δ15N Fractionation Research Articles

Trophic role and predatory interactions between the blue crab, Callinectes sapidus, and native species in open waters of the Ebro Delta

The Ebro Delta has become a major blue crab (Callinectes sapidus) fishery area in the NW Mediterranean, but there is limited information on factors controlling the abundance of populations in open waters, a crucial habitat for ovigerous females. Here, we use a stable isotope approach (δ15N and δ13C), to assess blue crab trophic position, the potential consumption of food items using mixing models, and the isotopic niche overlap with local commercial species. For Octopus vulgaris, a potential blue crab key predator, a trophic enrichment experiment was also conducted to further assess predation control in wild populations. The blue crab showed 1.6 times higher trophic position than in other habitats of the Ebro Delta, and similar to that found in the harbor crab, Liocarcinus depurator, and several predatory fish. Additionally, the isotopic niche of blue crabs showed overlaps from 46.2 to 14.9% with native predators, and mixing models also suggest even dietary contributions throughout the food web. For O. vulgaris, field results showed a trophic position of 3.93, lower than that of blue crab, and lower δ15N signatures were also obtained in a captivity experiment drawing negative fractionation (−1.1‰). We conclude that high dietary contribution of animal prey might provide a high protein diet that could be crucial for allowing the maintenance of a large local population, but the overall functional trophic similarity could also disfavor local native species. The similarity between experimental fractionation and field differences between predator and prey (−1.6‰) suggests that predation of blue crab is possible, but further research is needed to clarify the metabolic routes involved in octopus δ15N fractionation.

Assessing the Impact of Biodiversity (Species Evenness) on the Trophic Position of an Invasive Species (Apple Snails) in Native and Non-Native Habitats Using Stable Isotopes

Invasive apple snails negatively impact non-native habitats and human well-being. Here, the trophic position of Pomacea canaliculata in native habitats (Maldonado, Uruguay) and non-native habitats (Hangzhou, China and Hawaii, USA) are compared. Detritus samples and tissue samples from apple snails were collected in all sites. Trophic levels were calculated as the difference between the mean δ15N values of detritus samples and corresponding apple snail tissue samples, divided by the mean δ15N fractionation for nitrogen per trophic level in freshwater habitats. The mean δ15N values of detritus in sites served as a baseline (i.e., zero trophic level), allowing direct comparisons. Linear regression analysis established a correlation between species evenness and apple snail trophic level (R2 = 0.8602) in line with a Pearson’s product-moment correlation value (−0.83) and 95% confidence interval (−0.87, −0.77). Normal quartile plots indicated two normally distributed subsets of apple snail trophic-level data: (1) a biodiverse subset containing the Uruguayan and Chinese lake sites and (2) the homogenized Hawaiian and Chinese creek sites. A precipice value for species evenness (separating biodiversity from homogenization), between (3.7) and (2.4), once descended to or surpassed separates statistically distinct, normal distributions of invasive apple snail trophic-level data from diverse versus homogenized habitats.

Compound-Specific Carbon, Hydrogen, and Nitrogen Isotope Analysis of Nitro- and Amino-Substituted Chlorobenzenes in Complex Aqueous Matrices.

Compound-specific isotope analysis (CSIA) is an established tool to study the fate of legacy groundwater contaminants but is only emerging for nonconventional contaminants, e.g., nitro- and amino-substituted chlorobenzenes that are widely used as industrial feedstock and the target of this work. To date, CSIA of the target compound groups used special combustion interfaces and the potential matrix interferences in environmental samples has not been assessed. We validated CSIA methods for δ13C, δ2H, and δ15N of four analytes from each chemical group and developed a solid-phase extraction (SPE) method to minimize matrix interferences during preconcentration of complex aqueous samples. The SPE recovery was >80% and the method quantification limits of SPE-CSIA for δ13C, δ2H, and δ15N were 0.03-0.57, 1.3-2.7, and 3.4-10.2 μM aqueous-phase concentrations, respectively, using 2 L of spiked MQ water. The SPE-CSIA procedure showed negligible isotope fractionation for δ13C (≤0.5‰), δ15N (≤0.5‰), and δ2H (≤5‰ for nitroaromatics and ≤10‰ for aminoaromatics). In addition, solvent evaporation, water sample storage up to 7 months, and SPE extract storage for 1.5 years did not change analytes' δ13C signatures beyond ±0.5‰. However, to avoid significant δ2H and δ15N fractionation of aminoaromatics, cartridge breakthrough should be avoided and SPE preconcentration must be conducted at pH > pKa + 2. Application of the method at a contaminated site showed excellent precision, at ≤0.3‰ for C and N, and ≤1.5‰ for H. The methods validated here now allow the use of multielement CSIA to track the environmental fate of nitro- and amino-substituted chlorobenzenes in complex aqueous samples.

