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Hydrogen isotope assimilation and discrimination in green turtles
Abstract
Although hydrogen isotopes (δ ² H) are commonly used as tracers of animal movement, minimal research has investigated the use of δ ² H as a proxy to quantify resource and habitat use. While carbon and nitrogen are ultimately derived from a single source (food), the proportion of hydrogen in consumer tissues originates from two distinct sources: body water and food. Before hydrogen isotopes can be effectively used as a resource and habitat tracer, we need estimates of (net) discrimination factors (Δ ² H Net ) that account for the physiologically mediated differences in the δ ² H values of animal tissues relative to that of the food and water sources they use to synthesize tissues. Here we estimated Δ ² H Net in captive green turtles ( Chelonia mydas ) by measuring δ ² H values of tissues (epidermis and blood components) and dietary macromolecules collected in two controlled feeding experiments. Tissue δ ² H and Δ ² H Net values varied systematically among tissues, with epidermis having higher δ ² H and Δ ² H Net values than blood components, which mirrors patterns between keratinaceous tissues (feathers, hair) and blood in birds and mammals. Serum/plasma of adult female green turtles had significantly lower δ ² H values compared to that of juveniles, likely due increased lipid mobilization associated with reproduction. This is the first study to quantify Δ ² H Net values in a marine ectotherm, and we anticipate our results will further refine the use of δ ² H analysis to better understand animal resource and habitat use in marine ecosystems, especially coastal areas fueled by a combination of marine (e.g., micro/macroalgae and seagrass) and terrestrial (e.g., mangroves) primary production.
Stable isotope ratios in organisms can be used to estimate source contributions to the organism. However, during lipid synthesis light isotopes of carbon ( ¹² C) and hydrogen ( ¹ H) are preferentially incorporated into the lipids, potentially causing source contributions to be poorly estimated. Contrary to expectations and other published examples in animals, larval lampreys, which are basal vertebrates, have lipids which are enriched in heavy isotopes of carbon ( ¹³ C), but still depleted in heavy hydrogen (deuterium; ² H). Four lamprey species were collected and their isotopes ratios of δ ² H, δ ¹³ C, δ ¹⁵ N were measured in their muscle before and after lipid extraction. Larval lamprey of one species was collected every three months for a year from two streams in Maryland and the isotope ratios of muscle before and after lipid extraction, as well as the extracted lipid were measured. Muscle δ ¹³ C was positively related to C:N ratios in samples when lipids were not removed and δ ² H was negatively associated with the percent hydrogen in a sample. As expected, the measured difference between muscle and lipid δ ² H (Δ ML δ ² H) was the same for all months and was 111‰ (SE = ± 21, n = 35), but the Δ ML δ ¹³ C was different between months (ANOVA, F 3,53 = 5.05, p < 0.005) and was always negative. Our work suggests that while lipids are often enriched in ¹² C relative to muscle, this is not a universal rule. The physiological mechanism(s) for generating heavy carbon-backbones in lipids remains unknown and requires exploration.
Statistical regression relationships between the hydrogen (H) and oxygen (O) isotope ratios (δ²H and δ¹⁸O, respectively) of animal organic tissues and those of environmental water have been widely used to reconstruct animal movements, paleoenvironments, and diet and trophic relationships. In natural populations, however, tissue–environment isotopic relationships are highly variable among animal types and geographic regions. No systematic understanding of the origin(s) of this variability currently exists, clouding the interpretation of isotope data. Here, we present and apply a model, based on fundamental metabolic relationships, to test the sensitivity of consumer tissue H and O isotope ratios, and thus tissue–environment relationships, to basic physiological, behavioral, and environmental parameters. We then simulate patterns in consumer tissue isotopic compositions under several ‘real-world’ scenarios, demonstrating that the new model can reproduce—and potentially explain—previously observed patterns in consumer tissue H isotope ratios, including between-continent differences in feather–precipitation relationships and ²H-enrichment with trophic level across species. The model makes several fundamental predictions about the organic O isotope system, which constitute hypotheses for future testing as new data are obtained. By highlighting potential sources of variability and bias in tissue–environment relationships and establishing a framework within which such effects can be predicted, these results should advance the application of H and O isotopes in ecological, paleoecological, and forensic research.
https://www.elsevier.com/books/tracking-animal-migration-with-stable-isotopes/hobson/978-0-12-814723-8
Stable isotope analysis has revolutionized the way ecologists study animal resource use from the individual to the community level. Recent interest has emerged in using hydrogen isotopes (2H/1H) as ecological tracers, because they integrate information from both abiotic and biotic processes. A better physiological understanding of how animals assimilate hydrogen and use it to synthesize tissues is needed to further refine this tool and broaden its use in animal ecology. We conducted a controlled-feeding experiment using laboratory mice (Mus musculus) in which we varied the hydrogen isotope (δ2H) values of water and the proportions of dietary protein and carbohydrates among nine experimental treatments. For each tissue, we calculated the percent of hydrogen derived from water and the percent hydrogen derived from dietary protein versus carbohydrates using linear relationships and isotope mixing models based on accompanying carbon isotope (δ13C) data. The net discrimination (∆2HNet) between mice tissues and potential water and dietary sources of hydrogen differed among tissues. ∆2HNet was positively correlated with dietary protein content in red blood cells (RBC) and muscle, but negatively correlated in liver and plasma. We also report the first estimates for hydrogen isotope discrimination factors (∆2H) for different sources of hydrogen (∆2HWater, ∆2HProtein, and ∆2HCarbs) available for tissue synthesis. This research provides a foundation for understanding how diet quality (e.g., protein content) influences hydrogen isotope assimilation and discrimination in different tissues of a terrestrial mammal, which is a first step towards using δ2H as a tracer of resource use in free-ranging mammals.
Green sea turtles (Chelonia mydas) arrive on the geographically isolated Bermuda Platform as small juveniles and remain until they are approaching sexual maturity, at which point individuals depart for distant feeding and nesting sites. It has been reported that younger green turtles generally tend to carnivory or omnivory and that seagrasses become a significant food source as the turtles grow. Evidence indicates that grazing by green sea turtles in Bermuda is negatively impacting seagrass beds, thus understanding their diets is important to both conserving the turtles and their food. Stable isotope methods were used to investigate ontogenetic diet shifts of green sea turtles and to determine reliance on seagrass by larger turtles. Skin samples from 157 individual turtles and samples of known turtle foods, plants and animals, were collected for determination of consumer and food 13C and 15N values. A Bayesian stable isotope mixing model analysis indicated a wide range among individual turtles’ diets, with the greatest differences occurring between small and large turtles; larger turtles consumed seagrass almost exclusively. We also examined diet changes in 12 turtles captured in two successive years; these recapture data confirmed the changes in diet suggested by the relationship between size of turtles and diet composition. Very limited evidence was found of any diet variation among larger turtles that would indicate a shift away from declining seagrasses as their major food source.
World populations or stock distinction of Tursiops truncatus has been difficult to assess, because of large variations in morphology, habitat, feeding habits, and social structure among areas, which may reflect phylogenetic segregation or ecological plasticity. In the Gulf of California, Mexico, two common bottlenose dolphin ecotypes (inshore and offshore) have been reported. The offshore ecotype is frequently observed in association with sperm whales (Physeter macrocephalus) but the reason for this is still unknown. To explore the degree of resource partitioning/overlap between these species and stocks, we used skin stable isotope values (δ13C, δ15N) to estimate quantitative metrics of isotopic niche width (Bayesian standard ellipse areas, SEAB) and estimated their diet composition using Bayesian isotopic mixing models. The inshore ecotype in different regions (north, central, and south) of the Gulf of California exhibited distinct δ15N values and SEAB, suggesting a latitudinal gradient in nitrogen sources of coastal localities. The SEAB of inshore and offshore bottlenose dolphin ecotypes was completely distinct, indicating resource partitioning. Associated offshore ecotype and sperm whales had overlapping SEAB. The isotopic mixing model indicates that a considerable proportion of both species’ diet is large Humbolt squid. Our results suggest that resource partitioning and species association are two strategies that bottlenose dolphin ecotypes use in this zone.
