Trophic Discrimination Factors Research Articles

Isotopic trophic discrimination factors (Δ13C, Δ15N) and their determinants in a subtropical macroinvertebrate food chain

The accurate estimation of trophic discrimination factors (TDFs) for stable carbon and nitrogen isotopes (Δ13C and Δ15N) requires studies for each particular species or taxonomic group, because the use of general TDFs values for all animals obtained from the literature represents an important bias in isotopic modelling. Values for Δ13C and Δ15N were estimated in an aquatic food chain (periphyton-Chironomini-Perithemis sp.), in a subtropical region of South America, under experimental conditions using two approaches: i) Traditional arithmetic equation and ii) Bayesian inference from mixing models. The effect of diet quality on TDF variability was also evaluated. We report values for Δ13C and Δ15N for Chironomini when feeding on periphyton (1.12 ± 1.31‰ and 0.92 ± 1.94‰ for C and N, respectively using the arithmetic equation) and for Perithemis sp. when feeding on Chironomini (0.65 ± 1.52‰ and 0.90 ± 1.08‰ for C and N, respectively, according to the arithmetic equation). We obtained similar results when using Bayesian inference. We did not find effects of diet quality on Δ13C and Δ15N values; although we highlighted that, unexpectedly, the taxonomic composition of periphyton strongly affected the isotopic values of C and N. These reported values improve the accuracy of isotopic modeling for subtropical aquatic macroinvertebrates in future food web research.

Regulation of nitrogen isotopic ratios of cellular components

This paper is an attempt to generalize the nitrogen isotope information of the molecules that make up the cell from our point of view. Nitrogen in the cell exists as 20 proteinaceous amino acids, nucleobases, hemes, chlorophylls, and others, and their composition is similar among organisms. Based on a physiologically simple autonomous system that maintains a balance between inputs and outputs, it is theoretically predicted that 15N is distributed to each cellular compound with a certain regularity, and thus a specific relationship in nitrogen isotopic ratios among compounds. Previous studies essentially confirm this. The nitrogen isotopic ratio of compound i constituting the cell can be generalized as δ15Ni = δ15Nplant + Δi (TP − 1) + γi, where TP represents the trophic position, and Δi and γi are the trophic discrimination factor and the intracellular 15N distribution of compound i, respectively. Knowing reliable values of Δi and γi will help us to better understand nitrogen dynamics in the biosphere and advance our understanding of the picture of the Earth’s environment through biogeochemistry.

Improving trophic position estimates from amino acid stable isotopes by accounting for physiology and environment

AbstractNitrogen isotope analyses of amino acids (δ15N‐AA) are being increasingly used to decipher trophic dynamics. Interpretation of δ15N‐AA in consumers relies on the assumption that consumer physiological status and nutritional status of prey have negligible influences on the trophic discrimination factor (TDF), hence a constant TDF value is used in trophic position (TP) equations. Recent experiments have shown that this is not always the case and there is also a need to validate derived TP estimates in the field. We take advantage of the uniquely long time series of environmental monitoring data and archived (frozen) samples from the species‐poor Baltic Sea. We analyzed δ15N‐AA in similar sized individuals of cod and in its prey herring from four decades, 1980–2018; including time periods where dramatic reduction in condition status of cod has occurred. We expected that TDF in trophic AAs would increase during periods of poor cod condition, resulting in inflated TP estimates. We found that calculated TP and empirical estimates of TDF (difference in δ15N in trophic AAs between cod and herring) for cod increased in recent decades and that this was linked to condition status, herring (prey) lipid content and the hypoxic state of the ecosystem. Statistically adjusting TP for condition and prey lipid content as well as environmental stress (hypoxia) resulted in lower cod TP which better resembled the observed decrease in herring TP in recent decades. TP calculated from stomach analysis data in cod individuals over the same period showed no trend over time and confirmed that adjusted TP estimates mirror the real dietary TP better than unadjusted. By simultaneously measuring condition/nutritional status in both predator and prey it is possible to adjust for them as confounding variables and decipher actual consumer TP, partly overcoming the issues of unknown and variable TDF‐values. Our study also highlights the importance of including environmental stressors (here hypoxia) when interpreting TP and reconstructing food webs.

Using Stable Isotope Techniques to Analyze the Trophic Relationship between Argentine Hake (Merluccius hubbsi) and Anisakidae.

