The use of stable isotopes for food web analysis

Compound-specific nitrogen isotope of amino acids: Toward an improved understanding of mercury trophic transfer in different habitats

Herein, we investigated the trophic transfer of mercury (Hg) through food chains in different habitats (namely aquatic, riparian, and terrestrial) through bulk stable isotope analysis of nitrogen (δ15Nbulk) and compound-specific isotope analysis of nitrogen in amino acids (δ15NAA) using bird feathers and their potential food sources from a Hg-contaminated site in southwest China. Results showed similar δ15Nphe for water birds (4.7 ± 2.6 ‰) and aquatic food sources (5.2 ± 2.1 ‰) and for land-based food sources (10.1 ± 0.4 ‰) and terrestrial birds (11.6 ± 3.0 ‰), verifying δ15Nphe as a potential discriminant indicator for different food sources. The trophic positions (TPs) of most organisms based on δ15Nbulk (TPbulk) tended to overestimate compared with those based on δ15NAA (TPAA), especially for predators (such as kingfisher: ΔTP = 1.3). Additionally, significant differences were observed in the aquatic, riparian, and terrestrial food webs between trophic magnification slope (TMS)bulk and TMSAA (p < 0.05). The trophic magnification factor (TMF)AA-multiple based on multiple-AAs in three food webs were higher than the TMFAA and TMFbulk, probably because of the greater variation of δ15Nbaseline, complex food sources or the notably different in individual organisms. Altogether, our results improve the understanding of Hg trophic transfer in aquatic, riparian, and terrestrial food webs.

Consistencies in the dietary and isotopic niche of spotted seatrout, Cynoscion nebulosus, across a salinity gradient within a coastal Louisiana estuary

Estuaries are essential habitats for recreational and commercial fish that are shaped by both natural and anthropogenic processes. In Louisiana a combination of climate change and planned coastal restoration actions is predicted to increase freshwater introduction to coastal estuaries. As such there is a need to quantify the relationships between estuarine fish ecology and salinity to aid in predicting how species will respond to shifts in salinity. We investigated the relative abundance and dietary niches of adult (24.5 ± 5.4 cm standard length) spotted seatrout Cynoscion nebulosus across varying salinity regimes (oligohaline, mesohaline, and polyhaline) within Barataria Bay, Louisiana, using a combination of net sampling and gut content and stable isotopes analysis. We found that the relative abundance of C. nebulosus was lowest at the oligohaline site, translating to approximately five fewer fish captured for every single psu decrease in a site's average annual salinity. In contrast, we found that diets and, to a lesser extent, isotopic niches had a high degree of overlap across sites with differing salinity regimes. Fish and penaeid shrimp were the most common and important prey taxa recovered from guts at all sites. The small isotopic differences found among sites were likely due to spatial variation in hydrogeochemical baselines, and the observed isotopic overlap provides support for the idea that C. nebulosus move between adjacent salinity regimes and forage throughout Barataria Bay. Our results contribute to a greater understanding of the salinity preference and trophic ecology of C. nebulosus that can aid in predicting their responses to future salinity and habitat changes within Barataria Bay associated with predicted climate change and planned coastal restoration actions.

Organophosphate esters (OPEs) in a coral reef food web of the Xisha Islands, South China Sea: Occurrence, trophodynamic, and exposure risk

Despite organophosphate esters (OPEs) are widely prevalent in the environment, however, limited information is available regarding their occurrence, trophodynamics, and exposure risks in coral reef ecosystems. In this study, 11 OPEs were investigated in a tropical marine food web (7 fish species and 9 benthos species) from the Xisha (XS) Islands, South China Sea (SCS). The ∑₁₁OPEs were 1.52 ± 0.33 ng/L, 2227 ± 2062 ng/g lipid weight (lw), 1024 ± 606 ng/g lw, and 1800 ± 1344 ng/g lw in seawater, fish, molluscs, and corals, respectively. Tris (2-chloroisopropyl) phosphate (TCIPPs) were the dominant OPEs in seawater, fish, and molluscs, while tris (2-butoxyethyl) phosphate (TBOEP) predominated in coral tissues. Abiotic and biotic factors jointly affect the OPEs enrichment in marine organisms. Trophic magnification factors (TMFs) (range: 1.31-39.2) indicated the biomagnification potency of OPEs. A dietary exposure risk assessment indicated that OPEs at current levels in coral reef fish posed a low risk to human health but were not negligible. Overall, this study contributes to a further understanding of the environmental behaviors of OPEs in coral reef ecosystems.

Feeding Ecology of Odontaster validus under Different Environmental Conditions in the West Antarctic Peninsula

To study how Odontaster validus can influence the spatial structure of Antarctic benthic communities and how they respond to disturbance, it is necessary to assess potential dietary shifts in different habitats. We investigated the diets of O. validus from Maxwell Bay and South Bay in the West Antarctic Peninsula. A multifaceted approach was applied including in situ observations of cardiac stomach everted contents, isotopic niche, and trophic diversity metrics. Results confirm the flexible foraging strategy of this species under markedly different environmental conditions, suggesting plasticity in resource use. The data also showed evidence of isotopic niche expansion, high δ¹⁵N values, and Nacella concinna as a common food item for individuals inhabiting a site with low seasonal sea ice (Ardley Cove), which could have significant ecological implications such as new trophic linkages within the Antarctic benthic community. These results highlight the importance of considering trophic changes of key species to their environment as multiple ecological factors can vary as a function of climatic conditions.

Effects of depth and overgrowth of ephemeral macroalgae on a remote subtidal NE Atlantic eelgrass (Zostera marina) community

We conducted a short-term field sampling complemented with time integrating stable isotope analysis to holistically investigate status and ecological interactions in a remote NE Atlantic Zostera marina meadow. We found high nutrient water concentrations, large biomass of fast-growing, ephemeral macroalgae, low abundance, and biodiversity of epifauna and a food web with thornback ray (Raja clavata) as intermediate and cod (Gadus morhua) as top predator. We observed no variation with increasing depth (3.5-11 m) except for decreasing shoot density and biomass of Zostera and macroalgae. Our results indicate that the Finnøya Zostera ecosystem is eutrophicated. During the past three to four decades, nutrients from aquaculture have steadily increased to reach 75% of anthropogenic input while the coastal top predator cod has decreased by 50%. We conclude that bottom-up regulation is a predominant driver of change since top-down regulation is generally weak in low density and exposed Zostera ecosystems such as Finnøya.