A survey of amphetamine type stimulant nitrogen sources by isotope ratio mass spectrometry

The purpose of this study was to determine what variations exist in δ13C and δ15N values of nitrogen sources used in the clandestine production of amphetamine type stimulants (ATS). A survey of previously reported data revealed a lack of information on the novel nitrogen source hydroxylamine. HCl, which can be utilised in the production of MDA from helional. Samples of nitromethane, nitroethane and hydroxylamine.HCl were collected or synthesised for analysis by isotope ratio mass spectrometry (IRMS). Unlike many other nitrogen sources, nitromethane is an uncontrolled substance with legitimate applications. Nitromethane samples were collected from hobby stores around Brisbane, Australia. An investigation into the synthesis of nitromethane.HCl by the acid catalysed hydrolysis of nitromethane is also presented.The δ15N values for nitromethane (–37.21 to –0.10‰) and hydroxylamine.HCl (–97.87 to +2.19‰) expand considerably on the previously published δ15N values of nitrogen sources. The broad range of δ15N values identified should provide a useful tool in the comparison of ATS, especially when synthesised from the novel precursor helional and utilising hydroxylamine.HCl as a nitrogen source. Relatively consistent δ15N fractionation was observed for hydroxylamine.HCl synthesised from nitromethane.

Nitrate Isotopic Composition in Precipitation at a Chinese Megacity: Seasonal Variations, Atmospheric Processes, and Implications for Sources

AbstractNitrogen stable isotope composition (δ15N) of nitrate (NO3−) deposition can aid in source apportionment of its precursor emissions, nitrogen oxides (NOx), with implications in mitigation policy to address serious air pollution. However, potential δ15N fractionation during atmospheric NO3− formation may hinder accurate quantification of NOx contributions. Previously, NOx photochemical reactions have been suggested to be the dominant δ15N fractionation process in NO3− formation. Here, we have quantified the potential fractionation effects associated with NOx photochemical reactions based upon δ15N‐NO3− measured in 1 year of daily‐based bulk deposition samples in Shenyang, a megacity with distinct seasonal fossil fuel combustion in northeastern China. The mean δ15N was 0.9 ± 4.0‰ and ranged from −4.9 to 8.3‰, with the lowest and highest values observed during summer and winter, respectively. Calculated NOx photochemical equilibrium fractionation may account for up to 42% of the observed seasonal δ15N change in NO3− deposition. However, the relative NOx source contribution trends, estimated based upon a δ15N Bayesian mixing model, were insensitive to NOx photochemical fractionation considerations. Overall, NOx source partitioning based upon averaged year‐long δ15N of bulk NO3− deposition estimated the following relative trend in NOx emission sources: coal combustion > biomass burning = vehicle emissions ≫ soil emissions. Seasonally, the increase of coal combustion emissions from summer to winter drives the seasonality of δ15N in NO3− deposition, indicating a necessity to control NOx emissions from coal combustion to improve wintertime air quality.

