‰, and 35.6 ± 1.4‰ for Nitrosomonas europaea, Nitrosospira multiformis, and Methylosinus trichosporium, respectively, indicating similar site preferences for methane and ammonia oxidizers. The site preference of N₂O from ammonia oxidation by N. europaea (31.4 ± 4.2‰) was similar to that produced during hydroxylamine oxidation (33.5 ± 1.2‰) and distinct from that produced during nitrifier denitrification by N. multiformis (0.1 ± 1.7‰), indicating that isotopomers differentiate between nitrification and nitrifier denitrification. The site preferences of N₂O produced during nitrite reduction by the denitrifiers Pseudomonas chlororaphis and Pseudomonas aureofaciens (−0.6 ± 1.9‰ and −0.5 ± 1.9‰, respectively) were similar to those during nitrate reduction (−0.5 ± 1.9‰ and −0.5 ± 0.6‰, respectively), indicating no influence of either substrate on site preference. Site preferences of ~33‰ and ~0‰ are characteristic of nitrification and denitrification, respectively, and provide a basis to quantitatively apportion N₂O.     

How protein quality drives incorporation rates and trophic discrimination of carbon and nitrogen stable isotope ratios in a freshwater first-feeding fish

1. Using stable isotope ratios to explore the trophic ecology of freshwater animals requires knowledge about effects of food quality on isotopic incorporation dynamics. The aim of this experimental study was to: (1) estimate carbon and nitrogen isotopic incorporation rates and trophic discrimination factors (TDFs) of a freshwater first-feeding fish (i.e. salmonid fry) fed three diets that differed only in protein quality (animal or plant or a blend of both); (2) investigate effects of fasting and; (3) evaluate the proportion of each source assimilated when fry were fed a 50:50 animal:plant-based diet.
2. For each diet, incorporation rates of δ¹³C and δ¹⁵N values were estimated using a time or growth-dependent isotopic incorporation model. Effects of fasting on isotope ratio values were measured regularly until the death of fry. Bayesian stable-isotope mixing models were used to estimate the contribution of animal and plant material to fish fed a blend of both food types.
3. Our results show that incorporation rates were lower for fry fed a plant-based diet than for those fed an animal-based diet as growth rate decreased. Time- and growth-dependent models indicated that growth was solely responsible for isotopic incorporation in fry fed an animal-based diet, whereas catabolism increased in fry fed a plant-based diet. After lipid extraction, carbon TDFs were similar regardless of the diet, whereas nitrogen TDFs increased for fry fed a plant-based diet. Long-term fasting induced an increase of 0.63‰ in δ¹³C values of fry in 23 days, whereas δ¹⁵N values did not vary significantly. Proportions of food sources assimilated by fry fed an animal:plant-based diet were similar to those consumed when using a mixing model with the estimated TDFs, while proportions were unrealistic when using mean TDFs extrapolated from the literature.
4. The results of our study indicate that the quality of food must be considered to use an appropriate timescale to detect changes in fry diets in the field. Moreover, we recommend using different carbon and nitrogen TDFs, one for animal-derived sources and one for plant-derived sources, to increase the accuracy of mixing models.

     

Stable Carbon Isotope Fractionation by Sulfate-Reducing Bacteria

Biogeochemical transformations occurring in the anoxic zones of stratified sedimentary microbial communities can profoundly influence the isotopic and organic signatures preserved in the fossil record. Accordingly, we have determined carbon isotope discrimination that is associated with both heterotrophic and lithotrophic growth of pure cultures of sulfate-reducing bacteria (SRB). For heterotrophic-growth experiments, substrate consumption was monitored to completion. Sealed vessels containing SRB cultures were harvested at different time intervals, and δ¹³C values were determined for gaseous CO₂, organic substrates, and products such as biomass. For three of the four SRB, carbon isotope effects between the substrates, acetate or lactate and CO₂, and the cell biomass were small, ranging from 0 to 2‰. However, for Desulfotomaculum acetoxidans, the carbon incorporated into biomass was isotopically heavier than the available substrates by 8 to 9‰. SRB grown lithoautotrophically consumed less than 3% of the available CO₂ and exhibited substantial discrimination (calculated as isotope fractionation factors [α]), as follows: for Desulfobacterium autotrophicum, α values ranged from 1.0100 to 1.0123; for Desulfobacter hydrogenophilus, the α value was 0.0138, and for Desulfotomaculum acetoxidans, the α value was 1.0310. Mixotrophic growth of Desulfovibrio desulfuricans on acetate and CO₂ resulted in biomass with a δ¹³C composition intermediate to that of the substrates. The extent of fractionation depended on which enzymatic pathways were used, the direction in which the pathways operated, and the growth rate, but fractionation was not dependent on the growth phase. To the extent that environmental conditions affect the availability of organic substrates (e.g., acetate) and reducing power (e.g., H₂), ecological forces can also influence carbon isotope discrimination by SRB.     