Radiocarbon signature reveals that most springtails depend on carbon from living plants

Terrestrial carbon cycling is largely mediated by soil food webs. Identifying the carbon source for soil animals has been desired to distinguish their roles in carbon cycling, but it is challenging for small invertebrates at low trophic levels because of methodological limitations. Here, we combined radiocarbon (¹⁴C) analysis with stable isotope analyses (¹³C and ¹⁵N) to understand feeding habits of soil microarthropods, especially focusing on springtail (Collembola). Most Collembola species exhibited lower Δ¹⁴C values than litter regardless of their δ¹³C and δ¹⁵N signatures, indicating their dependence on young carbon. In contrast with general patterns across all taxonomic groups, we found a significant negative correlation between δ¹⁵N and Δ¹⁴C values among the edaphic Collembola. This means that the species with higher δ¹⁵N values depend on C from more recent photosynthate, which suggests that soil-dwelling species generally feed on mycorrhizae to obtain root-derived C. Many predatory taxa exhibited higher Δ¹⁴C values than Collembola but lower than litter, indicating non-negligible effects of collembolan feeding habits on the soil food web. Our study demonstrated the usefulness of radiocarbon analysis, which can untangle the confounding factors that change collembolan δ¹⁵N values, clarify animal feeding habits and define the roles of organisms in soil food webs.

Modernizing the Toolkit for Arthropod Bloodmeal Identification

Simple Summary

The ability to identify the source of vertebrate blood in mosquitoes, ticks, and other blood-feeding arthropod vectors greatly enhances our knowledge of how vector-borne pathogens are spread. The source of the bloodmeal is identified by analyzing the remnants of blood remaining in the arthropod at the time of capture, though this is often fraught with challenges. This review provides a roadmap and guide for those considering modern techniques for arthropod bloodmeal identification with a focus on progress made in the field over the past decade. We highlight genome regions that can be used to identify the vertebrate source of arthropod bloodmeals as well as technological advances made in other fields that have introduced innovative new ways to identify vertebrate meal source based on unique properties of the DNA sequence, protein signatures, or residual molecules present in the blood. Additionally, engineering progress in miniaturization has led to a number of field-deployable technologies that bring the laboratory directly to the arthropods at the site of collection. Although many of these advancements have helped to address the technical challenges of the past, the challenge of successfully analyzing degraded DNA in bloodmeals remains to be solved.

Abstract

Understanding vertebrate–vector interactions is vitally important for understanding the transmission dynamics of arthropod-vectored pathogens and depends on the ability to accurately identify the vertebrate source of blood-engorged arthropods in field collections using molecular methods. A decade ago, molecular techniques being applied to arthropod blood meal identification were thoroughly reviewed, but there have been significant advancements in the techniques and technologies available since that time. This review highlights the available diagnostic markers in mitochondrial and nuclear DNA and discusses their benefits and shortcomings for use in molecular identification assays. Advances in real-time PCR, high resolution melting analysis, digital PCR, next generation sequencing, microsphere assays, mass spectrometry, and stable isotope analysis each offer novel approaches and advantages to bloodmeal analysis that have gained traction in the field. New, field-forward technologies and platforms have also come into use that offer promising solutions for point-of-care and remote field deployment for rapid bloodmeal source identification. Some of the lessons learned over the last decade, particularly in the fields of DNA barcoding and sequence analysis, are discussed. Though many advancements have been made, technical challenges remain concerning the prevention of sample degradation both by the arthropod before the sample has been obtained and during storage. This review provides a roadmap and guide for those considering modern techniques for arthropod bloodmeal identification and reviews how advances in molecular technology over the past decade have been applied in this unique biomedical context.

Natural history study of an understudied sea catfish species from Panama (Siluriformes: Ariidae)

ABSTRACT The semi-anadromous sea catfish species Cathorops tuyra (Ariidae, Besudo sea catfish) from the Tropical Eastern Pacific has been found reproductively active in the freshwater rivers and lakes of the Panama Canal. Despite growing concerns for biodiversity, reports on natural history are lacking for many Neotropical sea catfishes. We aimed to provide data on the diet and seasonal timing of spawning of C. tuyra for an autochthonous, semi-anadromous, brackish water population from Rio Santa Maria and an allochthonous freshwater non-migrating population from Rio Chagres, an affluent to the Panama Canal, to understand how changing from semi-anadromous to residential lifestyle affects the natural history of a species. Fish from both sampling sites were dissected and information on stomach content, size, weight, parasitic load, sex, maturity, and number of eggs were recorded. In Rio Chagres, there was a female bias and individuals were larger and in pre-spawning mode compared to Rio Santa Maria. Parasite prevalence was low in Rio Chagres and zero in Rio Santa Maria. The diets were very similar between populations: gastropods, bivalves, and insects were the most important prey items in both rivers representing a diverse omnivorous diet that is similar to that of other catfishes.

Differentiating wild, lake-farmed and pond-farmed carp using stable isotope and multi-element analysis of fish scales with chemometrics

Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios and multi-element signatures of pooled fish scales were used to differentiate wild, lake-farmed and pond-farmed carp from Dongting Lake, China. Fish scales were found to be important archives for the dietary history and trophic level of wild and farmed fish, indicating their food sources. δ¹³C and δ¹⁵N values of pond-farmed carp scales were comparatively enriched to wild and lake-farmed carp due to their animal protein-derived feeds. Multi-element compositions of fish scales also showed significant differences between wild and farmed fish. A partial least squares discriminant analysis (PLS-DA) model correctly discriminated the three carp groups. Discrimination accuracies of wild, lake-farmed and pond-farmed carp were 100%, 95%, and 100% for the training set, and 100% for the testing set. This strategy provides a promising non-lethal alternative method to combat mislabeling of freshwater carp from different farming methods.