The Argentine hake (Merluccius hubbsi) is a vital fishery species in the Southwest Atlantic, recognized for its substantial economic importance. Previous studies have identified Anisakidae larvae as common parasites of M. hubbsi. However, the nutritional relationships between these parasites and their host remain poorly understood. This study employs stable isotope techniques to investigate the specific nutritional relationships between Anisakidae larvae and different tissues of M. hubbsi. The findings reveal notable differences in δ13C and δ15N compositions between the parasites and their host. The lower δ13C values in parasites compared to host tissues indicate the utilization of different carbon sources. The δ15N values of the parasites partially overlap with those of the host's stomach, indicating that the parasites primarily derive nutrients from the host's stomach. Nutritional niche indicators show that parasites have a broad carbon range (CR) and nitrogen range (NR), suggesting a high diversity in nutritional sources. The trophic discrimination factor (ΔTDF), which represents the difference in stable isotope values between host tissues and parasites, was analyzed for both δ13C and δ15N. The ΔTDFδ13C between the host liver and the parasites showed the greatest variation, indicating a strong dependence of the parasites on the liver's carbon sources. In contrast, variations in ΔTDFδ15N between host tissues and parasites were minimal. Analyzing ΔTDF across different stages of gonadal maturity in the host fish indicates that, as the gonads of the host fish mature, ΔTDFδ13C between host tissues and parasites significantly decreases (p < 0.01). The Kruskal-Wallis test showed significant differences in ΔTDFδ13C values among different parasite infection levels in muscle, liver, and stomach tissues, while no significant differences were found for ΔTDFδ15N values. These findings offer valuable insights into the nutritional relationships between parasites and hosts, aiding in a better understanding of the growth conditions and habitats of M. hubbsi.

Can biochemical tracers reveal ontogenetic trophic shift and individual prey selection in white sharks from Guadalupe Island, Northeast Pacific?

Refining the role of apex predators in marine food webs is a necessary step in predicting the consequences of their global decline under the footprint of fishing activities. White sharks (Carcharodon carcharias) are vulnerable predators, performing large migrations and able to forage on a variety of prey in different habitats. In the Northeast Pacific, juvenile and adult white sharks are found seasonally at the same aggregation sites, such as Guadalupe Island off Mexico. While adults are thought to target local pinniped colonies, very few predator-prey interactions have been documented and the diet of juveniles in this area remains poorly understood. Here we used carbon/nitrogen stable isotopes and fatty acids to characterize the trophic ecology of white sharks at Guadalupe Island. In contrast to the ontogenetic trophic shift paradigm, we detected no influence of size on muscle stable isotope and fatty acid composition, revealing no significant dietary variation between juvenile and adult sharks. Stable isotopes did not allow definitive conclusions to be drawn regarding the diet of white sharks at Guadalupe Island, due to significant variability in the contribution of different potential prey depending on the trophic discrimination factors used. However, most sharks were rich in polyunsaturated fatty acids (such as long-chain omega 3), suggesting a local diet of mainly pelagic prey (potentially large fish or cephalopods). A few individuals appeared to show recent consumption of pinnipeds, with higher proportions of saturated and monounsaturated fatty acids. These individual differences in fatty acid composition could reflect an ecological trade-off between consumption of prey rich in fat (marine mammals) versus prey rich in polyunsaturated fatty acids (pelagic prey), respectively meeting the energetic and physiological needs of white sharks. Although ontogenetic trophic changes were not able to be discerned, our results provide new insights into the physiological drivers of predator-prey interactions, which can benefit the definition of conservation strategies in a changing ocean.

Body size predicts ontogenetic nitrogen stable-isotope (δ15N) variation, but has little relationship with trophic level in ectotherm vertebrate predators

Large predators have disproportionate effects on their underlying food webs. Thus, appropriately assigning trophic positions has important conservation implications both for the predators themselves and for their prey. Large-bodied predators are often referred to as apex predators, implying that they are many trophic levels above primary producers. However, theoretical considerations predict both higher and lower trophic position with increasing body size. Nitrogen stable isotope values (δ15N) are increasingly replacing stomach contents or behavioral observations to assess trophic position and it is often assumed that ontogenetic dietary shifts result in higher trophic positions. Intraspecific studies based on δ15N values found a positive relationship between size and inferred trophic position. Here, we use datasets of predatory vertebrate ectotherms (crocodilians, turtles, lizards and fishes) to show that, although there are positive intraspecific relationships between size and δ15N values, relationships between stomach-content-based trophic level (TPdiet) and size are undetectable or negative. As there is usually no single value for 15N trophic discrimination factor (TDF) applicable to a predator species or its prey, estimates of trophic position based on δ15N in ectotherm vertebrates with large size ranges, may be inaccurate and biased. We urge a reconsideration of the sole use of δ15N values to assess trophic position and encourage the combined use of isotopes and stomach contents to assess diet and trophic level.