Rationale:
The ecological application of stable isotope analysis (SIA) relies on taxa- and tissue-specific stable carbon (Δ(13) C) and nitrogen (Δ(15) N) isotope discrimination factors, determined with captive animals reared on known diets for sufficient time to reflect dietary isotope ratios. However, captive studies often prohibit lethal sampling, are difficult with endangered species, and reflect conditions not experienced in the wild.
Methods:
We overcame these constraints and determined the Δ(13) C and Δ(15) N values for skin and cortical bone from green sea turtles (Chelonia mydas) that died in captivity and evaluated the utility of a mathematical approach to predict discrimination factors. Using stable carbon (δ(13) C values) and nitrogen (δ(15) N values) isotope ratios from captive and wild turtles, we established relationships between bone stable isotope (SI) ratios and those from skin, a non-lethally sampled tissue, to facilitate comparisons of SI among studies using multiple tissues.
Results:
The mean (±SD) Δ(13) C and Δ(15) N values (‰) between skin and bone from captive turtles and their diet (non-lipid extracted) were 2.3±0.3 and 4.1±0.4 and 2.1±0.6 and 5.1±1.1, respectively. The mathematically predicted Δ(13) C and Δ(15) N values were similar (within 1‰) to the experimentally derived values. The mean δ(15) N values from bone were higher than those from skin for captive (+1.0±0.9‰) and wild (+0.8±1.0‰) turtles; the mean δ(13) C values from bone were lower than those from skin for wild turtles (-0.6±0.9‰), but the same as for captive turtles. We used linear regression equations to describe bone vs skin relationships and create bone-to-skin isotope conversion equations.
Conclusions:
For sea turtles, we provide the first a) bone-diet SI discrimination factors, b) comparison of SI ratios from individual-specific bone and skin, and c) evaluation of the application of a mathematical approach to predict stable isotope discrimination factors. Our approach opens the door for future studies comparing different tissues, and relating SI ratios of captive to wild animals.
Background
Selective pressures that occur during long-distance migration can influence morphological traits across a range of taxa. In flying insects, selection should favour individuals that have wing morphologies that increase energy efficiency and survival. In monarch butterflies, differences in wing morphology between migratory and resident populations suggest that migratory populations have undergone selection for larger (as measured by length and area) and more elongated (as measured by roundness and aspect ratio) forewings. However, selection on wing morphology may also occur within migratory populations, particularly if individuals or populations consistently migrate different distances.
Results
Using 613 monarch butterflies that were collected on the Mexican wintering grounds between 1976 – 2014, we tested whether monarch wing traits were associated with migratory distance from their natal areas in eastern North America (migration range: 774–4430 km), as inferred by stable-hydrogen (δ²H) and -carbon (δ¹³C) isotopic measurements. Monarchs that migrated farther distances to reach their overwintering sites tended to have longer and larger wings, suggesting positive selective pressure during migration on wing length and area. There was no relationship between migration distances and either roundness or aspect ratio.
Conclusions
Our results provide correlative evidence that the migratory period may act as a selective episode on monarch butterfly wing morphology, although selection during other portions of the annual cycle, as well as extensive mixing of individuals from various natal locations on the breeding grounds, likely counteracts directional selection of migration on morphology.
Electronic supplementary material
The online version of this article (doi:10.1186/s40462-017-0098-9) contains supplementary material, which is available to authorized users.
Significance
Hydrogen isotope (δ ² H) values of bulk tissues have become valuable tracers for studying migration and movement patterns in animals, but the biochemical mechanism for how hydrogen is incorporated into heterotrophic organisms is not well understood. We grew Escherichia coli as a model organism on two different substrates, and then measured δ ² H values of individual amino acids (AAs) in cellular material. The δ ² H values of AAs were highly variable within a simple microbial culture. Using isotopic fractionation models, we show that AA δ ² H provide tracers of an organism’s environmental (e.g., drinking) water, as well as its food, information of prime interest to ecologists. The work is also of significance to microbial physiologists studying metabolic pathways in microbes from extreme environments.
Developing an understanding of how differences in the dietary ecology and physiology of different species can influence the incorporation of hydrogen and oxygen from resources into consumer tissues is an important factor to consider when designing, or interpreting data from, ecological studies using stable isotope analyses of these elements. Here, we present the results of an experiment designed to examine the relationship between the delta H-2 and delta O-18 of drinking water and the delta H-2 and delta O-18 values of body water, blood plasma, red blood cells, intestine, liver, muscle, and feathers of Japanese Quail (Cortunix japonica). Because Japanese Quail have high drinking-water requirements compared with many other bird species, we hypothesized that the relative contribution of drinking water to tissue delta H-2 and delta O-18 values would be higher than that of bird species with lower drinking-water requirements. Our results demonstrate that the contribution of drinking water to the delta H-2 and delta O-18 values of tissues is generally higher in Japanese Quail than in other birds with lower drinking-water rates. However, we failed to find significant relationships between drinking-water isotope values and tissue isotope values in many of the tissues that we examined. We suspect that this lack of significant relationships is the result of variation in tissue isotope values caused by differences in drinking-water consumption rates among individual birds. Given these results, we recommend that researchers use caution when interpreting data from ecological investigations using delta H-2 and delta O-18 analyses.
Hydrogen isotopes have significantly enhanced our understanding of the biogeography of migratory animals. The basis for this methodology lies in predictable, continental patterns of precipitation δD values that are often reflected in an organism's tissues. δD variation is not expected for oceanic pelagic organisms whose dietary hydrogen (water and organic hydrogen in prey) is transferred up the food web from an isotopically homogeneous water source. We report a 142 ‰ range in the δD values of flight feathers from the Hawaiian petrel (Pterodroma sandwichensis), an oceanic pelagic North Pacific species, and inquire about the source of that variation. We show δD variation between and within four other oceanic pelagic species: Newell's shearwater (Puffinus auricularis newellii), Black-footed albatross (Phoebastria nigripes), Laysan albatross (Phoebastria immutabilis) and Buller's shearwater (Puffinus bulleri). The similarity between muscle δD values of hatch-year Hawaiian petrels and their prey suggests that trophic fractionation does not influence δD values of muscle. We hypothesize that isotopic discrimination is associated with water loss during salt excretion through salt glands. Salt load differs between seabirds that consume isosmotic squid and crustaceans and those that feed on hyposmotic teleost fish. In support of the salt gland hypothesis, we show an inverse relationship between δD and percent teleost fish in diet for three seabird species. Our results demonstrate the utility of δD in the study of oceanic consumers, while also contributing to a better understanding of δD systematics, the basis for one of the most commonly utilized isotope tools in avian ecology.