Weaning and stunting affect nitrogen and carbon stable isotope natural abundances in the hair of young children

Natural abundances of stable nitrogen and carbon isotopes (δ¹⁵N and δ¹³C) can vary with both dietary intake and metabolic (specifically catabolic) state. In low-income countries, weaning is a period of dietary transition from milk to plant-based foods and a high-risk period for malnutrition. We explored how diet and malnutrition impact hair δ¹⁵N and δ¹³C in young children by an observational, cross-sectional study in Cox’s Bazar District, Bangladesh [255 children, 6–59 months with 19.6% wasted (7.1% severely) and 36% stunted (9.8% severely)]. Hair δ¹⁵N and δ¹³C exhibited exponential decreases with age, with the loss of one trophic level (3.3‰ and 0.8‰, respectively) from 6 to 48 months, which we associate with the shift from exclusive breastfeeding to complete weaning. After adjustment for age and breastfeeding status, hair isotopic values were unaffected by wasting but lower in severe stunting (−0.45‰ to −0.6‰, P < 0.01). In this population of young children, whose isotopic values in hair primarily depended on age, we failed to observe any effect of wasting, likely due to opposite, compensating effects between dietary and metabolic changes involved. In contrast, we evidenced low δ¹⁵N and δ¹³C values in severely stunted children that likely indicate chronic exposure to diets low in animal products.

Variation in arsenolipid concentrations in seafood consumed in Japan

Variation in arsenolipid concentrations was assessed in 18 seafood samples including fish, shellfish, and crustaceans purchased in Japan. Analyses were performed by high performance liquid chromatography-inductively coupled plasma mass spectrometry/electrospray ionization tandem mass spectrometry. Stable isotope ratios for nitrogen and carbon were also measured in the samples for obtaining trophic level information of the species. Arsenic-containing hydrocarbons (AsHCs) and arsenic-containing fatty acids (AsFAs) were detected in the seafood samples; the toxic AsHCs were found in all of the seafood samples with large variation in the concentrations (83 ± 73 ng As/g fw, coefficient of variation = 88%). Our previous point estimate of health risk of AsHCs intake via seafood consumption in Japan, based on average AsHC concentration in seafood, suggested insignificant risk, and the present study supports our previous estimate. AsHC concentrations significantly correlated with lipid content of the seafood samples (r = 0.67, p < 0.01), a result expected because of the fat solubility of the compounds. The AsHCs concentrations, however, were not significantly correlated with nitrogen stable isotope ratios suggesting that AsHCs do not biomagnify. The source of the observed large variation in AsHC concentrations will be the subject of further investigation.

Caste-specific nutritional differences define carbon and nitrogen fluxes within symbiotic food webs in African termite mounds

Fungus-growing termites of the genus Macrotermes cultivate symbiotic fungi (Termitomyces) in their underground nest chambers to degrade plant matter collected from the environment. Although the general mechanism of food processing is relatively well-known, it has remained unclear whether the termites get their nutrition primarily from the fungal mycelium or from plant tissues partly decomposed by the fungus. To elucidate the flows of carbon and nitrogen in the complicated food-chains within the nests of fungus-growing termites, we determined the stable isotope signatures of different materials sampled from four Macrotermes colonies in southern Kenya. Stable isotopes of carbon revealed that the termite queen and the young larvae are largely sustained by the fungal mycelium. Conversely, all adult workers and soldiers seem to feed predominantly on plant and/or fungus comb material, demonstrating that the fungal symbiont plays a different nutritional role for different termite castes. Nitrogen stable isotopes indicated additional differences between castes and revealed intriguing patterns in colony nitrogen cycling. Nitrogen is effectively recycled within the colonies, but also a presently unspecified nitrogen source, most likely symbiotic nitrogen-fixing bacteria, seems to contribute to nitrogen supply. Our results indicate that the gut microbiota of the termite queen might be largely responsible for the proposed nitrogen fixation.

Shift in trophic niches of soil microarthropods with conversion of tropical rainforest into plantations as indicated by stable isotopes (15N, 13C)

Land-use change is threatening biodiversity worldwide, affecting above and below ground animal communities by altering their trophic niches. However, shifts in trophic niches with changes in land use are little studied and this applies in particular to belowground animals. Oribatid mites are among the most abundant soil animals, involved in decomposition processes and nutrient cycling. We analyzed shifts in trophic niches of six soil-living oribatid mite species with the conversion of lowland secondary rainforest into plantation systems of different land-use intensity (jungle rubber, rubber and oil palm monoculture plantation) in two regions of southwest Sumatra, Indonesia. We measured stable isotope ratios (13C/12C and 15N/14N) of single oribatid mite individuals and calculated shifts in stable isotope niches with changes in land use. Significant changes in stable isotope ratios in three of the six studied oribatid mite species indicated that these species shift their trophic niches with changes in land use. The trophic shift was either due to changes in trophic level (δ15N values), to changes in the use of basal resources (δ13C values) or to changes in both. The trophic shift generally was most pronounced between more natural systems (rainforest and jungle rubber) on one side and monoculture plantations systems (rubber and oil palm plantations) on the other, reflecting that the shifts were related to land-use intensity. Although trophic niches of the other three studied species did not differ significantly between land-use systems they followed a similar trend. Overall, the results suggest that colonization of very different ecosystems such as rainforest and intensively managed monoculture plantations by oribatid mite species likely is related to their ability to shift their trophic niches, i.e. to trophic plasticity.

Gut content and stable isotope analysis of tadpoles in floodplain wetlands

Larval amphibians (tadpoles) are an important link in aquatic food webs, as they can be highly abundant consumers and prey for a wide variety of predators. Most tadpoles are considered omnivores, predominately grazing on algae, detritus and macrophytes, though recent work has identified greater plasticity and breadth in diet than previously considered. We used gut content and stable isotope analysis (SIA) in a baseline study to determine the important dietary items (ingested material) and food sources (assimilated material) for tadpoles of two abundant generalist frog species in regulated floodplain wetlands of the Murrumbidgee River, south-east Australia. We identified a wide variety of dietary items in the gut contents, including whole microcrustaceans, filamentous algae and macrophytes. The composition of several ingested food items was correlated with their availability in each wetland. However, SIA identified biofilm as the food source most consistently assimilated across several wetlands, though microcrustaceans and algae contributed when abundant. Biofilm is likely the most important basal food item for tadpoles in floodplain wetlands because it is ubiquitous and has a high nutritional quality. Identifying important food sources is a crucial step towards developing management strategies for promoting tadpole recruitment in regulated wetlands.

Stable C and N isotope analysis of hair suggest undernourishment as a factor in the death of a mummified girl from late 19th century San Francisco, CA

The chance discovery of a 1.5–3.5 years old mummified girl presents a unique opportunity to further our understanding of health and disease among children in 19^th Century San Francisco. This study focuses on carbon and nitrogen stable isotope signatures in serial samples of hair that cover the last 14 months of her life. Results suggest an initial omnivorous diet with little input from marine resources or C4 plants. Around six months before death δ¹⁵N starts a steady increase, with a noticeable acceleration just two months before she died. The magnitude of δ¹⁵N change, +1.5‰ in total, is consistent with severe undernourishment or starvation. Cemetery records from this time period in San Francisco indicate high rates of infant and child mortality, mainly due to bacterial-borne infectious diseases, about two orders of magnitude higher than today. Taken together, we hypothesize that the girl died after a prolonged battle with such an illness. Results highlight the tremendous impacts that modern sanitation and medicine have had since the 1800s on human health and lifespan in the United States.