Use of stable isotope ratio analysis to investigate the biology and clinical significance of seal parasites.

Stranded seals are often infected with a broad range of parasites, although whether they are the cause of significant morbidity or an incidental finding is usually unclear. In this study we used stable isotope ratio analysis, a method frequently used to investigate food webs, to explore the extent to which common seal parasites feed directly on host tissue and fluids or absorb host-derived metabolites, which in turn may give an indication of their potential impact on the host's health. The trophic discrimination factor Δ15N for the nasal mite, Halarachne halichoeri, was mostly positive, ranging between −0.015 and 3.2‰ (n = 6), while for the Acanthocephalan worm, Corynosoma strumosum and the anisakid nematode, Pseudoterranova decipiens, Δ15N ranged between −4.2 and −2.0‰ (n = 7), and between −6.7 and −0.8‰ (n = 5) respectively. In the case of the lungworm, Otostrongylus circumlitus, Δ15N measured between −5.6 and 0‰ for worms collected in the stomach (n = 5), between −1.1 and 0.2‰ for worms collected from the heart (n = 3), between −0.7 and 1.9‰ for worms situated in the lungs (n = 4). Based on Δ15N, parasites could be clearly divided into those that were on a higher trophic level than their host suggesting a predator–prey-like relationship, and those that were not. It is hypothesized that Δ15N may be indicative of the clinical significance of parasite–host associations.

Species-specific trophic discrimination factors can reduce the uncertainty of stable isotope analyses

Stable isotope analysis has been broadly used to study food webs, but often relies on inaccurate assumptions of trophic isotopic discriminations, which could lead to misinterpretation of obtained results. While many taxa exhibit similar trophic discrimination factors (TDFs), crayfish, exhibit omnivorous feeding strategies, yet TDFs are missing. In this study, we determined TDFs and tissue biokinetic parameters of the marbled crayfish Procambarus virginalis as a model species. Moreover, we compared commonly used TDFs and those determined from this study and applied them across species and ecosystems as a first attempt to compare the effect of species-specific TDFs in Bayesian trophic mixing models. Our results revealed differences between the TDFs of different tissues and biokinetic parameters of crayfish. Our result also revealed TDFs differences between crayfish relying mostly on plants versus those relying on an animal diet. We found differences of TDF suitability among species and ecosystems, highlighting the need for specific TDFs for different crayfish species. This study improves our understanding and the need for species-calibrated TDFs for robust statistical analysis of stable isotope data. Our approach is widely applicable across taxa and ecosystems to reduce the bias introduced by using unspecific TDFs in Bayesian mixing models.

Critical evaluation of trophic discrimination factors using compound-specific stable isotopes in a Northwest Atlantic shelf marine food web

Critical evaluation of trophic discrimination factors using compound-specific stable isotopes in a Northwest Atlantic shelf marine food web

Integrated approach for the isotope trophic position of black-tailed gull (Larus crassirostris) eggs over a decade: Combining stable isotopes of amino acids and fatty acids composition

Recently, compound-specific isotope analysis (CSIA) using the amino acid nitrogen stable isotope ratio (δ15NAAs) has been widely used for accurate estimation of trophic position (TP). In addition, a quantitative fatty acid signature analysis (QFASA) offers insights into diet sources. In this study, we used these techniques to estimate the TP for seabirds that rely on diverse food sources across multiple ecosystems. This allows for the proper combination of factors used in TP calculation which are different for each ecosystem. The approach involved the application of a multi-mixing trophic discrimination factor (TDF) and mixing β which is a Δδ15N between trophic and source amino acid of primary producer. Since the black-tailed gulls (BTGs) are income-breeding seabirds, which rely on energy sources obtained around their breeding sites, they and their eggs could be useful bioindicators for environmental monitoring. However, the ecological properties of BTGs such as habitats, diets, and TP are not well known due to their large migration range for wintering or breeding and their feeding habits on both aquatic and terrestrial prey. In this study, the eggs were used for estimating TP and for predicting TP of mother birds to overcome difficulties such as capturing birds and collecting non-invasive tissue samples. Eggs, sampled over a decade from three Korean islands, showed spatial differences in diet origin. Considering both the food chain and physiology of BTG, the TP of eggs was estimated to be 3.3–4.0. Notably, the TP was significantly higher at site H (3.8 ± 0.1) than at site B (3.5 ± 0.2), which indicated a higher contribution of marine diet as confirmed by QFASA. Using a reproductive shift of δ15NAAs, the TP of the mother birds was predicted to be 3.6–4.3, positioning them as the top predator in the food web. The advanced integration of multiple approaches provides valuable insights into bird ecology.