Diet items and habitat constitute some of the environmental resources that may be used differently by individuals within a population. Long-term fidelity by individuals to particular resources exemplifies individual specialization, a phenomenon that is becoming increasingly recognized across a wide range of species. Less is understood about the consequences of such specialization. Here, we investigate the effects of differential foraging ground use on reproductive output in 183 loggerhead sea turtles (Caretta caretta) nesting at Wassaw Island, Georgia (31.89°N, 80.97°W), between 2004 and 2011 with resulting possible fitness effects. Stable isotope analysis was used to assign the adult female loggerheads to one of three foraging areas in the Northwest Atlantic Ocean. Our data indicate that foraging area preference influences the size, fecundity, and breeding periodicity of adult female loggerhead turtles. We also found that the proportion of turtles originating from each foraging area varied significantly among the years examined. The change in the number of nesting females across the years of the study was not a result of uniform change from all foraging areas. We develop a novel approach to assess differential contributions of various foraging aggregations to changes in abundance of a sea turtle nesting aggregation using stable isotopes. Our approach can provide an improved understanding of the influences on the causes of increasing or decreasing population trends and allow more effective monitoring for these threatened species and other highly migratory species.
ABSTRACT: While many migratory marine organisms converge at breeding areas, identifying foraging strategies away from these reproductive sites can be challenging. Adult female green turtles Chelonia mydas regularly migrate thousands of kilometers between nesting and foraging areas, making it difficult to identify foraging habitats that support nesting populations and to understand their feeding strategies. In this study, we use stable isotope analysis to investigate the trophic ecology and spatial distribution of foraging green turtles in the Greater Caribbean. Further, we explore the possibility that adult green turtles, originally considered to be herbivores, may, like their counterparts in the Pacific Ocean, display carnivorous feeding strategies. The wide range of carbon and nitrogen isotope values in bulk epidermis observed in the nesting population at Tortuguero, Costa Rica, could indicate that these turtles feed over several trophic levels. Isotopic niches—or the range of δ13C and δ15N values, which can be used as a proxy for ecological niche—varied among the 5 green turtle foraging aggregations sampled. Similarly, the isotopic composition of the primary producer Thalassia testudinum also varied substantially with geographic location. However, compound-specific stable isotope analysis of amino acids (AA-CSIA) indicated that individuals in the nesting population with different bulk δ15N values feed at the same trophic position. The combined results suggest that spatial differences in the isotopic composition of seagrass at the base of the food web, rather than differences in turtle foraging strategy, contribute to the isotopic variation in the nesting population. This study improves understanding of the foraging ecology of a highly dispersed and migratory species.
1. Tracking insect migration at continental scales is intractable using exogenous markers because of tiny body size and high improbability of recapture. Naturally occurring endogenous isotopic markers, such as tissue δ2H and δ18O, are a means of assigning origins to both vertebrate and invertebrate populations, but the success depends upon derivation of a robust algorithm linking measured tissue isotope values with large-scale geospatial isotopic patterns (isoscapes) in the terrestrial hydrosphere.
2. We derived a North American dragonfly wing δ2H and δ18O isoscape from known-origin dragonflies of three species (Aeshna interrupta, Aeshna umbrosa and Pachydiplax longipennis) obtained across North America. A strong relationship (r2 = 0·75) was found between wing δ2H and hydrologic geospatial δ2H patterns, and between wing δ2H and δ18O (r2 = 0·92). The strong coupling between emergent insect tissue and hydrologic spatial patterning suggested that this dragonfly isoscape may be applicable to other aquatic emergent migratory insects in North America and elsewhere.
3. As a proof of concept, we used the wing isoscape algorithm to map the probability of natal origin of Common Green Darners (Anax junius) migrating through southern Texas. Results showed that these Texan dragonflies were a mix of local and far-distant migrant (e.g. northern United States) individuals. We suggest that this isoscape algorithm opens new opportunities to quantify the migration and natal origins of dragonflies and other aquatic emergent insects where conventional methods have failed.
The stable hydrogen and oxygen isotope (2H and 18O) composition of animal tissues are well-established tracers for terrestrial migration ecology and wildlife forensics. However, the behaviour of these isotopes in aquatic ecosystems and their potential as tracers of diet and provenance are complicated because of inputs from ambient H2O and diet. We conducted controlled aquaria-based experiments to quantify the mechanisms that drive the H and O isotopic flow within and among aquatic species. The isotopic composition of water and diet of two aquatic species (Chironomus tentans and Poecilia reticulata), representing two trophic levels, was varied in six isothermal treatments. Both species were raised from juvenile to adult to ensure that tissues were in isotopic equilibrium with their dietary and environmental conditions (ambient water, food, dissolved oxygen). We measured water, dissolved O2, diet, tissue protein and lipids for 2H or 18O. The flows of H and O isotopes for tissue formation in aquatic organisms were parameterized using a steady-state multi-pool mass-balance model. The ambient H2O contribution to tissue protein H and O isotopes in both species was significant (3050% for 2H and >80% for 18O). An apparent trophic effect for 2H and isotopic discrimination between water and protein for 18O (c. 15 parts per thousand) were identified. Our isotopic data and model revealed potential applications and cautions in using 2H and 18O measurements for ecological studies in aquatic food webs. Tissue 2H values may be a complementary trophic tracer in aquatic food webs, but only when the main controlling mechanisms are properly accounted for (i.e., H isotopic exchange with water during protein synthesis and influence of metabolic water). Measurements of 18O, on the other hand, reflect that of water and so can be used for predicting isotopic assignment to origin of aquatic organisms as there is no complicating trophic effect, but more 18O field data and improved analytical precision may be required to better establish the strengths to ecological applications.
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 (Delta(dt) 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 Delta C-13(dt) ranged from -1.11%. (red blood cells) to +0.1.7%o (epidermis), Delta N-15(dt) 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 VC and 3 to 5%, for delta N-15. Comprising the initial enrichment factors available for green turtles, these data will be useful in future interpretations of field isotopic values for this species.
The relative contribution of terrestrial and aquatic primary energy sources in food webs along a stream continuum in the Miramichi River system, New Brunswick, was investigated through the use of stable carbon and nitrogen isotope ratios. In sites where these primary energy sources were isotopically distinct, quantitative mixing models were used to identify the relative importance of allochthonous carbon in the diets of wild juvenile Atlantic salmon (Salmo salar) and other resident stream fishes. The δ13C data of the stream fauna ranged from -34‰ to -20‰, suggesting variable assimilation of allochthonous and autochthonous carbon sources at the four study sites. Results from the mixing model were congruent with the stream continuum hypothesis, indicating that fishes in the headwaters of Catamaran Brook were more dependent (>85%) on allochthonous carbon sources than those in sites located downstream (36-52%). Stable nitrogen isotope distributions successfully described food web structure in this study, suggesting at least 2.5-3.5 trophic levels in these lotic ecosystems. Stable isotope ratios of carbon were particularly useful for illustrating seasonal changes in food resources of recently emerged age 0+ salmon as maternally derived marine carbon was subsequently diluted by freshwater carbon over the growing season.
We studied the diet of green sea turtles (Chelonia mydas) at Bahía de los Angeles in the Gulf of California, México. From 1995–1999, we collected esophageal lavage and fecal samples from live-captured turtles and analyzed stomach contents from stranded carcasses encountered in the study area. Foods ingested did not vary with carapace length. Turtles consumed diverse marine algae, with the filamentous red alga Gracilariopsis lemaneiformis the most abundant; other common genera included Gracilaria, Codium, Ulva, and Chaetomorpha. Turtles also augmented their diet with animal matter; 25 nonalga food items were identified, including sponges, tube worms, sea pens, and sea hares. Substratum and anthropogenic debris such as plastic bags and nylon cord were commonly recovered in diet samples.