Nitrogen resorption and fractionation during leaf senescence in typical tree species in Japan

In northeastern Japan, an area of high precipitation and mountains, beech (Fagus creanata Blume), larch (Larix kaempferi Lamb.), cedar (Cryptomeria japonica D. Don) and black locust (Robinia pseudoacacia L.) were evaluated for N resorption and N isotope fractionation in pre- and post-abscission leaves in comparison to green leaves. The highest leaf N concentration in summer corresponded to the N-fixing black locust, followed in decreasing order by the deciduous beech and larch and evergreen cedar. On the other hand, the lowest N resorption efficiency corresponded to black locust and the highest to beech, in increasing order by larch and cedar. All tree species returned significant amounts of N before leaf abscission; however, N isotope fractionation during leaf N resorption was only found for beech, with a depleted N isotope value from green to pre-abscission leaf. The most N, however, was resorbed from pre-abscission to post-abscission. This result may indicate that δ15N fractionation took place during N transformation processes, such as protein hydrolysis, when the concentration of free amino acids increased sharply. The difference in the type of amino acid produced by each species could have influenced the N isotope ratio in beech but not in the other tree species. The results of this study showed that it is possible to infer the type and timing of processes relevant to N resorption by analyzing leaf δ15N variation during senescence.

Application of stable isotopic analyses for fish host–parasite systems: an evaluation tool for parasite-mediated material flow in aquatic ecosystems

Parasites potentially have important roles in aquatic ecosystems, although relatively little is known about their contributions to the complexity of food web structure. In this study, stable carbon and nitrogen isotope analyses (δ13C and δ15N) were applied for fish host–parasite systems in a shallow swamp–lake ecosystem to assess the validity of stable isotope technics to reveal the parasite-mediated trophic linkages in the food web. Forty host–parasite pairs, including seven parasite taxa (cestodes, trematodes, crustaceans, and hirudinids), found on six host fish species (cyprinids and percids) were examined. The parasites showed unusual δ15N fractionation, with an overall average of − 1.9‰, suggesting the intake of 14N-rich ammonia for amino acid synthesis and/or selective absorption of 15N-depleted amino acids in the host fluid. The isotopic signatures of fish hosts and their parasites were positively correlated, suggesting the absorption and transfer of host-derived nutrients during infection. A δ13C-based isotope mixing model showed that each host fish species exhibited unique dependencies on POM, land-derived organic matter, and macrophytes, suggesting the host-specific trophic niche of the associated parasites in the lake–swamp food web. These emphasized that parasites are potential pathways of material and energy flows in aquatic ecosystems, contributing substantially to the food web complexity. Stable isotope analyses are the useful tools to elucidate the host–parasite trophic linkages, and case-specific isotopic fractionation factors are the mandatory information for a better understanding of the parasite-mediated material flow in ecosystems.

Estimating δ15N fractionation and adjusting the lipid correction equation using Southern African freshwater fishes.

Stable isotope analysis is an important tool for characterising food web structure; however, interpretation of isotope data can often be flawed. For instance, lipid normalisation and trophic fractionation values are often assumed to be constant, but can vary considerably between ecosystems, species and tissues. Here, previously determined lipid normalisation equations and trophic fractionation values were re-evaluated using freshwater fish species from three rivers in the Upper Zambezian floodplain ecoregion in southern Africa. The parameters commonly used in lipid normalisation equations were not correct for the 18 model species (new D and I parameters were estimated as D = 4.46‰ [95% CI: 2.62, 4.85] and constant I = 0 [95% CI: 0, 0.17]). We suggest that future isotopic analyses on freshwater fishes use our new values if the species under consideration do not have a high lipid content in their white muscle tissue. Nitrogen fractionation values varied between species and river basin; however, the average value closely matched that calculated in previous studies on other species (δ15N fractionation factor of 3.37 ± 1.30 ‰). Here we have highlighted the need to treat stable isotope data correctly in food web studies to avoid misinterpretation of the data.