Functional roles of an engineer species for coastal benthic invertebrates and demersal fish

Through their tissues or activities, engineer species create, modify, or maintain habitats and alter the distribution and abundance of many plants and animals. This study investigates key ecological functions performed by an engineer species that colonizes coastal ecosystems. The gregarious tubiculous amphipod Haploops nirae is used as a biological model. According to previous studies, the habitat engineered by H. nirae (i.e., Haploops habitat) could provide food and natural shelter for several benthic species such as benthic diatoms belonging to the gender Navicula, the micrograzer Geitodoris planata, or the bivalve Polititapes virgineus. Using data from scientific surveys conducted in two bays, this study explored whether (1) the Haploops sandy‐mud community modifies invertebrate and ichthyologic community structure (diversity and biomass); (2) H. nirae creates a preferential feeding ground; and (3) this habitat serves as a refuge for juvenile fish. Available Benthic Energy Coefficients, coupled with more traditional diversity indices, indicated higher energy available in Haploops habitat than in two nearby habitats (i.e., Sternaspis scutata and Amphiura filiformis/Owenia fusiformis habitats). The use of isotopic functional indices (IFIs) indicated (1) a higher functional richness in the Haploops habitat, related to greater diversity in food sources and longer food chains; and (2) a higher functional divergence, associated with greater consumption of a secondary food source. At the invertebrate‐prey level, IFIs indicated little specialization and little trophic redundancy in the engineered habitat, as expected for homogenous habitats. Our results partly support empirical knowledge about engineered versus nonengineered habitats and also add new perspectives on habitat use by fish and invertebrate species. Our analyses validated the refuge‐area hypothesis for a few fish species. Although unique benthic prey assemblages are associated with Haploops habitat, the hypothesis that it is a preferential feeding area was not verified. However, specialist feeding behavior was observed for predators, which calls for further investigation.

Stable isotopes reveal spatial variability in the trophic structure of a macro-benthic invertebrate community in a tropical coral reef

RATIONALE

Studies of organic matter fluxes in coral reefs are historically based on physical and biogeochemical approaches. It is important to link these approaches to community analysis as the abundance and behaviour of species, populations or trophic groups can have a profound effect on nutrient budgets.

METHODS

We determined the carbon and nitrogen isotopic compositions of coral reef organic matter sources and macro-benthic invertebrate communities using a Europa Geo 20/20 isotope ratio mass spectrometer interfaced to an ANCA-SL elemental analyzer in continuous flow mode. Isotopic ecology metrics and a mixing model were used to analyze and interpret the data.

RESULTS

The coral reef macro-invertebrate community principally relies on detrital or recycled food sources. An increased reliance on reef nitrogen-derived sources was observed in the cold-dry season. The community food-web lengths differ noticeably across the coral reef and reflect the characteristics and origin of organic matter reservoirs.

CONCLUSIONS

Anthropogenic and terrestrial inputs lead to a loss of biological diversity. Exclusive dominance of suspension-feeding species is observed in areas receiving direct surface riverine particulate organic matter. The accumulation of sediment organic matter in eutrophic areas leads to dominance of deposit-feeding species. Copyright © 2016 John Wiley & Sons, Ltd.

Eelgrass (Zostera marina) Food Web Structure in Different Environmental Settings

This study compares the structure of eelgrass (Zostera marina L.) meadows and associated food webs in two eelgrass habitats in Denmark, differing in exposure, connection to the open sea, nutrient enrichment and water transparency. Meadow structure strongly reflected the environmental conditions in each habitat. The eutrophicated, protected site had higher biomass of filamentous algae, lower eelgrass biomass and shoot density, longer and narrower leaves, and higher above to below ground biomass ratio compared to the less nutrient-enriched and more exposed site. The faunal community composition and food web structure also differed markedly between sites with the eutrophicated, enclosed site having higher biomass of consumers and less complex food web. These relationships resulted in a column shaped biomass distribution of the consumers at the eutrophicated site whereas the less nutrient-rich site showed a pyramidal biomass distribution of consumers coupled with a more diverse consumer community. The differences in meadow and food web structure of the two seagrass habitats, suggest how physical setting may shape ecosystem response and resilience to anthropogenic pressure. We encourage larger, replicated studies to further disentangle the effects of different environmental variables on seagrass food web structure.

Stable isotopes confirm a coastal diet for critically endangered Mediterranean monk seals

Understanding the ecology and behaviour of endangered species is essential for developing effective management and conservation strategies. We used stable isotope analysis to investigate the foraging behaviour of critically endangered Mediterranean monk seals (Monachus monachus) in Greece. We measured carbon and nitrogen isotope ratios (expressed as δ(13)C and δ(15)N values, respectively) derived from the hair of deceased adult and juvenile seals and the muscle of their known prey to quantify their diets. We tested the hypothesis that monk seals primarily foraged for prey that occupy coastal habitats in Greece. We compared isotope values from seal hair to their coastal and pelagic prey (after correcting all prey for isotopic discrimination) and used these isotopic data and a stable isotope mixing model to estimate the proportion of coastal and pelagic resources consumed by seals. As predicted, we found that seals had similar δ(13)C values as many coastal prey species and higher δ(13)C values than pelagic species; these results, in conjunction with mean dietary estimates (coastal=61 % vs. pelagic=39 %), suggest that seals have a diverse diet comprising prey from multiple trophic levels that primarily occupy the coast. Marine resource managers should consider using the results from this study to inform the future management of coastal habitats in Greece to protect Mediterranean monk seals.