1.The conservation of highly mobile and migratory species remains one of the most serious challenges to resource managers.2.Intense mortality of immature green turtles has been identified at principal developmental–feeding habitats in Baja California, and is considered a great threat to the stability of the eastern Pacific population. Thus, coastal lagoons of the Baja California peninsula, such as Bahía Magdalena, have been identified as high priority areas for their protection.3.Conservation efforts to date have focused on creating a sea turtle refuge in the northwestern part of the bay, but this area may not be sufficient for protecting all of the critical feeding areas used by green turtles.4.The diet of green turtles was investigated through analysis of digestive tract contents from 24 green turtles that were drowned incidentally in fishing nets at feeding grounds in Bahía Magdalena and adjacent areas along the Pacific coast of the Baja California peninsula, Mexico.5.Mean straight carapace length (SCL) differed significantly between localities: mean: 55.5 cm (range: 47.7–76.9 cm, n=11) in Bahía Magdalena; mean: 67.7 >cm (range: 49–87 cm, n=11) in adjacent coastal waters.6.Green turtle diet consisted largely of marine algae and seagrasses, but food items varied spatially. The red algae Gracilaria pacifica, Gracilariopsis lemaneiformis and Hypea johnstonii were the most abundant diet items inside Bahía Magdalena, and the seagrass Phyllospadix torreyi was the most commonly ingested food in coastal areas outside of the bay.7.Results indicate that green turtles utilize spatially distinct foraging habitats within this region. Therefore, it is recommended that any design of protected areas for sea turtles in Bahía Magdalena should consider a regional approach instead of a local approach, taking into account the most important feeding areas used by green turtles at different life stages. Copyright © 2005 John Wiley & Sons, Ltd.
To determine whether stable isotopes can be used for identifying the geographic origins of migratory bird populations, we
examined the isotopic composition of hydrogen (deuterium, δD), carbon (δ13C), and strontium (δ87Sr) in tissues of a migratory passerine, the black-throated blue warbler (Dendroica caerulescens), throughout its breeding range in eastern North America. δD and δ13C values in feathers, which are grown in the breeding area, varied systematically along a latitudinal gradient, being highest
in samples from the southern end of the species’ breeding range in Georgia and lowest in southern Canada. In addition, δD
decreased from east to west across the northern part of the breeding range, from New Brunswick to Michigan. δ87Sr ratios were highest in the Appalachian Mountains, and decreased towards the west. These patterns are consistent with geographical
variation in the isotopic composition of the natural environment, i.e., with that of precipitation, plants, and soils for
δD, δ13C, and δ87Sr, respectively. Preliminary analyses of the δD and δ13C composition of feathers collected from warblers in their Caribbean winter grounds indicate that these individuals were mostly
from northern breeding populations. Furthermore, variances in isotope ratios in samples from local areas in winter tended
to be larger than those in summer, suggesting that individuals from different breeding localities may mix in winter habitats.
These isotope markers, therefore, have the potential for locating the breeding origins of migratory species on their winter
areas, for quantifying the degree of mixing of breeding populations on migratory and wintering sites, and for documenting
other aspects of the population structure migratory animals – information needed for studies of year-round ecology of these
species as well as for their conservation. Combining information from several stable isotopes will help to increase the resolution
for determining the geographic origins of individuals in such highly vagile populations.
To understand the ecology of migratory animals it is important to link geographic regions used by individuals including breeding,
wintering, and intermediate stopover sites. Previous conventional approaches used to track animal movements have relied on
extrinsic markers and typically the subsequent recovery of individuals. This approach has generally been inappropriate for
most small, or non-game animals. The use of intrinsic markers such as fatty acid profiles, molecular DNA analyses, and the
measurement of naturally occurring stable isotopes in animal tissues offer alternative approaches. This paper reviews the
use of stable isotope analyses (primarily δ13C, δ15N, δ34S, δD, δ87Sr) to trace nutritional origin and migration in animals. This approach relies on the fact that foodweb isotopic signatures
are reflected in the tissues of organisms and that such signatures can vary spatially based on a variety of biogeochemical
processes. Organisms moving between isotopically distinct foodwebs can carry with them information on the location of previous
feeding. Such an approach has been used to track animal use of inshore versus offshore, marine versus freshwater, terrestrial
C3 versus marine, terrestrial mesic versus xeric, and C3 versus C4 or Crassulacean acid metabolism foodwebs. More recently, the use of stable hydrogen isotope analyses (δD) to link organisms
to broad geographic origin in North America is based on large-scale isotopic contours of growing-season average δD values
in precipitation. This technique, especially when combined with the assay of other stable isotopes, will be extremely useful
in helping to track migration and movement of a wide range of animals from insects to birds and mammals. Future research to
refine our understanding of natural and anthropogenic-induced isotopic gradients in nature, and to explore the use of stable
isotopes of other elements, is recommended.
The analysis of hydrogen stable isotopes (δD) is a potentially powerful tool for studying animal ecology. Unlike other stable isotopes used in ecological research, however, we are less familiar with the physiological processes that influence the incorporation of hydrogen isotopes from dietary resources to animal tissues. Here we present the results of a controlled feeding experiment utilizing Japanese quail (Cortunix japonica) that was designed to: (1) estimate the relative contributions of diet to the δD signatures of blood plasma, red blood cells, intestine, liver, muscle and feathers; (2) investigate possible differences among these same tissues in diet to tissue discrimination; and (3) explore the differences in incorporation dynamics between deuterium ((2)H) and a well-studied isotope, (13)C, for blood plasma solids and red blood cells. Tissues differed in both the relative contribution of diet to tissue δD and diet to tissue discrimination. The average residence time of both hydrogen and carbon was significantly lower in plasma solids than in red blood cells. The average residence time of hydrogen was significantly lower than that of carbon in plasma solids, but not in red blood cells. Although the average residence times of hydrogen and carbon were positively correlated, the correlation was weak. Hence the incorporation of carbon seems to be a poor predictor of that of hydrogen.
The use of stable isotopes in ecological studies requires that we know the magnitude of discrimination factors between consumer and element sources. The causes of variation in discrimination factors for carbon and nitrogen have been relatively well studied. In contrast, the discrimination factors for hydrogen have rarely been measured. We grew cabbage looper caterpillars (Trichoplusia ni) on cabbage (Brassica oleracea) plants irrigated with four treatments of deuterium-enriched water (δD = -131, -88, -48, and -2‰, respectively), allowing some of them to reach adulthood as moths. Tissue δD values of plants, caterpillars, and moths were linearly correlated with the isotopic composition of irrigation water. However, the slope of these relationships was less than 1, and hence, discrimination factors depended on the δD value of irrigation water. We hypothesize that this dependence is an artifact of growing plants in an environment with a common atmospheric δD value. Both caterpillars and moths were significantly enriched in deuterium relative to plants by ∼45‰ and 23‰ respectively, but the moths had lower tissue to plant discrimination factors than did the caterpillars. If the trophic enrichment documented here is universal, δD values must be accounted for in geographic assignment studies. The isotopic value of carbon was transferred more or less faithfully across trophic levels, but δ(15)N values increased from plants to insects and we observed significant non-trophic (15)N enrichment in the metamorphosis from larvae to adult.