Carbon and Nitrogen Isotopes from Top Predator Amino Acids Reveal Rapidly Shifting Ocean Biochemistry in the Outer California Current

Climatic variation alters biochemical and ecological processes, but it is difficult both to quantify the magnitude of such changes, and to differentiate long-term shifts from inter-annual variability. Here, we simultaneously quantify decade-scale isotopic variability at the lowest and highest trophic positions in the offshore California Current System (CCS) by measuring δ¹⁵N and δ¹³C values of amino acids in a top predator, the sperm whale (Physeter macrocephalus). Using a time series of skin tissue samples as a biological archive, isotopic records from individual amino acids (AAs) can reveal the proximate factors driving a temporal decline we observed in bulk isotope values (a decline of ≥1 ‰) by decoupling changes in primary producer isotope values from those linked to the trophic position of this toothed whale. A continuous decline in baseline (i.e., primary producer) δ¹⁵N and δ¹³C values was observed from 1993 to 2005 (a decrease of ∼4‰ for δ¹⁵N source-AAs and 3‰ for δ¹³C essential-AAs), while the trophic position of whales was variable over time and it did not exhibit directional trends. The baseline δ¹⁵N and δ¹³C shifts suggest rapid ongoing changes in the carbon and nitrogen biogeochemical cycling in the offshore CCS, potentially occurring at faster rates than long-term shifts observed elsewhere in the Pacific. While the mechanisms forcing these biogeochemical shifts remain to be determined, our data suggest possible links to natural climate variability, and also corresponding shifts in surface nutrient availability. Our study demonstrates that isotopic analysis of individual amino acids from a top marine mammal predator can be a powerful new approach to reconstructing temporal variation in both biochemical cycling and trophic structure.     

Exploring the Potential of Laser Ablation Carbon Isotope Analysis for Examining Ecology during the Ontogeny of Middle Pleistocene Hominins from Sima de los Huesos (Northern Spain)

Laser ablation of tooth enamel was used to analyze stable carbon isotope compositions of teeth of hominins, red deer, and bears from middle Pleistocene sites in the Sierra de Atapuerca in northern Spain, to investigate the possibility that this technique could be used as an additional tool to identify periods of physiological change that are not detectable as changes in tooth morphology. Most of the specimens were found to have minimal intra-tooth variation in carbon isotopes (< 2.3‰), suggesting isotopically uniform diets through time and revealing no obvious periods of physiological change. However, one of the two sampled hominin teeth displayed a temporal carbon isotope shift (3.2‰) that was significantly greater than observed for co-occurring specimens. The δ¹³C value of this individual averaged about -16‰ early in life, and -13‰ later in life. This isotopic change occurred on the canine crown about 4.2 mm from the root, which corresponds to an approximate age of two to four years old in modern humans. Our dataset is perforce small owing to the precious nature of hominid teeth, but it demonstrates the potential utility of the intra-tooth isotope profile method for extracting ontogenetic histories of human ancestors.     

Isotopic Discrimination in the Double-Crested Cormorant (Phalacrocorax auritus)

The diet-tissue discrimination factor is the amount by which a consumer’s tissue varies isotopically from its diet, and is therefore a key element in models that use stable isotopes to estimate diet composition. In this study we measured discrimination factors in blood (whole blood, red blood cells and plasma), liver, muscle and feathers of Double-crested Cormorants (Phalacrocorax auritus) for stable isotope ratios of carbon, nitrogen and sulfur. Cormorants exhibited discrimination factors that differed significantly among tissue types (for carbon and nitrogen), and differed substantially (in the context of the isotopic variation among relevant prey species) from those observed in congeneric species. The Double-crested Cormorant has undergone rapid population expansion throughout much of its historic range over the past three decades, leading to both real and perceived conflicts with fisheries throughout North America, and this study provides an essential link for the use of stable isotope analysis in researching foraging ecology, diet, and resource use of this widespread and controversial species.     