Stable isotope and signature fatty acid analyses suggest reef manta rays feed on demersal zooplankton

Assessing the trophic role and interaction of an animal is key to understanding its general ecology and dynamics. Conventional techniques used to elucidate diet, such as stomach content analysis, are not suitable for large threatened marine species. Non-lethal sampling combined with biochemical methods provides a practical alternative for investigating the feeding ecology of these species. Stable isotope and signature fatty acid analyses of muscle tissue were used for the first time to examine assimilated diet of the reef manta ray Manta alfredi, and were compared with different zooplankton functional groups (i.e. near-surface zooplankton collected during manta ray feeding events and non-feeding periods, epipelagic zooplankton, demersal zooplankton and several different zooplankton taxa). Stable isotope δ¹⁵N values confirmed that the reef manta ray is a secondary consumer. This species had relatively high levels of docosahexaenoic acid (DHA) indicating a flagellate-based food source in the diet, which likely reflects feeding on DHA-rich near-surface and epipelagic zooplankton. However, high levels of ω6 polyunsaturated fatty acids and slightly enriched δ¹³C values in reef manta ray tissue suggest that they do not feed solely on pelagic zooplankton, but rather obtain part of their diet from another origin. The closest match was with demersal zooplankton, suggesting it is an important component of the reef manta ray diet. The ability to feed on demersal zooplankton is likely linked to the horizontal and vertical movement patterns of this giant planktivore. These new insights into the habitat use and feeding ecology of the reef manta ray will assist in the effective evaluation of its conservation needs.

Identifying migrations in marine fishes through stable-isotope analysis

The isotopic composition of many elements varies across both land and ocean surfaces in a predictable fashion. These stable-isotope ratios are transferred into animal tissues, potentially providing a powerful natural geospatial tag. To date, most studies using stable isotopes as geolocators in marine settings have focussed on mammals and seabirds conducting large ocean-basin scale migrations. An increasing understanding of isotopic variation in the marine environment, and improved sampling and analytical techniques, however, means that stable isotopes now hold genuine promise as a natural geolocation tag in marine fishes. Here, the theoretical background underpinning the use of stable isotopes of C, N and O in otolith, scale and muscle tissues as geolocation tools in the marine environment is reviewed, and examples of their applications are provided.

A 700-year record of mercury in avian eggshells of Guangjin Island, South China Sea

Ancient eggshells over the past 700 years were extracted from an ornithogenic sediment profile on Guangjin Island, South China Sea. Based on SEM and nitrogen isotope analyses, we determined that neither post-depositional processes nor seabirds' dietary changes had a large influence on eggshell Hg levels. The historical change of Hg in these eggshells was reconstructed. Eggshell Hg was a marker for past Hg deposition in marine environment. The eggshell Hg showed three small peaks at around 1300AD, 1600 AD and 1700-1750AD and rapid increase since 1800 AD. Before 1970 AD the Hg deposition in the Xisha area had global distribution characteristics, with increased Hg emissions due to global anthropogenic activities in industrial times. However, after 1970 AD, a further sharp increase up to present day occurred, implying that the Hg production center had gradually shifted from Europe and America to Asia.

Food partitioning and diet temporal variation in two coexisting sparids, Pagellus erythrinus and Pagellus acarne

Resource partitioning in two congeneric sparids, pandora Pagellus erythrinus and axillary seabream Pagellus acarne, was investigated using stomach content analysis integrated with data on stable isotopes (δ(15) N and δ(13) C). The study was carried out on coastal muddy bottoms in the Gulf of Castellammare (southern Tyrrhenian Sea, western Mediterranean Sea) in seasons (autumn, November 2004; winter, March 2005; spring, early June 2005), at depths between 50 and 100 m. Stomach content analysis suggested low trophic niche overlap between the two species. Pagellus erythrinus mainly preyed on strictly benthic organisms (polychaetes, brachyuran crabs and benthic crustaceans). Although it consumed benthic prey, P. acarne preferred suprabenthic prey such as peracarid crustaceans from the benthic boundary layer a few metres above the bottom. The two species showed different isotopic values, with P. erythrinus exhibiting higher δ(15) N and more enriched δ(13) C than P. acarne, in accordance with its marked benthic behaviour and high predation on carnivore polychaetes. Significant temporal variability in both diet and isotopic values caused trophic differences between the two species. The autumn and winter diet differed from the spring diet and the trophic levels of both species increased from autumn and winter to spring, in accordance with variations in food availability and changes in prey δ(15) N and δ(13) C. These temporal variations may be linked to an increase in energy requirements for reproduction, together with the differing availability of preferred prey throughout the year. Significantly, lower δ(13) C was recorded in fishes collected in winter (March), suggesting the influence of river inputs as a source of particulate organic matter in this zone after the flooding season. In conclusion, these sympatric congeneric fish species displayed clear food partitioning throughout the temporal scale analysed.

Intraspecific comparison of diet of California sea lions (Zalophus californianus) assessed using fecal and stable isotope analyses

The diet of juvenile and adult female California sea lions ( Zalophus californianus (Lesson, 1828)) at San Miguel Island, California, was estimated and compared using fecal and stable isotope analyses to determine dietary differences by age. Fecal samples were collected during 2002–2006 and prey remains were identified. Stable carbon (δ13C) and stable nitrogen (δ15N) isotope values were determined from plasma and fur obtained from yearlings, 2- to 3-year-old juveniles, and adult females during 2005 and 2006. Juveniles ate more than 15 prey taxa, whereas adult females consumed more than 33 taxa. Relative importance of prey was determined using percent frequency of occurrence (%FO). Engraulis mordax Girard, 1854, Sardinops sagax (Jenyns, 1842), Merluccius productus (Ayres, 1855), genus Sebastes Cuvier, 1829, and Loligo opalescens Berry, 1911 were the most frequently occurring (%FO > 10%) prey in the feces of both juvenile and adult female sea lions, although their importance varied between age groups. Only yearlings had significantly different isotopic values than older conspecifics, indicating that older juveniles were feeding at a similar trophic level and in similar habitats as adult females. Whereas each method had biases, combining the two provided a better understanding of the diet of California sea lions and intraspecific differences.

Differences in urbanization and degree of marine influence are reflected in delta13C and delta15N of producers and consumers in seagrass habitats of Puerto Rico

Couplings between land use and marine food webs in tropical systems are poorly understood. We compared land-sea coupling in seven sites around Puerto Rico, differing in the degree of precipitation and urbanization, by measuring delta(13)C and delta(15)N in producers and consumers. delta(15)N values were influenced by human activity: the food web from sites near urbanized centers was on average 1 per thousand heavier in delta(15)N compared to undeveloped sites. This is most likely due to wastewater inputs from septic systems relatively near the shoreline. Changes in delta(13)C were best explained by differences in the degree of marine influence. Where terrestrial inputs from a major river dominated, delta(13)C values were lighter, whereas sites further from land and in locations exposed to oceanic currents had heavier delta(13)C values, characteristic of a marine source of dissolved organic carbon. We found no significant effect of precipitation on connectivity in spite of a twofold difference in annual average rainfall between the north and south coast. The results suggest there is some connectivity between land and sea in Puerto Rico, despite high rates of evaporation relative to precipitation.