We investigated the relationships between the δdeuterium (δD) and the δ(18)oxygen (δ(18)O) of drinking water and the δD and δ(18)O of blood plasma, red blood cells and feathers in house sparrows (Passer domesticus) fed on diets with identical hydrogen and oxygen isotopic compositions and five isotopically distinct drinking water treatments. We expected and, with only one exception ((18)O in blood plasma), found linear relationships between the δD and δ(18)O values of drinking water and those of bird tissues. The slopes of these relationships, which estimate the percentage contributions of drinking water to the tissue isotopic signatures, were lower than those of previous studies. We found significant differences in the δD and δ(18)O values of feathers, red blood cells and plasma solids. In feathers and red blood cells, δD and δ(18)O values were linearly correlated. Our results have three implications for isotopic field studies: (1) if the isotopic composition of drinking water differs from that of food, its effect on tissue isotope values can confound the assignment of animals to a site of origin; (2) comparisons of the δD and δ(18)O values of different tissues must account for inter-tissue discrimination factors; and (3) δD/δ(18)O linear relationships are probably as prevalent in animal systems as they are in geohydrological systems. These relationships may prove to be useful tools in animal isotopic ecology.
Hydrogen isotope (δ²H) analysis has become a valuable tool in the study of animal migration; however, the biochemical framework required for ecologists to confidently apply δ²H to quantify resource use has yet to be adequately resolved. In contrast to carbon (d13C) and nitrogen (d15N) isotopes where food is the only source of these elements, there are two distinct sources of hydrogen available to consumers for tissue synthesis: food and water. To effectively use tissue δ²H values as a tracer of food and water resources, two fundamental questions need to be examined in animals that inhabit different environments: (1) What proportion of hydrogen in tissues is derived from sources of food vs. water? and (2) What is the magnitude and degree of variation in total δ²H discrimination (Δ²Hnet) between consumer tissues and these sources? We completed a 3 9 3 controlled feeding experiment on Nile tilapia (Oreochromis niloticus) in which we varied the δ²H of tank water and dietary macromolecules to examine these questions in two tissues commonly analyzed by ecologists: muscle and liver. We found that the proportion of hydrogen in tilapia tissue derived from tank water was similar for muscle (∼23%) and liver (∼25%). We then used linear regression and an isotope mixing model based on accompanying d13C data to estimate the proportion of hydrogen in muscle and liver tissue derived from dietary protein (34-44%), cornmeal (21-27%), corn syrup (4-5%), and lipids (≤1%). With this information and the δ²H values of water, protein, carbohydrates, and lipids supplied to fish in each treatment, we calculated Δ²Hnet values of-47& + 5& for muscle and-41& + 5& for liver. Our experiment is the first to quantify the relative proportion of hydrogen from different dietary macromolecules used by an omnivore to synthesize its tissues. Such information is needed to further refine the use of δ²H analysis as a dietary tracer for aquatic animals.
A common use of stable isotope analysis in mammalogy is to make inferences about diet from isotope values (typically 13C and 15N) measured in tissues and food sources of a consumer. Mathematical mixing models are used to estimate the proportional contributions of food sources to the isotopic composition of the tissues of a consumer, which reflect the assimilated diet. This paper reviews basic mixing models and how they work; additional refinements also are described that include addressing uncertainty, larger numbers of sources, combining sources, concentration effects, and Bayesian statistical frameworks. Information is provided on where to access software for the various models. Numerous examples are cited to show application of these models in the mammal research literature.
The use of stable carbon isotopes for diet reconstruction is predicated on the assumption that you are what you eat. In other words, the carbon isotopic composition of animal tissues is assumed to be a direct and constant function of the diet. Is this assumption valid? Precise dietary reconstruction requires as accurate knowledge of the isotopic composition of locally available dietary resources, as well as an adequate understanding of the effects of nutrition, environment, and physiology on the diet-tissue function (van der Merwe 1982, 1989; Chisholm 1989; Norr 1990; Matson and Chisholm 1991; Tieszen 1991; Ambrose 1992). There is a systematic but poorly defined difference between the isotopic composition of the consumer tissues and that of the diet (an enrichment factor, expressed as Δ diet-tissue). Given the isotopic composition of a specific tissue, that of the diet or of other tissues may be calculated if the Δ diet-tissue difference factors are known. The dietary proportions of isotopically distinct food resources (e.g., C3 vs C4, or C3 vs marine) have thus been calculated from the δ
13C value of bone collagen (Δ13Cd-co) and bone apatite carbonate (Δ13Cd-ca). Deviations from actual or assumed average δ
13C values for dietary endmembers, and incorrect values for diet-to-tissue isotopic relationships, will lead to errors in the estimation of consumption of specific classes of resources. Experiments and observations designed to determine the diet-to-collagen stable isotope functions (Δ13Cd-co) however, have provided widely different values.
Bivalves influence both the ecology and the economy of coastal regions. By filterfeeding on particles in the water column, these organisms reduce turbidity and link benthic and pelagic production. In addition, production and sales of harvested bivalves are a source of income in coastal areas like the Eastern Shore of Virginia (USA). Phytoplankton are known to be a main food source to many bivalves; however, ocean-side lagoons off the coast of Virginia support extensive aquaculture of Mercenaria mercenaria (hard clams) in waters with relatively low chlorophyll concentrations. The ultimate energy sources supporting these clams are uncertain but significant because seagrass restoration, sea level rise, and climate change will potentially change the quality and quantity of primary production available to these populations. We measured the C, N, and H isotopic ratios of aquaculture clams and a variety of primary producers in a Virginia coastal lagoon over an annual cycle and conducted a Bayesian mixing model analysis to identify current energy sources for clams. By adding a third isotopic ratio (hydrogen), we were able to improve precision over a 2-isotope model based on C and N isotopes. Our analysis reveals that field-cultured clams in Virginia coastal lagoons are significantly supported by microalgae (23 to 44%) but gain most of their energy from macroalgae (55 to 66%), and only a small fraction from macrophytes (0 to 14%). While macroalgae are often an indicator of coastal eutrophication, these algae can be an important food source to bivalves when abundant in low nitrogen, oligotrophic systems. Our results also indicate hydrogen stable isotopes are useful in concert with other isotopes for tracing sources in coastal food webs.
The importance of terrestrial-derived organic matter for lake zooplankton communities remains debated, partly because little is known about the basic pathways by which allochthonous carbon is transferred to zooplankton, and whether these vary among the major taxonomic and functional groups. We quantified allochthony of three zooplankton groups (Cladocera, Calanoida, and Cyclopoida) across 18 lakes in Quebec, spanning broad gradients of dissolved organic matter (DOM) and lake trophy, using a multi-isotope (delta2H + delta13C), multi-source (terrestrial, phytoplanktonic, benthic) approach. All three zooplankton groups had significant levels of allochthony, but differed greatly in their respective patterns across lakes. Allochthony in Calanoida and Cyclopoida was linked to detrital food chains based on particulate organic matter (POM) and on DOM, respectively, whereas in Cladocera it appeared related to both pathways; not surprisingly this latter group had the highest mean allochthony (0.31; compared to 0.18 in Cyclopoida and 0.16 in Calanoida). This study highlights the complexity of the pathways of delivery and transfer of terrestrial organic matter in freshwaters, and underscores the role that microbial food webs play in this transfer.