Spatially Explicit Estimates of Prey Consumption Reveal a New Krill Predator in the Southern Ocean

Development in foraging behaviour and dietary intake of many vertebrates are age-structured. Differences in feeding ecology may correlate with ontogenetic shifts in dispersal patterns, and therefore affect foraging habitat and resource utilization. Such life-history traits have important implications in interpreting tropho-dynamic linkages. Stable isotope ratios in the whiskers of sub-yearling southern elephant seals (Mirounga leonina; n = 12) were used, in conjunction with satellite telemetry and environmental data, to examine their foraging habitat and diet during their first foraging migration. The trophic position of seals from Macquarie Island (54°30′S, 158°57′E) was estimated using stable carbon (δ^{1 3}C) and nitrogen (δ¹⁵N) ratios along the length of the whisker, which provided a temporal record of prey intake. Satellite-relayed data loggers provided details on seal movement patterns, which were related to isotopic concentrations along the whisker. Animals fed in waters south of the Polar Front (>60°S) or within Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) Statistical Subareas 88.1 and 88.2, as indicated by both their depleted δ^{1 3}C (<−20‰) values, and tracking data. They predominantly exploited varying proportions of mesopelagic fish and squid, and crustaceans, such as euphausiids, which have not been reported as a prey item for this species. Comparison of isotopic data between sub-yearlings, and 1, 2 and 3 yr olds indicated that sub-yearlings, limited by their size, dive capabilities and prey capture skills to feeding higher in the water column, fed at a lower trophic level than older seals. This is consistent with the consumption of euphausiids and most probably, Antarctic krill (Euphausia superba), which constitute an abundant, easily accessible source of prey in water masses used by this age class of seals. Isotopic assessment and concurrent tracking of seals are successfully used here to identify ontogenetic shifts in broad-scale foraging habitat use and diet preferences in a highly migratory predator.     

Experimentally Derived δ13C and δ15N Discrimination Factors for Gray Wolves and the Impact of Prior Information in Bayesian Mixing Models

Stable isotope analysis of diet has become a common tool in conservation research. However, the multiple sources of uncertainty inherent in this analysis framework involve consequences that have not been thoroughly addressed. Uncertainty arises from the choice of trophic discrimination factors, and for Bayesian stable isotope mixing models (SIMMs), the specification of prior information; the combined effect of these aspects has not been explicitly tested. We used a captive feeding study of gray wolves (Canis lupus) to determine the first experimentally-derived trophic discrimination factors of C and N for this large carnivore of broad conservation interest. Using the estimated diet in our controlled system and data from a published study on wild wolves and their prey in Montana, USA, we then investigated the simultaneous effect of discrimination factors and prior information on diet reconstruction with Bayesian SIMMs. Discrimination factors for gray wolves and their prey were 1.97‰ for δ¹³C and 3.04‰ for δ¹⁵N. Specifying wolf discrimination factors, as opposed to the commonly used red fox (Vulpes vulpes) factors, made little practical difference to estimates of wolf diet, but prior information had a strong effect on bias, precision, and accuracy of posterior estimates. Without specifying prior information in our Bayesian SIMM, it was not possible to produce SIMM posteriors statistically similar to the estimated diet in our controlled study or the diet of wild wolves. Our study demonstrates the critical effect of prior information on estimates of animal diets using Bayesian SIMMs, and suggests species-specific trophic discrimination factors are of secondary importance. When using stable isotope analysis to inform conservation decisions researchers should understand the limits of their data. It may be difficult to obtain useful information from SIMMs if informative priors are omitted and species-specific discrimination factors are unavailable.     

Carbon isotope fractionation by ribulose-1,5-bisophosphate carboxylase from various organisms

Carbon isotope fractionation by structurally and catalytically distinct ribulose-1,5-bisphosphate carboxylases from one eucaryotic and four procaryotic organisms has been measured under nitrogen. The average fractionation for 40 experiments was −34.1 ‰ with respect to the δ¹³C of the dissolved CO₂ used, although average fractionations for each enzyme varied slightly: spinach carboxylase, −36.5 ‰; Hydrogenomonas eutropha, −38.7 ‰; Agmenellum quadruplicatum, −32.2 ‰; Rhodospirillum rubrum, −32.1 ‰; Rhodopseudomonas sphaeroides peak I carboxylase, −31.4 ‰; and R. sphaeroides peak II carboxylase, −28.3 ‰. The carbon isotope fractionation value was largely independent of method of enzyme preparation, purity, or reaction temperature, but in the case of spinach ribulose-1,5-bisphosphate carboxylase fractionation, changing the metal cofactor used for enzyme activation had a distinct effect on the fractionation value. The fractionation value of −36.5 ‰ with Mg²⁺ as activator shifted to −29.9 ‰ with Ni²⁺ as activator and to −41.7 ‰ with Mn²⁺ as activator. These dramatic metal effects on carbon isotope fractionation may be useful in examining the catalytic site of the enzyme.