Isotopic composition of nitrogenous compounds with emphasis on anthropogenic loading in river ecosystems

Nitrogenous compounds with high delta(15)N values were recently found in human-dominated small rivers in the Lake Biwa area. A detailed survey was performed to determine the distribution and variation of delta(15)N values in nitrogenous compounds in a representative small river (Hebisuna River) that flows into Lake Nishinoko, an inner bay of Lake Biwa. A high delta(15)N value was detected in the lower reaches of the river and the inner bay, most likely due to denitrification. These results strongly suggest that denitrification in small river systems such as the Hebisuna watershed has contributed to (15)N enrichment in the Lake Biwa ecosystem during the past 40 years. We also observed a clear, stepwise, positive correlation between population density and delta(15)N values for particulate organic matter or sediments. These results demonstrate that delta(15)N (POM) and delta(15)N (sediment) are helpful indicators for assessing nitrogen loading from domestic sewage. Moreover, they will aid in the development of new concepts in the environmental capacity of river ecosystems and its relationship to redox conditions. Finally, our data suggest that a population density of 100-200 persons per km(2) is the upper limit for a watershed in which only simple conventional sewage treatment is in effect.

The loss of aquatic and riparian plant communities: Implications for their consumers in a riverine food web

Abstract  Human induced alterations to rivers and steams have resulted in significant changes to the structure and diversity of riparian and aquatic plant communities. These changes will impact on the dynamics of riverine carbon cycles and food web structure and function. Here we investigate the principal sources of organic carbon supporting local shredder communities across a gradient in different levels of anthropogenic development along riverine reaches, in South Australia. In forested/wooded reaches with minimum to limited development, semi-emergent macrophytes were the principal sources of organic carbon supporting the local shredder communities. However, in developed reaches, course particulate organic matter and filamentous algae were the principal food sources. The C:N ratios of the food sources in developed reaches were higher than those of their consumers indicating a stoichiometric mismatch. This imbalanced consumer-resource nutrient ratio in those developed reaches is likely to impose constraints on the growth and reproduction of their aquatic shredder communities with probable knock-on effects to higher trophic levels.

Stable isotope analysis can potentially identify completely-digested bloodmeals in mosquitoes

Background

Vertebrate bloodfeeding is a critical component of a mosquito's ability to transmit pathogens that cause diseases such as malaria, dengue fever and viral encephalitis. Due to degradation by the digestive process, current methods to identify mosquito bloodmeal sources are only useful for approximately 36 hours post-feeding. A critical need exists for technologies to extend this window and gain a more complete picture of mosquito feeding behavior for epidemiological studies. Stable isotopes are useful for investigating organism feeding behavior because the isotopic ratio of an organism's tissues reflects that of the material it ingests.

Methodology/Principal Findings

Proof-of-principle data indicates that after bloodfeeding, Aedes albopictus mosquitoes acquire diagnostic Carbon and Nitrogen stable isotope profiles from their vertebrate hosts that can be accurately identified one week post-feeding, approximately 4 days after the entire bloodmeal has been digested. Total C/N ratio served as a biomarker marker for bloodfeeding (P<0.02), while δN was the most informative variable which could distinguish between unfed, chicken-fed and human-fed mosquitoes (P<0.01). By plotting C/N vs. δN, all feeding treatments could be identified in a double-blind analysis.

Conclusions/Significance

These proof-of-principle experiments indicate that analysis of stable isotopes can be used to distinguish bloodfed from unfed mosquitoes, and also distinguish between different vertebrate bloodmeal sources even after all blood has been digested. The development of stable isotope-based assays for mosquito bloodmeal identification may be a powerful tool to investigate mosquito feeding ecology and the dynamics of vector-borne pathogens.

Food web analysis in two permanently open temperate estuaries: consequences of saltmarsh loss?

Saltmarsh vegetation, seston and microphytobenthos are all conspicuous components of most temperate estuaries and they potentially contribute to the estuarine food chain. Yet their relative contributions are unclear, as is the significance of saltmarsh losses through natural and human-induced impacts. This study aimed to quantitatively determine the contribution of various types of primary producers to detritus in the Walpole-Nornalup Estuary and Leschenault Inlet, two permanently open estuaries in SW Australia, and, estimate the flow of different types of detritus to higher trophic levels, using carbon ((13)C) and nitrogen ((15)N) stable isotopes as tracers. Results of the mixing model indicated that seston, microphytobenthos and to some extent seagrass and fringing saltmarsh were the main contributors to the detrital pool in both estuaries. However, the relative contribution of different primary producers varied both within and between estuaries. The contribution of saltmarsh was higher at sites close to rivers and dense fringing vegetation, while seston, microphytobenthos and seagrass dominated the detrital material at other sites. Benthic harpacticoid copepods were shown to feed on detritus though they appeared to actively select for different components of the detritus depending on site and estuary. Isotopic signatures of other consumers indicated that fish and invertebrates derived nutrients from MPB and detritus, either directly as food or indirectly through feeding on invertebrates. The overall contribution of saltmarsh to detritus was lower in Leschenault Inlet than in Walpole-Nornalup Estuary, possibly as a result of increased clearing of fringing vegetation around Leschenault Inlet. This pattern was however not reflected in harpacticoid food. Therefore, although losses of fringing saltmarsh around estuaries have the potential to significantly affect estuarine food webs, the significance of such losses will be site- and estuary-dependent.