Diet-tissue discrimination factors of three neotropical freshwater fishes and a comparison of the trophic position

The trophic discrimination factor (TDF) is a key parameter for stable isotope analysis and due to a lack of species-specific TDFs, mean universal values have been used, resulting in uncertainties about the trophic position of species and a call for more experiments. In this study, we have addressed the lack of experimental species-specific TDFs conducting three experiments of 128 days each to determine the TDF (muscle and liver) of three species, the piscivore Pseudoplatystoma corruscans (Spix &amp; Agassiz, 1829), and the omnivores Piaractus mesopotamicus (Holmberg, 1887) and Astyanax lacustris (Lütken, 1875), tropical fishes native to the La Plata River basin. Then, we calculated the trophic position (TP) using the mean universal TDF from literature and the species-specific TDF produced in this study for Pseudoplatystoma corruscans. We estimated the TDFs for the three species through experiment and the values found differed from the mean universal TDF in the literature. Moreover, the TP was lower when using the species-specific TDFs. The TP is important for several analyses, including its use in functional diversity. Therefore, we recommend using species-specific TDF values for calculating TP once it differs from the results calculated with mean universal TDF.

Climate change-informed dietary modeling in Pacific cod: Experimentally-derived effects of temperature and dietary quality on carbon and nitrogen stable isotope trophic discrimination factors

Stable isotope analysis is a powerful tool for dietary modeling and trophic ecology research. A crucial piece of information for isotopic dietary modeling is the accurate estimation of trophic discrimination factors (TDFs), or the isotopic offset between a consumer’s tissue and its diet. In order to parameterize stable isotope dietary models for future climate scenarios, we investigated the effect of water temperature and dietary protein and lipid content on TDFs in juvenile Pacific cod (Gadus macrocephalus). Pacific cod are a commercially and ecologically important species, with stock numbers in the northeast Pacific recently having dropped by more than 70%. We tested four water temperatures (6, 8, 10, and 12°C) and two dietary regimens (low and high lipid content), representing a range of potential ocean temperature and prey quality scenarios, in order to determine carbon and nitrogen TDFs in juvenile Pacific cod. Additionally, we assessed dietary intake and proximate composition of the experimental fish in order to estimate consumption, assimilation, and retention of dietary nutrients. The results of this study suggest that dietary protein catabolism is a primary driver of nitrogen TDF variability in juvenile Pacific cod. Across all temperature treatments from 6 to 12°C, fish reared on the lower quality, lower lipid content diet had higher nitrogen TDFs. The mean TDFs for fish raised on the higher lipid, lower protein diet were +3.40 ‰ for nitrogen (Δ15N) and +0.36 ‰ for lipid-corrected carbon (Δ LC 13C). The mean TDFs for fish raised on the lower lipid, higher protein diet were +4.09 ‰ for nitrogen (Δ15N) and 0.00 ‰ for lipid-corrected carbon (Δ LC 13C). Lipid-corrected carbon isotope data showed that, regardless of temperature, fish consuming the lower lipid diet had essentially no trophic discrimination between diet and bulk tissues. We found no ecologically meaningful differences in TDFs due to water temperature across the 6°experimental range. The results of this experiment demonstrate that dietary quality, and more specifically the use of dietary protein for energetic needs, is a primary driver of trophic discrimination factors. The TDFs determined in this study can be applied to understanding trophic ecology in Pacific cod and closely related species under rapidly changing prey availability and ocean temperature conditions.

Trophic Transfer of Halogenated Organic Pollutants in a Wetland Food Web: Insights from Compound-Specific Nitrogen Isotope of Amino Acids and Food Source Analysis.