Benthic foodweb structure in a large shallow lake studied by stable isotope analysis

Abstract: The benthic foodweb structure of Lake Vortsjarv, a large (270 km2), shallow, and turbid Estonian lake, was evaluated based on C and N stable-isotope signatures (δ13C, δ15N). Variation in δ13C between sampling sites was not related to site proximity to the littoral zone or the more vegetated southern part of the lake, but rather appeared to be influenced by in-situ site peculiarities. δ13C was stable temporally and between functional feeding groups, a result implying that the whole benthic food web of the lake relies largely on the same C source admixture, essentially particulate organic matter (POM). Thus, the foodweb composition of Lake Vortsjarv is remarkably homogeneous given the lake's large surface area. Apparent trophic-level δ15N fractionation between total collectors and total predators (mean 1.7‰) was lower than the value of 3.4‰ generally adopted in foodweb studies, but the higher value was valid for specific prey—predator links. The low δ13C signature of some chironomid samples indica...

Compound-specific amino acid δ15N patterns in marine algae: Tracer potential for cyanobacterial vs. eukaryotic organic nitrogen sources in the ocean

Stable nitrogen isotopic analysis of individual amino acids (δ15N-AA) has unique potential to elucidate the complexities of food webs, track heterotrophic transformations, and understand diagenesis of organic nitrogen (ON). While δ15N-AA patterns of autotrophs have been shown to be generally similar, prior work has also suggested that differences may exist between cyanobacteria and eukaryotic algae. However, δ15N-AA patterns in differing oceanic algal groups have never been closely examined. The overarching goals of this study were first to establish a more quantitative understanding of algal δ15N-AA patterns, and second to examine whether δ15N-AA patterns have potential as a new tracer for distinguishing prokaryotic vs. eukaryotic N sources. We measured δ15N-AA from prokaryotic and eukaryotic phytoplankton cultures and used a complementary set of statistical approaches (simple normalization, regression-derived fractionation factors, and multivariate analyses) to test for variations. A generally similar δ15N-AA pattern was confirmed for all algae, however significant AA-specific variation was also consistently identified between the two groups. The relative δ15N fractionation of Glx (glutamine+glutamic acid combined) vs. total proteinaceous N appeared substantially different, which we hypothesize could be related to differing enzymatic forms. In addition, the several other AA (most notably glycine and leucine) appeared to have strong biomarker potential. Finally, we observed that overall patterns of δ15N values in algae correspond well with the Trophic vs. Source-AA division now commonly used to describe variable AA δ15N changes with trophic transfer, suggesting a common mechanistic basis. Overall, these results show that autotrophic δ15N-AA patterns can differ between major algal evolutionary groupings for many AA. The statistically significant multivariate results represent a first approach for testing ideas about relative eukaryotic vs. prokaryotic ON sources in the sea.

Diet-dependent δ13C and δ15N fractionation among sea urchin Lytechinus variegatus tissues: implications for food web models