Intraguild interactions between spiders and ants and top-down control in a grassland food web

In most terrestrial ecosystems ants (Formicidae) as eusocial insects and spiders (Araneida) as solitary trappers and hunters are key predators. To study the role of predation by these generalist predators in a dry grassland, we manipulated densities of ants and spiders (natural and low density) in a two-factorial field experiment using fenced plots. The experiment revealed strong intraguild interactions between ants and spiders. Higher densities of ants negatively affected the abundance and biomass of web-building spiders. The density of Linyphiidae was threefold higher in plots without ant colonies. The abundance of Formica cunicularia workers was significantly higher in spider-removal plots. Also, population size of springtails (Collembola) was negatively affected by the presence of wandering spiders. Ants reduced the density of Lepidoptera larvae. In contrast, the abundance of coccids (Ortheziidae) was positively correlated with densities of ants. To gain a better understanding of the position of spiders, ants and other dominant invertebrate groups in the studied food web and important trophic links, we used a stable isotope analysis ((15)N and (13)C). Adult wandering spiders were more enriched in (15)N relative to (14)N than juveniles, indicating a shift to predatory prey groups. Juvenile wandering and web-building spiders showed delta(15)N ratios just one trophic level above those of Collembola, and they had similar delta(13)C values, indicating that Collembola are an important prey group for ground living spiders. The effects of spiders demonstrated in the field experiment support this result. We conclude that the food resource of spiders in our study system is largely based on the detrital food web and that their effects on herbivores are weak. The effects of ants are not clear-cut and include predation as well as mutualism with herbivores. Within this diverse predator guild, intraguild interactions are important structuring forces.

A 2,000-year record of mercury and ancient civilizations in seal hairs from King George Island, West Antarctica

The concentrations of total mercury (Hg(T)) and three bio-essential elements (phosphor, potassium, sodium) were analyzed in Antarctic seal hairs from a lake core spanning the past 2,000 years and collected from King George Island (63 degrees 23'S, 57 degrees 00'W), West Antarctica. The Hg(T) concentration shows a significant fluctuation while the levels of the three bio-essential elements remain almost constant. The rise and fall of the Hg(T) concentration in the seal hairs are found to be closely coincided with ancient activities of gold and silver mining using Hg-amalgamation process around the world, especially in the Southern Hemisphere. Briefly, Hg(T) levels are high during five episodes of extensive gold and silver mining activities--Rome Empire and China Han Dynasty (approximately 18-300 A.D.), Maya period and China Tang (750-900 A.D.), Incas civilization and Christian Kingdom (1200-1500 A.D.), New world (1650-1800 A.D.), and modern industry period (1840 A.D.-present); they are low during four time periods of reduced gold and silver mining activities--the China Han and Rome fall (since 300 A.D.), Maya fall and Wartime period in China (1050-1250 A.D.), Pizarro coming (ca. 1532 A.D.) and Independence War of South America (1800-1830 A.D.). Two profiles of Hg(T) in other two lake cores, one affected by seal excrements and the other by penguin droppings, from the same region are similar to the one in seal hairs. The Hg concentration profile in the seal hairs is significantly correlated with the one in a peat bog of Southern Chile near King George Island. Since Hg is existent mainly at the form of methyl-mercury in seal hairs, this correlation supports a relationship and link between atmospheric mercury concentration and methyl-mercury production. Comparing with samples from American and European continents, the Antarctic seal hairs provide an archive of total mercury concentration in surface seawater of the South Ocean less affected by regional human activities, and this archive may provide a good reference for assessing the global Hg emissions, depositions and recycling in the past thousand years.

Stable isotope analysis of production and trophic relationships in a tropical marine hard-bottom community

Seagrasses produce much of the organic carbon in the shallow waters of the Caribbean and it has long been assumed that a substantial portion of this carbon is exported to nearby habitats, contributing substantially to their food webs. In the shallow coastal waters of the Florida Keys (USA), seagrass intersperses with hard-bottom habitat where bushy, red macroalgae are the most prominent primary producers. However, the relative importance of seagrass-derived carbon versus autochthonous algal production or phytoplankton in supporting higher trophic levels within hard-bottom communities has never been investigated. We compared the carbon and nitrogen isotopic values of potential primary producers and representative higher trophic level taxa from hard-bottom sites on the bay-side and ocean-side of the Florida Keys. We also included in our study a set of bay-side sites that experienced significant ecological disturbances over the past decade (e.g., cyanobacteria blooms, seagrass die-off, and sponge die-offs) that may have altered trophic relationships in those regions. We did not detect any differences among regions in the trophic status of hard-bottom taxa that might be associated with ecosystem disturbance. However, our results suggest that autochthonous production of algal detritus is an important source of secondary production in these hard-bottom communities, with seagrass and phytoplankton contributing smaller fractions.

Carbon and nitrogen stable isotopic composition of hair protein and amino acids can be used as biomarkers for animal-derived dietary protein intake in humans

The stable nitrogen (15N) and carbon (13C) isotopic composition of tissues reflects the isotopic pattern of food sources. We investigated whether the isotopic composition of human hair can be used as a biomarker to predict the dietary intake of animal-derived food. Hair samples were collected from subjects during a 1987-1988 German nutrition survey (VERA) in which dietary information was collected using a 7-d dietary record. Samples of 50 men and 50 women were randomly selected, in addition to 27 samples of subjects with a reported low meat intake. Isotope ratio MS was used to analyze hair bulk and amino acid-specific isotopic composition. Its relation with and feasibility for predicting animal protein intake were tested using regression analysis and cross-tabulation of observed and predicted dietary data and comparison of the individual values for the binary categories of high and low intake. 15N and 13C abundances strongly predicted relative animal protein and meat intake (R2= 0.31, P < 0.01 and R2= 0.20, P <0.01, respectively). Distinct patterns of individual hair amino acid 15N and 13C abundances were observed. In contrast to bulk values, the isotopic abundances in individual amino acids did not show discriminating ability across sex and isotope-specific categories. We conclude that hair 13C values are as predictive for animal protein consumption as hair 15N values. Bulk isotopic abundance of hair can be used as a biomarker for animal protein intake to validate dietary assessment methods provided that the correlation between isotopic abundances and dietary protein intake is verified in dietary intervention studies.

Background and interpretation of the 'Marine Trophic Index' as a measure of biodiversity

Since the demonstration, in 1998, of the phenomenon now widely known as 'fishing down marine food webs', and the publication of a critical rejoinder by Food and Agricultural Organization (FAO) staff, a number of studies have been conducted in different parts of the world, based on more detailed data than the global FAO fisheries statistics originally used, which established the validity and ubiquity of this phenomenon. In this contribution, we briefly review how, rather than being an artefact of biased data, this phenomenon was in fact largely masked by such data, and is in fact more widespread than was initially anticipated. This is made visible here by comparing two global maps of trophic level (TL) changes from the early 1950s to the present. The first presents the 50-year difference of the grand mean TL values originally used to demonstrate the fishing down effect, while the second is based on means above a cut-off TL (here set at 3.25), thus eliminating the highly variable and abundant small pelagic fishes caught throughout the world. Based on this, we suggest that using mean TL as 'Marine Trophic Index' (MTI), as endorsed by the Convention on Biological Diversity, always be done with an explicitly stated cut-off TL (i.e. cutMTI), chosen (as is the case with our proposed value of 3.25) to emphasize changes in the relative abundance of the more threatened, high-TL fishes. We also point out the need to improve the taxonomic resolution, completeness and accuracy of the national and international fisheries catch data series upon which the cutMTI is to be based.