The relative global 2H-content of natural plant products is correlated to that of the primary hydrogen source, i.e. water, to the site of their biosynthesis (C3-, C4- and CAM-plants; chloroplasts, cytosol), and to their biosynthetic pathways. A relative global 2H-content sequence can be established in the order phenylpropanoids > carbohydrates > bulk material > hydrolysable lipids > steroids. A detailed analysis of the 2H-patterns of the main groups of secondary compounds reveals regularities, in that they are correlated to the primary precursors and to the origin of hydrogen from four main pools with the mean d2H-values [‰]V-SMOW: leaf H2O ~+30; carbohydrates ~-70; NADPH ~-250; flavoproteins ~-350. Aside from the 2H-discrimination between these pools, kinetic isotope effects on defined reactions only become effective in connection with metabolic branching events. So, the 2H-pattern of natural aromatic compounds can be correlated to the 2H-pattern of the precursor carbohydrates and a reduction step in the course of the shikimic acid pathway, furthermore to the implication of the NIH-shift. The pattern of aromatic compounds from the polyketide is different from that of the shikimate pathway. The alternating 2H-abundance of fatty acid chains is caused by the origin of their hydrogen atoms from carbohydrates and from NADPH, directly or via a flavoprotein, respectively. This is similar for isoprenoids, and the natural 2H-patterns permit their assignment to the mevalonate or non-mevalonate biosynthetic pathway. Generally, the correlations and regularities of the 2H-patterns of organic compounds found are a new reliable tool for the elucidation of biosynthetic pathways and origin assignments.
The ability to quantify dietary inputs using stable isotope data depends on accurate estimates of isotopic differences between a consumer (c) and its diet (d), commonly referred to as trophic discrimination factors (TDFs) and denoted by Δ c-d . At present, TDFs are available for only a few mammals and are usually derived in captive settings. The magnitude of TDFs and the degree to which they vary in wild populations is unknown. We determined δ¹³C and δ¹⁵N TDFs for vibrissae (i.e., whiskers), a tissue that is rapidly becoming an informative isotopic substrate for ecologists, of a wild population of sea otters for which individual diet has been quantified through extensive observational study. This is one of the very few studies that report TDFs for free-living wild animals feeding on natural diets. Trophic discrimination factors of 2.2‰ ± 0.7‰ for δ¹³C and 3.5‰ ± 0.6‰ for δ¹⁵N (mean ± SD) were similar to those reported for captive carnivores, and variation in individual δ¹³C TDFs was negatively but significantly related to sea urchin consumption. This pattern may relate to the lipid-rich diet consumed by most sea otters in this population and suggests that it may not be appropriate to lipid-extract prey samples when using the isotopic composition of keratinaceous tissues to examine diet in consumers that frequently consume lipid-rich foods, such as many marine mammals and seabirds. We suggest that inherent variation in TDFs should be included in isotopically based estimates of trophic level, food chain length, and mixing models used to quantify dietary inputs in wild populations; this practice will further define the capabilities and limitations of isotopic approaches in ecological studies.
Determination of amino acid enantiomers in environmental samples is difficult, because metals and organic impurities interfere with the analyses. We developed a new gas chromatographic method to assess amino acid enantiomer concentrations in complex soil matrices following hot HCl hydrolysis (6M, 12h, 105°C). The crucial focus was the establishment of a simple, reliable sample clean-up procedure. The presented method involved the adsorption of the enantiomers on a Dowex 50 W X8 cation exchange resin and the removal of interfering compounds with 0.1M oxalic acid prior to amino acid elution with 2.5M NH4OH. After conversion to N-pentafluoropropionyl-amino acid iso-propyl esters, the diastereomers were separated by a Chirasil l-Val capillary column and quantified by a flame ionization or mass selective detector. The lower limit of quantification tested here was ≤1pg injection amount. The recovery of amino acid enantiomers averaged 99±11% for pure standards and 97±11% for spiked soil hydrolysates. The general applicability of the method was demonstrated by determination of amino acid enantiomers in taxonomically different soils from different geographic regions. The coefficients of variation presented by d/l ratios were 12% for alanine and
This review is concerned with the isotopic relationships between organic compounds produced by a single organism, specifically their enrichments or depletions in 13C relative to total-biomass carbon. These relationships are biogeochemically significant because
1. An understanding of biosynthetically controlled, between-compound isotopic contrasts is required in order to judge whether plausibly related carbon skeletons found in a natural mixture might come from a single source or instead require multiple sources.
2. An understanding of compound-to-biomass differences must underlie the interpretation of isotopic differences between individual compounds and total organic matter in a natural mixture.
My approach is pedagogic. The coverage is meant to be thorough, but the emphases and presentation have been chosen for readers approaching this subject as students rather than as research specialists. In common with the geochemists in my classes, many readers of this paper may not be very familiar with biochemistry and microbiology. I have not tried to explain every concept from those subjects and I have not inserted references for points that appear in standard texts in biochemistry or microbiology. Among such books, I particularly recommend the biochemistry text by Garrett and Grisham (1999) and the microbiology text by Madigan et al. (2000). The biochemistry text edited by Zubay (1998) is also particularly elegant and detailed. White (1999) has written a superb but condensed text on the physiology and biochemistry of prokaryotes.
A schematic overview of the relevant processes is shown in Figure 1⇓. Plants and other autotrophs fix CO2. Animals and other heterotrophs utilize organic compounds. If the assimilated carbon is a small molecule (like CO2, CH4, or acetate), significant isotopic fractionation is likely to accompany the fixation or assimilation of C. Such fractionations establish the isotopic relationship between an organism and its carbon source. Those associated …
We present a new 3-dimensional 1° × 1° gridded data set
for the annual mean seawater oxygen isotope ratio
(δ18O) to use in oceanographic and paleoceanographic
applications. It is constructed from a large set of observations made
over the last 50 years combined with estimates from regional
δ18O to salinity relationships in areas of sparse data.
We use ocean fronts and water mass tracer concentrations to help define
distinct water masses over which consistent local relationships are
valid. The resulting data set compares well to the GEOSECS data (where
available); however, in certain regions, particularly where sea ice is
present, significant seasonality may bias the results. As an example
application of this data set, we use the resulting surface
δ18O as a boundary condition for isotope-enabled GISS
ModelE to yield a more realistic comparison to the isotopic composition
of precipitation data, thus quantifying the `source effect' of
δ18O on the isotopic composition of precipitation.
Stable isotope analysis (SIA) has proven to be a useful tool in reconstructing
diets, characterizing trophic relationships, elucidating patterns of resource
allocation, and constructing food webs. Consequently, the number
of studies using SIA in trophic ecology has increased exponentially over the
past decade. Several subdisciplines have developed, including isotope mixing
models, incorporation dynamics models, lipid-extraction and correction
methods, isotopic routing models, and compound-specific isotopic analysis.
As with all tools, there are limitations to SIA. Chief among these are
multiple sources of variation in isotopic signatures, unequal taxonomic and
ecosystem coverage, over-reliance on literature values for key parameters,
lack of canonical models, untested or unrealistic assumptions, low predictive
power, and a paucity of experimental studies. We anticipate progress in SIA
resulting from standardization of methods and models, calibration of model
parameters through experimentation, and continued development of several
recent approaches such as isotopic routing models and compound-specific
isotopic analysis.
We examine inherent variation in carbon and nitrogen stable isotope values of multiple soft tissues from a population of captive green turtles Chelonia mydas to determine the extent of isotopic variation due to individual differences in physiology. We compare the measured inherent variation in the captive population with the isotopic variation observed in a wild population of juvenile green turtles. Additionally, we measure diet-tissue discrimination factors to determine the offset that occurs between isotope values of the food source and four green turtle tissues. Tissue samples (epidermis, dermis, serum, and red blood cells) were collected from captive green turtles in two life stages (40 large juveniles and 30 adults) at the Cayman Turtle Farm, Grand Cayman, and analyzed for carbon and nitrogen stable isotopes. Multivariate normal models were fit to the isotope data, and the Bayesian Information Criterion was used for model selection. Inherent variation and discrimination factors differed among tissues and life stages. Inherent variation was found to make up a small portion of the isotopic variation measured in a wild population. Discrimination factors not only are tissue and life stage dependent but also appear to vary with diet and sea turtle species, thus highlighting the need for appropriate discrimination factors in dietary reconstructions and trophic-level estimations. Our measures of inherent variation will also be informative in field studies employing stable isotope analysis so that differences in diet or habitat are more accurately identified.