A trophic position (TP) model (TPmix model) that simultaneously considered trophic discrimination factor and βGlu/Phe variations was developed in this study and was first applied to investigate the trophic transfer of halogenated organic pollutants (HOPs) in wetland food webs. The TPmix model characterized the structure of the wetland food web more accurately and significantly improved the reliability of TMF compared to the TPbulk, TPAAs, and TPsimmr models, which were calculated based on the methods of stable nitrogen isotope analysis of bulk, traditional AAs-N-CSIA, and weighted βGlu/Phe, respectively. Food source analysis revealed three interlocking food webs (kingfisher, crab, and frogs) in this wetland. The highest HOP biomagnification capacities (TMFmix) were found in the kingfisher food web (0.24-82.0), followed by the frog (0.08-34.0) and crab (0.56-11.7) food webs. The parabolic trends of TMFmix across combinations of log KOW in the frog food web were distinct from those of aquatic food webs (kingfisher and crab), which may be related to differences in food web composition and HOP bioaccumulation behaviors between aquatic and terrestrial organisms. This study provides a new tool to accurately study the trophic transfer of contaminants in wetlands and terrestrial food webs with diverse species and complex feeding relationships.

Isotopic Turnover and Fractionation of δ15N and δ13C in Captive Pseudopleuronectes americanus (Walbaum)

Stable isotope ratios of nitrogen (δ15N) and carbon (δ13C) are ubiquitous ecological tracers used to elucidate an organism’s diet and habitat. However, the application of stable isotope ratios to reconstruct a consumer’s ecology relies upon accurate rates for isotopic turnover at both a tissue and species-specific level. This study estimated isotope turnover rates and trophic discrimination factors in four different tissues (liver, digestive tissue, muscle, and skin) with variable metabolic activity in winter flounder Pseudopleuronectes americanus using a controlled diet-switch experiment. Differences in half-lives were noted among the tissues and between the experimental diets for both δ15N and δ13C. The experimental diets of krill and mysis had variability in nutritional composition, resulting in similar turnovers in δ15N but slower turnovers in δ13C for fish fed krill. Turnovers in both δ15N and δ13C were strongly influenced by metabolism, with the contribution reaching up to 98%, as fish exhibited minimal overall growth. The results of this study demonstrate the importance of considering differences in the catabolic activity of tissue maintenance for fish exhibiting minimal growth, as well as differences in metabolic assimilation of dietary sources that vary in their protein and lipid contents.

Trophic discrimination factors and stable isotope variability in a captive feeding trial of the southern rock lobster Jasus edwardsii () (Decapoda: Palinuridae) in Tasmania, Australia

Abstract Trophic discrimination or fractionation factors (TDFs), such as ∆15N and ∆13C, are used in stable isotope mixing models to account for differences between source tissues (diet/prey) and consumer tissues (predator). We aimed firstly to obtain TDF values for a spiny lobster, the southern rock lobster Jasus edwardsii (Hutton, 1875), to better understand lobster diet in the wild and secondly to investigate variability in isotope signature within tissues of individuals and across a temporal scale to test if non-lethal sampling can be used in an ecological context. We conducted an 18-mo captive feeding trial with juvenile lobsters using three diet treatments and analysed dorsal and leg muscle, along with dorsal and leg exoskeleton for δ13C and δ15N values. Average TDFs for the three diet treatments were 3.86 ± 0.98‰ (∆13C) and 5.06 ± 0.65‰ (∆15N) for leg muscle, and 4.45 ± 1.04‰ (∆13C) and 4.36 ± 0.6‰ (∆15N) for dorsal muscle. When tested against wild lobsters and prey, these TDFs outperformed multi-taxa TDFs found in the literature. Isotope values from lobster leg muscle were not identical to associated dorsal muscle but the two were highly correlated, indicating that non-lethal sampling is acceptable. Values for exoskeleton isotope were significantly different from muscle, likely due to the exoskeleton not being in a constant state of growth and replacement, unlike the muscle tissue, which constantly incorporates new material. We conclude that our experimentally derived TDFs are suitable for mixing model analysis for J. edwardsii and when tested on a wild sample of lobsters they outperformed other TDFs reported in the literature. We show that non-lethal sampling using leg muscle is an appropriate sampling method, since this tissue is highly correlated to the commonly used dorsal muscle. This option for non-lethal sampling enhances the potential to widely sample wild populations or sample during industrial processing without the need to sacrifice whole animals.