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 462:175-190 (2012) - DOI: https://doi.org/10.3354/meps09786 Diet-dependent δ13C and δ15N fractionation among sea urchin Lytechinus variegatus tissues: implications for food web models Patricia Prado1,2,*, Ruth H. Carmichael3,4, Stephen A. Watts5, Just Cebrian3,4, Kenneth L. Heck Jr.3,4 1Institut de Recerca i Tecnología Agroalimentàries (IRTA), Aquatic Ecosystems, Ctra Poble Nou km 5.5, 43540 Sant Carles de la Ràpita, Tarragona, Spain 2Department of Biology, East Carolina University, Greenville, North Carolina 27858, USA 3Dauphin Island Sea Lab, 101 Bienville Boulevard, Dauphin Island, Alabama 36528, USA 4Department of Marine Sciences, University of South Alabama, LSCB 25, Mobile, Alabama 36688, USA 5Department of Biology, University of Alabama at Birmingham, University Station, Birmingham, Alabama 35294-1170, USA *Email: patricia.prado@irta.cat ABSTRACT: Consumer-diet discrimination factors (Δ15N and Δ13C) are often applied without corroboration from laboratory experiments. Deviations in Δ15N and Δ13C that may occur from different diet types were quantified by raising 120 sea urchins Lytechinus variegatus in laboratory tanks on 3 different diets: seagrass Thalassia testudinum, red foliose macroalgae (Grauteloupia sp. and Palmaria palmata) and a mixed diet specifically formulated for L. variegatus. Patterns of δ13C and δ15N and resulting fractionation factors were then determined in muscle, gonad, gut, test organic matrix and whole individuals. Tissue δ13C values showed a strong positive association with δ13C of diets and estimates of absorption efficiency (R2 = 0.81). The seagrass diet consistently resulted in negative Δ13C values in all tissues (from −0.86‰ in muscle to −1.63‰ in gonad) and whole individuals (−1.19‰), whereas macroalgal and formulated diets showed positive values (0.11 and 0.19‰, respectively). Only individuals on the formulated diet clearly reached isotopic equilibrium for δ13C, suggesting that other lower quality diets may have resulted in more continuous reallocation of internal resources. δ15N values increased as the nitrogen content of these diets decreased (3.18, 1.21 and 0.82‰ for seagrass, macroalgae and formulated diets, respectively). Overall differences in the biochemical composition of diets and a robust relationship between δ13C and δ15N signatures suggest that protein quantity and quality could be central in driving isotope fractionation. The influence of macrophyte material type in the diet can be stronger than that of trophic level for both δ13C and δ15N, so further compound-specific isotope analyses are necessary to determine reliable values of Δ13C and Δ15N for ecological applications. KEY WORDS: Seagrass · Macroalgae · Omnivory · Diet quality · Absorption efficiency Full text in pdf format PreviousNextCite this article as: Prado P, Carmichael RH, Watts SA, Cebrian J, Heck KL Jr (2012) Diet-dependent δ13C and δ15N fractionation among sea urchin Lytechinus variegatus tissues: implications for food web models. Mar Ecol Prog Ser 462:175-190. https://doi.org/10.3354/meps09786 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 462. Online publication date: August 21, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Light-mediated 15N fractionation in Caribbean gorgonian octocorals: implications for pollution monitoring

The stable nitrogen isotope ratio (δ15N) of coral tissue is a useful recorder of anthropogenic pollution in tropical marine ecosystems. However, little is known of the natural environmentally induced fractionations that affect our interpretation of coral δ15N values. In symbiotic scleractinians, light affects metabolic fractionation of N during photosynthesis, which may confound the identification of N pollution between sites of varied depth or turbidity. Given the superiority of octocorals for δ15N studies, our goal was to quantify the effect of light on gorgonian δ15N in the context of monitoring N pollution sources. Using field collections, we show that δ15N declined by 1.4‰ over 20 m depth in two species of gorgonians, the common sea fan, Gorgonia ventalina, and the slimy sea plume, Pseudopterogorgia americana. An 8-week laboratory experiment with P. americana showed that light, not temperature causes this variation, whereby the lowest fractionation of the N source was observed in the highest light treatment. Finally, we used a yearlong reciprocal depth transplant experiment to quantify the time frame over which δ15N changes in G. ventalina as a function of light regime. Over the year, δ15N was unchanged and increased slightly in the deep control colonies and shallow colonies transplanted to the deep site, respectively. Within 6 months, colonies transplanted from deep to shallow became enriched by 0.8‰, mirroring the enrichment observed in the shallow controls, which was likely due to the combined effect of an increase in the source δ15N and reduced fractionation. We conclude that light affects gorgonian δ15N fractionation and should be considered in sampling designs for N pollution monitoring. However, these fractionations are small relative to differences observed between natural and anthropogenic N sources.

Use of a δ13C–δ15N relationship to determine animal trophic positions in a tropical Australian estuarine wetland

AbstractStable isotope composition of organisms from different trophic groups collected from a semi‐isolated wetland pool in the Ross River estuary, northern Australia, was analysed to determine if there was a consistent relationship between δ13C, δ15N and trophic level that could be used to assign trophic positions. A strong linear negative relationship between δ13C and δ15N was detected for the three trophic levels considered (primary producers, primary consumers and secondary consumers). This relationship was consistent among trophic levels, differing only in height, that is, on δ15N values, which indicate trophic positions. A difference of 3.6–3.8‰ between trophic levels was present, suggesting a δ15N fractionation of approximately 3.7‰, a value slightly higher than the commonly assumed δ15N fractionation of approximately 3.4‰. The relationship between δ13C and δ15N was similar for invertebrate and fish primary consumers, indicating similar δ15N trophic fractionation for both groups, meaning trophic positions and trophic length could be reliably calculated based on either invertebrates or fish.