1. Use of the natural ratios of carbon and nitrogen stable isotopes as tracers of trophic interactions has some clear advantages over alternative methods for food web analyses, yet is limited to situations where organic materials of interest have adequate isotopic separation between potential sources. This constrains the use of natural abundance stable isotope approaches to a subset of ecosystems with biogeochemical conditions favourable to source separation.
2. Recent studies suggest that stable hydrogen isotopes (δD) could provide a robust tracer to distinguish contributions of aquatic and terrestrial production in food webs, but variation in δD of consumers and their organic food sources are poorly known. To explore the utility of the stable hydrogen isotope approach, we examined variation in δD in stream food webs in a forested catchment where variation in δ13C has been described previously.
3. Although algal δD varied by taxa and, to a small degree, between sites, we found consistent and clear separation (by an average of 67‰) from terrestrial carbon sources. Environmental conditions known to affect algal δ13C, such as water velocity and stream productivity did not greatly influence algal δD, and there was no evidence of seasonal variation. In contrast, algal δ13C was strongly affected by environmental factors both within and across sites, was seasonally variable at all sites, and partially overlapped with terrestrial δ13C in all streams with catchment areas larger than 10 km2.
4. While knowledge of isotopic exchange with water and trophic fractionation of δD for aquatic consumers is limited, consistent source separation in streams suggests that δD may provide a complementary food web tracer to δ13C in aquatic food webs. Lack of significant seasonal or spatial variation in δD is a distinct advantage over δ13C for applications in many aquatic ecosystems.
Summary • Carbon and nitrogen isotope ratios in consumer tissues are known to correlate with diet isotope composition, and nitrogen isotope ratios are observed to increase with increasing trophic level. • We analysed nitrogen and hydrogen isotope ratios of collagen from 19 species of British fish, birds and mammals to investigate how D also correlated with trophic level and with feeding environment (terrestrial or aquatic). • A strong relationship between trophic level and D was discovered for both terrestrial and aquatic consumers. • The correlation between trophic level and 15N was apparent for terrestrial consumers, but less so for aquatic consumers. • No differentiation was found between D of aquatic and terrestrial consumers at the same trophic level. • This observation should provide an additional tool in the study of current and ancient animal and human food web ecology. Journal of Animal Ecology (2005) 74, 877–881 doi: 10.1111/j.1365-2656.2005.00979.x
Linking foraging and breeding habitats is key to the understanding of behaviour, ecology and demography of migratory species. Establishing such connections has long been hampered by the logistical problems of following individuals between foraging and breeding areas, especially in the marine realm. We used variation in nitrogen stable isotope patterns between 2 foraging regions of loggerhead sea turtles Caretta caretta determined from samples of satellite-tracked individuals to assign untracked turtles to a foraging region. We sought to enhance determination of the relative importance of geographically separated foraging regions and to investigate the relationship between fitness correlates and inferred migratory strategies. Of 18 turtles followed by satellite tracking from Zakynthos (Greece), 10 moved north to foraging areas in the Adriatic Sea and the Gulf of Amvrakikos and 8 moved south to foraging areas off the coast of North Africa. Of 51 untracked individuals sampled for stable isotope analysis, we considered the stable isotope signature of 47 to qualify for assignment to foraging areas in the north (n = 22) and south (n = 25). Females foraging north were significantly larger (curved carapace length), and the former group laid larger clutches (even after correction for body length) than turtles foraging south, a fact that can be interpreted as a carry-over effect. Combining satellite tracking with stable isotope signatures in marine turtles opens new perspectives into how forensic tracking methodologies may be used to scale up knowledge from electronic tracking of a limited number of individuals to sample sizes that are more meaningful from a population perspective.
Extracting nutrients is of upmost importance to the survival of any individual or species. One of the distinguishing characteristics of the order Primates is the vast range of nutritional adaptations it exhibits. Within our own species all manner of adaptations are practiced and it has been a major focus of research to determine when and where these various patterns originated. We present one method based on stable isotope analysis in human tissues and discuss its contributions. The ratios of 13C/12C and 15N/14N vary among various pools (i.e., the atmosphere, the oceans, plant communities, trophic levels). These differences are transferred to humans via the foods they eat. The major differences in carbon occur between two photosynthetic pathways (C3 and C4), which in the New World permits tracing the introduction of maize (a C4 plant) and in Asia permits tracing the introduction of millet (also a C4 plant). The marine and terrestrial systems have distinctive isotope ratios of both carbon and nitrogen. Thus, the dependence on marine resources has been traced throughout several areas of the New and Old Worlds. We discuss several potential sources of variation including sex, age, nutritional status, among others. We conclude with some suggestions for future research.
Reconstruction of continental palaeoclimate and palaeohydrology is currently hampered by limited information about isotopic patterns in the modern hydrologic cycle. To remedy this situation and to provide baseline data for other isotope hydrology studies, more than 4800, depth- and width-integrated, stream samples from 391 selected sites within the USGS National Stream Quality Accounting Network (NASQAN) and Hydrologic Benchmark Network (HBN) were analysed for δ¹⁸O and δ²H (http://water.usgs.gov/pubs/ofr/ofr00-160/pdf/ofr00-160.pdf). Each site was sampled bimonthly or quarterly for 2·5 to 3 years between 1984 and 1987. The ability of this dataset to serve as a proxy for the isotopic composition of modern precipitation in the USA is supported by the excellent agreement between the river dataset and the isotopic compositions of adjacent precipitation monitoring sites, the strong spatial coherence of the distributions of δ¹⁸O and δ²H, the good correlations of the isotopic compositions with climatic parameters, and the good agreement between the ‘national’ meteoric water line (MWL) generated from unweighted analyses of samples from the 48 contiguous states of δ²H=8·11δ¹⁸O+8·99 (r²=0·98) and the unweighted global MWL of sites from the Global Network for Isotopes in Precipitation (GNIP) of the International Atomic Energy Agency and the World Meteorological Organization (WMO) of δ²H=8·17δ¹⁸O+10·35.
The apparent digestibility coefficients for 4 size classes of the green turtle Chelonia mydas feeding on the seagrass Thalassia testudinum were measured in Union Creek, Great Inagua, Bahamas, from September 1975 to August 1976. The values ranged from 32.6 to 73.9% for organic matter; from 21.5 to 70.7% for energy; from 71.5 to 93.7% for cellulose; from 40.3 to 90.8% for hemicellulose; and from 14.4 to 56.6% for protein. Digestive efficiency increased with increases in water temperature and body size. There was no seasonal variation in the nutrient composition of T. testudinum blades. Grazing on T. testudinum may be limited by its low quality as a forage, a result of its high fiber content and possible low protein availability. Turtles did not graze at random over the extensive beds of T. testudinum, but maintained grazing plots of young leaves by consistent recropping. They thus consumed a more digestible forage-higher in protein and lower in lignin-than the ungrazed, older leaves of T. testudinum. The selectivity of green turtles for either a seagrass or algal diet may reflect the specificity of their intestinal microflora.