Meta‐analysis and critical review of trophic discrimination factors (Δ13C and Δ15N): Importance of tissue, trophic level and diet source

Abstract Robustly quantifying dietary resource use and trophic position using stable isotopes requires accurate trophic discrimination factors (TDF; Δ13C and Δ15N for carbon and nitrogen, respectively), defined as the isotopic difference between consumer and diet. Early TDF studies converged on values of around 1.0‰ for Δ13C and 3.4‰ for Δ15N but more recent work indicates that TDF values may be more nuanced, depending on taxa, tissues, trophic level and diets. Yet, the relative importance of these factors remains unclear. Focusing on vertebrates (birds, fish, herptiles and mammals), we conducted a literature review of 279 studies that estimated TDF values and used a Bayesian framework to determine how tissue type, trophic level and diet source influence variation in Δ13C and Δ15N. Additionally, we reviewed 358 trophic ecology studies to determine if studies accounted for these factors during their TDF selection process. For Δ13C, vertebrates showed consistent patterns among tissue types (likely influenced by amino acid composition) and between trophic levels and diet sources (likely a result of dietary protein content and metabolic routing). Comparatively, for Δ15N, vertebrates showed considerable variation among tissue types and trophic levels, likely due to differences in tissue synthesis and physiological capabilities. Overall, Δ13C ranged from −5.1‰ to 9.1‰ and Δ15N from −3.3‰ to 9.7‰, underscoring that 1.0‰ for Δ13C and 3.4‰ for Δ15N are not universally appropriate. Moreover, both Δ13C and Δ15N varied by more than 9‰ within a single species and tissue type, demonstrating that using TDF values from the same, or similar, species may not be appropriate if diet and trophic level are not considered. Despite the importance of diet source on TDF values, most trophic ecology studies did not account for it. Further, most fish studies relied on literature review values that failed to account for tissue type, trophic level and diet source. To aid ecologists in diet and trophic assessments of vertebrates, we used our meta‐analysis to model taxon‐specific TDF estimates (mean ± SD) for each tissue type, trophic level and diet source combination. These more refined TDF values should improve ecological assessments that use stable isotopes. Read the free Plain Language Summary for this article on the Journal blog.

Food web differences between two neighboring tropical high mountain lakes and the influence of introducing a new top predator.

High mountain lakes (HMLs) are considered unique and comparable ecosystems for monitoring global climate change. The food web structure can indicate the response of these ecosystems to ecological threats, such as fish introduction, by analyzing the trophic dynamics. Nonetheless, the food webs of tropical HMLs are less well-studied than temperate HMLs. The present study assessed the food webs of two neighboring (600 m apart) tropical HMLs, El Sol and La Luna, inside the crater of the Nevado de Toluca volcano, Mexico. It used stable isotopes (δ13C and δ15N) and Bayesian mixing models with different trophic discrimination factors and priors to assess the impacts of introduced rainbow trout, persisting only in the larger lake, El Sol. The food web in Lake El Sol was more complex than in Lake La Luna, mainly due to its larger size, extensive vegetated littoral zone, and being fueled by autochthonous primary production. In contrast, the smaller and fishless Lake La Luna has a reduced and bare littoral zone that harbored a simple food web substantially sustained by allochthonous carbon inputs. The persistence of introduced rainbow trout in Lake El Sol but not in Lake La Luna accentuated the differences between the lakes. The models suggested that rainbow trout fed on key consumers of littoral macroinvertebrates (70-80%) and pelagic zooplankton (20-30%), increasing the linkage between sub-networks. In both tropical HMLs, the species richness and herbivorous fraction were elevated compared with temperate HMLs, while the linkage density and omnivorous fraction were lower. Basal nodes dominated these tropical HMLs, and the vegetated littoral zone of Lake El Sol had more intermediate (omnivore) nodes. Our results showed the convenience of food web analysis to compare the effects of introduced fish in originally fishless lakes in different latitudes.