Feeding ecology of pelagic fish larvae and juveniles in slope waters of the Gulf of Mexico

Stable isotope ratios of carbon (delta13C) and nitrogen (delta15N) were used to investigate feeding patterns of larval and early juvenile pelagic fishes in slope waters of the Gulf of Mexico. Contribution of organic matter supplied to fishes and trophic position within this pelagic food web was estimated in 2007 and 2008 by comparing dietary signatures of the two main producers in this ecosystem: phytoplankton [based on particulate organic matter (POM)] and Sargassum spp. Stable isotope ratios of POM and pelagic Sargassum spp. were significantly different from one another with delta13C values of POM depleted by 3-6 per thousand and delta15N values enriched by 2 relative to Sargassum spp. Stable isotope ratios were significantly different among the five pelagic fishes examined: blue marlin Makaira nigricans, dolphinfish Coryphaena hippurus, pompano dolphinfish Coryphaena equiselis, sailfish Istiophorus platypterus and swordfish Xiphias gladius. Mean delta13C values ranged almost 2 among fishes and were most depleted in I. platypterus. In addition, mean delta15N values ranged 4-5 with highest mean values found for both C. hippurus and C. equiselis and the lowest mean value for M. nigricans during both years. Increasing delta13C or delta15N with standard length suggested that shifts in trophic position and diet occurred during early life for several species examined. Results of a two-source mixing model suggest approximately an equal contribution of organic matter by both sources (POM=55%; pelagic Sargassum spp.=45%) to the early life stages of pelagic fishes examined. Contribution of organic matter, however, varied among species, and sensitivity analyses indicated that organic source estimates changed from 2 to 13% for a delta(13)C fractionation change of +/-0.25 per thousand or a delta15N fractionation change of +/-1.0 per thousand relative to original fractionation values.

Production of 15N‐depleted biomass during cyanobacterial N2‐fixation at high Fe concentrations

In this study we examine the effects of varying Fe, Mo, and P concentrations on δ15N fractionation during N2 fixation in the cyanobacterium Anabaena variabilis. We show that when grown in Fe‐enriched media ([Fe] ≥ 50 nM), this organism produces biomass up to 3‰ lower in δ15N than when grown in Fe‐limited media ([Fe] < 50 nM). A compilation of our data with previous measurements of δ15N in N2‐fixing cyanobacteria reveals a general trend toward the production of more 15N‐depleted biomass at higher Fe concentrations. We discuss our results in the context of negative δ15N values preserved in Archean and some Phanerozoic sediments, generally attributed to the production of marine organic matter with low δ15N by N2 fixation (and potentially NH4+ regeneration) during periods of fluctuating nutrient dynamics. We suggest that enhanced Fe availability during periods of widespread ocean anoxia can further stimulate the production of 15N‐depleted biomass by N2‐fixing organisms, contributing to the isotopic record.

Diet–Feather Stable Isotope (δ15N and δ13C) Fractionation in Common Murres and Other Seabirds