Stable isotopes show food web changes after invasion by the predatory cladoceran Cercopagis pengoi in a Baltic Sea bay

Cercopagis pengoi, a recent invader to the Baltic Sea and the Laurentian Great Lakes, is a potential competitor with fish for zooplankton prey. We used stable C and N isotope ratios to elucidate trophic relationships between C. pengoi, zooplankton (microzooplankton, 90-200 microm, mostly copepod nauplii and rotifers; mesozooplankton, >200 microm, mostly copepods), and zooplanktivorous fish (herring, size range 5-15 cm and sprat, 9-11 cm) in a coastal area of the northern Baltic Sea. The isotope ratios in C. pengoi and fish were much higher than those of zooplankton, showing general trends of enrichment with trophic level. Young-of-the-year (YOY) herring had a significantly higher (15)N/(14)N ratio than C. pengoi, suggesting of a trophic linkage between the two species. To evaluate the possible relative importance of different food sources for C. pengoi and YOY herring, two-source isotope-mixing models for N were used, with micro- and mesozooplankton as prey for C. pengoi and mesozooplankton and C. pengoi as prey for YOY herring. These models indicate that mesozooplankton was the major food source of both species. However, microzooplankton may be important prey for young stages of C. pengoi. Comparative analyses of the herring trophic position before and after the invasion by C. pengoi showed a trophic level shift from 2.6 to 3.4, indicating substantial alterations in the food web structure. Our findings contribute to a growing body of evidence, showing that C. pengoi can modify food webs and trophic interactions in invaded ecosystems.

Choice of dietary protein of vegetarians and omnivores is reflected in their hair protein 13C and 15N abundance

Stable isotopic (15N, 13C) composition of tissues depends on isotopic pattern of food sources. We investigated whether the isotopic compositions of human hair protein and amino acids reflect the habitual dietary protein intake. Hair samples were analyzed from 100 omnivores (selected randomly out of the 1987-1988 German nutrition survey VERA), and from 15 ovo-lacto-vegetarians (OLV), and from 6 vegans recruited separately. Hair bulk and amino acid specific isotopic compositions were analyzed by isotope-ratio mass spectrometry (EA/IRMS and GC/C/IRMS, respectively) and the results were correlated with data of the 7 day dietary records. Hair bulk 15N and 13C abundances clearly reflect the particular eating habits. Vegans can be distinguished from OLV and both are significantly distinct from omnivores in both 15N and 13C abundances. 15N and 13C abundances rose with a higher proportion of animal to total protein intake (PAPI). Individual proportions of animal protein consumption (IPAP) were calculated using isotopic abundances and a linear regression model using animal protein consumption data of vegans (PAPI = 0) and omnivores (mean PAPI = 0.639). IPAP values positively correlated with the intake of protein, meat, meat products, and animal protein. Distinct patterns for hair amino acid specific 15N and 13C abundances were measured but with lower resolution between food preference groups compared with bulk values. In conclusion, hair 13C and 15N values both reflected the extent of animal protein consumption. Bulk isotopic abundance of hair can be tested for future use in the validation of dietary assessment methods.

Nitrogen balance and delta15N: why you're not what you eat during pregnancy

Carbon (13C/12C) and nitrogen (15N/14N) stable isotope ratios were longitudinally measured in human hair that reflected the period from pre-conception to delivery in 10 pregnant women. There was no significant change in the delta13C results, but all subjects showed a decrease in delta15N values (-0.3 to -1.1 per thousand) during gestation. The mechanisms causing this decrease in hair delta15N have not been fully elucidated. However, since the delta15N values of dietary nitrogen and urea nitrogen are significantly lower compared to maternal tissues, it is hypothesized that the increased utilization of dietary and urea nitrogen for tissue synthesis during pregnancy resulted in a reduction of the steady state diet to a body trophic level effect by approximately 0.5-1 per thousand. An inverse correlation (R2 = 0.67) between hair delta15N and weight gain was also found, suggesting that positive nitrogen balance results in a reduction of delta15N values independent of diet. These results indicate that delta15N measurements have the ability to monitor not only dietary inputs, but also the nitrogen balance of an organism. A potential application of this technique is the detection of fertility patterns in modern and ancient species that have tissues that linearly record stable isotope ratios through time.

Stable-isotope ratios of blood components from captive northern fur seals (Callorhinus ursinus) and their diet: applications for studying the foraging ecology of wild otariids

Stable nitrogen and carbon isotope ratios (13C/12C and 15N/14N, respectively) of plasma, serum, clotted red blood cells, and unclotted red blood cells from six captive northern fur seals (Callorhinus ursinus) and two fish species in their diet, Atlantic herring (Clupea harengus) and Icelandic capelin (Mallotus villosus), were measured. The δ15N values from the fur seal blood components ranged from 15.5 ± 0.1‰ (mean ± SE; all RBCs) to 16.7 ± 0.1‰ (plasma), and δ13C values ranged from –18.3 ± 0.1‰ (serum) to –17.5 ± 0.1‰ (clotted RBCs). Fur seal blood components had higher δ15N and δ13C values than their diet. Mean enrichments of 15N between fur seal RBCs and plasma/serum and their prey were +4.1 and +5.2‰, respectively, while mean 13C enrichments were +0.6‰ (serum), +1.0‰ (plasma), and +1.3 to +1.4‰ (all RBCs). Fur seal blood components did not differ in δ15N or δ13C between the sexes. One female (Baabs) was pregnant at the time of sampling and was resampled 4 months later, when she was approximately 3 months post parturient. The δ15N values of the blood components taken from Baabs during pregnancy were all higher by 0.6‰ (unclotted RBCs) to 1.3‰ (plasma) than those of samples taken during lactation, while her δ13C values were nearly the same (RBCs) for the two time periods or lower during lactation by 0.5‰ (serum) and 0.7‰ (plasma). This study was the first to analyze isotope ratios of blood components from captive fur seals and their prey. The subsequent establishment of fractionation values between captive fur seals and their diet can be used to interpret trophic level and habitat usage of otariids feeding in the wild.