Tissue‐ and diet‐dependent stable carbon and nitrogen isotope discrimination: a calibration study in a captive shorebird species

In ecology, stable‐isotope ratios are widely used to determine diets of organisms and reconstruct food webs. This is usually done by analyzing the stable‐isotope ratios of nitrogen (δ15N), which increase with increasing trophic level, and those of carbon (δ13C), which correlate with the δ13C value of food source(s) and generally differ between terrestrial and marine food sources. Assimilation of food changes stable‐isotope ratios, resulting in different values between the food source and its consumer. These differences are known as isotope trophic discrimination factors and, if known, can be used to determine from the stable‐isotope ratios in the consumer's tissue what the consumer has been eating. What is often ignored is that discrimination factors can differ between consumer's food sources and also between tissue types. Therefore, we performed a controlled feeding study in red knots Calidris canutus to determine discrimination factors between different food sources and red knot tissues. We kept two groups of red knots in captivity on a stable diet, one group feeding on mudsnails and the other on Trouvit pellets, for several months, during which the birds molted their feathers. We analyzed δ13C and δ15N in both food sources and in five red knot tissues (blood cells, blood plasma and three feather types) and subsequently calculated the isotope discrimination factors. We confirmed that the discrimination factors differed between tissues, and also between diets. Our values deviated from general averages reported in reviews on a wide range of animals/birds, but were very similar to values from previous red knot and dunlin studies. We therefore think that our discrimination factors can be used in future stable isotope studies, not only on red knots, but also on other marine shorebird species and plea for careful consideration of using the right discrimination factors.Keywords: δ13C, δ15N, discrimination factor, red knot, shorebird, stable isotope

Isotopic ecology of Hawaiian green sea turtles (Chelonia mydas) and reliability of δ13C, δ15N, and δ34S analyses of unprocessed bone samples for dietary studies

This study conducted stable isotope analysis (δ13C, δ15N, and δ34S) on the epidermis and two skeletal elements (rib and squamosal bones) of Hawaiian green turtles (Chelonia mydas) and putative diet items obtained from two neritic sites: the Kona/Kohala coast and Oahu. Turtle tissues were collected in 2018–2020 and diet samples in 2018, 2019, and 2021. The effect of body size and sampling locality on individual bulk tissue isotope values was evaluated, and stable isotope mixing models based on δ13C, δ15N, and δ34S values from those tissues and four groups of food sources were used to reconstruct diet histories of the turtles. Mixing models indicated that green turtles along the Kona/Kohala coast consumed an omnivorous diet, whereas those from Oahu had an herbivorous diet. These diet make-ups are consistent with published gut content analyses. However, mixing models using the stable isotope ratios in rib and squamosal bone failed to yield reasonable diet histories, probably due to inadequacies of the applied trophic discrimination factor (TDF), a key model parameter. These results further establish that stable isotope ratios in the epidermis can be used effectively to study green turtle diet, but also reveal that more validation—and establishment of appropriate TDFs—is needed before bone can be used reliably to assess green turtle diet.

Isotopic turnover in polar cod (Boreogadus saida) muscle determined through a controlled feeding experiment.

Polar cod (Boreogadus saida) is an important trophic link within Arctic marine food webs and is likely to experience diet shifts in response to climate change. One important tool for assessing organism diet is bulk stable isotope analysis. However, key parameters necessary for interpreting the temporal context of stable isotope values are lacking, especially for Arctic species. This study provides the first experimental determination of isotopic turnover (as half-life) and trophic discrimination factors (TDFs) of both δ13 C and δ15 N in adult polar cod muscle. Using a diet enriched in both 13 C and 15 N, we measured isotopic turnover times of 61 and 49 days for δ13 C and δ15 N, respectively, with metabolism accounting for >94% of the total turnover. These half-life estimates are valid for adult polar cod (>3 years) experiencing little somatic growth. We measured TDFs in our control of 2.6‰ and 3.9‰ for δ13 C and δ15 N, respectively, and we conclude that applying the commonly used TDF of ~1‰ for δ13 C for adult polar cod may lead to misrepresentation of dietary carbon source, while the use of 3.8‰ for δ15 N is appropriate. Based on these results, we recommend that studies investigating seasonal shifts in the diet of adult polar cod sample at temporal intervals of at least 60 days to account for isotopic turnover in polar cod muscle. Although isotopic equilibrium was reached by the fish in this study, it was at substantially lower isotope values than the diet. Additionally, the use of highly enriched algae in the experimental feed caused very high variability in diet isotope values which precluded accurate calculation of TDFs from the enriched fish. As a result of the challenges faced in this study, we discourage the use of highly enriched diets for similar experiments and provide recommendations to guide the design of future isotopic turnover experiments.