Abstract We measured the fractionation of stable nitrogen (δ15N) and carbon (δ13C) isotopes in the breast and primary feathers of 11 Common Murres (Uria aalge) maintained on a diet of capelin (Mallotus villosus). Diet–feather δ15N fractionation from delipidated capelin muscle to murre feathers was 3.6‰ ± 0.2‰ in breast feathers and 3.7‰ ± 0.2‰ in primary feathers. Fractionation of δ13C was 2.5‰ ± 0.2‰ in breast feathers and 1.9‰ ± 0.3‰ in primary feathers. Prey–feather fractionation (for delipidated, muscle-only prey samples) for nine other species of seabirds ranged from 3.0‰ to 4.6‰ for δ15N and 0.1‰ to 2.5‰ for δ13C. Studies that did not remove lipids from prey samples showed higher δ15N and δ13C fractionation, and those that used whole prey items rather than muscle tissue alone showed higher δ15N fractionation. We suggest that: (1) prey samples be delipidated to facilitate interpretation of δ13C fractionation, (2) high interstudy and interspecific variation in δ13C makes species-specific studies essential, and (3) use of muscle tissue rather than whole bodies of fish will minimize unexplained variation in δ15N fractionation.

DIET–FEATHER STABLE ISOTOPE (δ15N AND δ13C) FRACTIONATION IN COMMON MURRES AND OTHER SEABIRDS

Abstract We measured the fractionation of stable nitrogen (δ15N) and carbon (δ13C) isotopes in the breast and primary feathers of 11 Common Murres (Uria aalge) maintained on a diet of capelin (Mallotus villosus). Diet–feather δ15N fractionation from delipidated capelin muscle to murre feathers was 3.6‰ ± 0.2‰ in breast feathers and 3.7‰ ± 0.2‰ in primary feathers. Fractionation of δ13C was 2.5‰ ± 0.2‰ in breast feathers and 1.9‰ ± 0.3‰ in primary feathers. Prey–feather fractionation (for delipidated, muscle-only prey samples) for nine other species of seabirds ranged from 3.0‰ to 4.6‰ for δ15N and 0.1‰ to 2.5‰ for δ13C. Studies that did not remove lipids from prey samples showed higher δ15N and δ13C fractionation, and those that used whole prey items rather than muscle tissue alone showed higher δ15N fractionation. We suggest that: (1) prey samples be delipidated to facilitate interpretation of δ13C fractionation, (2) high interstudy and interspecific variation in δ13C makes species-specific studies essen...

Effects of body size and environment on diet-tissue δ15N fractionation in fishes

Nitrogen stable isotope natural abundance data are often used in trophodynamic research. The assumed nitrogen diet-tissue fractionation (Δ δ 15N) determines conclusions about trophic level, potential food sources and ontogenetic diet shifts. Δ δ 15N is usually assumed to be 3.0–3.4‰ per trophic level and unaffected by the size or age of animals or their environment. To assess the effects of body size, experimental duration and environmental conditions on fish tissue Δ δ 15N, two populations of European sea bass ( Dicentrarchus labrax) were reared on constant diets of dab ( Limanda limanda) muscle or sandeel ( Ammodytes marinus) for 2 years under natural light and temperature regimes. Bass were sampled at approximately monthly intervals to determine Δ δ 15N for muscle, heart and liver tissue. Mean values of Δ δ 15N were 3.83‰, 3.54‰, 2.05‰ (sandeel diet) and 3.98‰, 3.32‰, 1.95‰ (dab diet) for muscle, heart and liver tissue respectively. The assumption that fractionation was independent of body mass was upheld for muscle and heart tissue, but not for liver. Time effects on muscle Δ δ 15N were explainable by a sinusoidal function with a period of 1 year and wave height ∼ 0.3‰. Time resulted in increases in heart δ 15N and decreases in liver δ 15N which were small compared to background variation, equating to 1/6 of a trophic level over 2 years, and unlikely to have great significance in ecological studies. Heart and liver δ 15N were also affected by temperature probably reflecting the metabolic functions of these tissues and their associated rates of turnover. However in heart the explanatory power of temperature appeared tied to that of time. Although the Δ δ 15N for bass muscle on both diets approached 4‰, the Δ δ 15N values from this study, when combined with those from the literature, suggest that where fish species specific data are not available, a mean Δ δ 15N for fish muscle of 3.2‰ should be applied (mean white muscle Δ δ 15N = 3.15). The literature based mean Δ δ 15N for whole fish was lower than that of white muscle suggesting that a separate Δ δ 15N (2.9‰) should be applied when sampling whole fish.