A new method for the simultaneous, quantitative determination of neutral and acidic sugars liberated from non-cellulosic soil carbohydrates is described. A single hydrolytic step with 4 M trifluoroacetic acid (TFA) at 105°C for 4 h is suggested to be more effective in releasing sugar monomers from soil than other recommended hydrolysis procedures (2.5 M H2SO4 reflux for 20 min followed by 12 M H2SO4 for total sugar hydrolysis; 1 M HCl for 5 h at 105°C or for 7 h at 100°C; 4 M TFA at 125°C for 1 h or 2 M TFA at 105°C for 2 h). Different materials were tested for purification (cation exchange resin, activated carbon, C-18, XAD-4, XAD-7), and the combination of the cation exchange resin and of XAD-7 is recommended. Analysis of purified monomers involved separation of O-methyl-oxime-trimethylsilyl derivatives by capillary gas-liquid chromatography in 21 min. Results were compared with those obtained from spiked soil samples, resulting in a high recovery of standard mixtures (70–109% for individual sugars) processed in the way proposed here. The method was sensitive and characterized by simple, accurate and rapid work-up and, therefore, is suitable for routine use.
Hydrogen-isotopic data are often interpreted using mathematical approximations originally intended for other isotopes. One of the most common, apparent in literature over the last several decades, assumes that delta values of reactants and products are separated by a constant fractionation factor: δp = δr + εp/r. Because of the large fractionations that affect hydrogen isotopes, such approximations can lead to substantial errors. Here we review and develop general equations for isotopic mass balances that include the differential fractionation of each component in a mixture and discuss their use in three geochemical applications. For the fractionation of a single component, the reactant and product are related by δp = αp/rδr + εp/r, where α and ε refer to the same fractionation. Regression of δp on δr should give equivalent fractionations based on the intercept and slope, but this has not generally been recognized in studies of D/H fractionation. In a mixture of two components, each of which is fractionated during mixing, there is no unique solution for the three unknown variables (two fractionation factors and the elemental mixing ratio of the two hydrogen sources). The flow of H from CH4 and H2O to bacterial lipids in the metabolism of Methylococcus capsulatus provides an example of such a case. Data and conclusions from an earlier study of that system (Sessions et al., 2002) are reexamined here. Several constraints on the variables are available based on plausible ranges for fractionation factors. A possible refinement to current experimental procedures is the measurement of three different isotopes, which would allow unique determination of all variables.
The influence of diet on the distribution of carbon isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant carbon isotopic composition.
The isotopic composition of the whole body of an animal reflects the isotopic composition of its diet, but the animal is on average enriched in δ^(13)C by about 1‰ relative to the diet. In three of the four cases examined, the ^(13)C enrichment of the whole body relative to the diet is balanced by a ^(13)C depletion of the respired CO_2. The isotopic relationships between the whole bodies of animals and their diets are similar for different species raised on the same diet and for the same species raised on different diets. However, the δ^(13)C values of whole bodies of individuals of a species raised on the same diet may differ by up to 2‰. The relationship between the ^(13)C/^(12)C ratio of a tissue and the ^(13)C/^(12)C ratio of the diet depends both on the type of tissue and on the nature of the diet. Many of the isotopic relationships among the major biochemical fractions, namely the lipid, carbohydrate and protein fractions, are qualitatively preserved as diet carbon is incorporated into the animal. However, the difference between the δ^(13)C values of a biochemical fraction in an animal and in its diet may be as large as 3‰. The δ^(13)C values of the biochemical components collagen, chitin and the insoluble organic fraction of shells, all of which are often preserved in fossil material, are related to the isotopic composition of the diet.
These results indicate that it will be possible to perform dietary analysis based on the determination of the ^(13)C/^(12)C ratio of animal carbon. Analysis of the total animal carbon will in most cases provide a better measure of diet than the analysis of individual tissues, biochemical fractions, or biochemical components. The limits of accuracy of this method will generally restrict its application to situations in which the diet is derived from sources with relatively large differences in their δ^(13)C values, such as terrestrial vs aquatic organisms or C_3 vs C_4 plants. The method should be applicable to fossil as well as to living material.
Explains isotope terminology and fractionation, and summarises isotopic distributions in the C, N and S biogeochemical cycles. Five case studies (delta 15N measures of N2 fixation; the global carbon cycle and the CO2 problem; sulphur and acid deposition; use in archaeology; and detrital organic matter in saltmarshes) show how stable isotope measurements can provide crucial information for ecosystem analysis.-P.J.Jarvis
The study of long-distance dispersal (LDD) in animals may be advanced by recent applications of stable isotope analyses designed to track migratory organisms and to link populations throughout their annual cycle. This approach depends on there being enough isotopic difference in tissues among potential source populations such that individuals can be unequivocally assigned to their source. The isotopic mapping of such populations will be feasible only for species occurring in relatively few disjunct populations. However, the identification of isotopic outliers within known populations will be an extremely useful first step in the forensic application of stable isotopes to identify dispersal in general, and LDD in particular. The use of deuterium isotope analysis (deltaD) of tissues that can be assigned to its source (e.g. feather moult or its hair growth location) has provided a recent breakthrough in our ability to associate individuals with geographical origins at continental scales. The combination of this stable isotope with others and the ultimate combination of a variety of techniques, including the measurement of trace elements and molecular genetics markers, will undoubtedly improve resolution. The isotopic cataloguing of known history (i.e. philopatric) individuals within populations will be an important step in applying isotope techniques to evaluating LDD in any species. For aquatic insects, the isotopic marking of large numbers of individuals is possible through isotopic enrichment of local food webs using labelled compounds.
Because there are no internationally distributed stable hydrogen and oxygen isotopic reference materials of human hair, the U.S. Geological Survey (USGS) has prepared two such materials, USGS42 and USGS43. These reference materials span values commonly encountered in human hair stable isotope analysis and are isotopically homogeneous at sample sizes larger than 0.2 mg. USGS42 and USGS43 human-hair isotopic reference materials are intended for calibration of δ(2)H and δ(18)O measurements of unknown human hair by quantifying (1) drift with time, (2) mass-dependent isotopic fractionation, and (3) isotope-ratio-scale contraction. While they are intended for measurements of the stable isotopes of hydrogen and oxygen, they also are suitable for measurements of the stable isotopes of carbon, nitrogen, and sulfur in human and mammalian hair. Preliminary isotopic compositions of the non-exchangeable fractions of these materials are USGS42(Tibetan hair)δ(2)H(VSMOW-SLAP) = -78.5 ± 2.3‰ (n = 62) and δ(18)O(VSMOW-SLAP) = +8.56 ± 0.10‰ (n = 18) USGS42(Indian hair)δ(2)H(VSMOW-SLAP) = -50.3 ± 2.8‰ (n = 64) and δ(18)O(VSMOW-SLAP) = +14.11 ± 0.10‰ (n = 18). Using recommended analytical protocols presented herein for δ(2)H(VSMOW-SLAP) and δ(18)O(VSMOW-SLAP) measurements, the least squares fit regression of 11 human hair reference materials is δ(2)H(VSMOW-SLAP) = 6.085δ(2)O(VSMOW-SLAP) - 136.0‰ with an R-square value of 0.95. The δ(2)H difference between the calibrated results of human hair in this investigation and a commonly accepted human-hair relationship is a remarkable 34‰. It is critical that readers pay attention to the δ(2)H(VSMOW-SLAP) and δ(18)O(VSMOW-SLAP) of isotopic reference materials in publications, and they need to adjust the δ(2)H(VSMOW-SLAP) and δ(18)O(VSMOW-SLAP) measurement results of human hair in previous publications, as needed, to ensure all results on are on the same scales.