Isotope Analysis Of Amino Acids Research Articles

Compound-specific amino acid δ15 N values in archaeological shell: Assessing diagenetic integrity and potential for isotopic baseline reconstruction.

Reconstructing stable isotope (SI) ratios at the base of paleo-food webs is often challenging. For coastal systems, the SI ratios of organic matter in archeological shell represents a possible solution, providing a direct record of primary consumer SI ratios in the littoral zone. However, shell is often porous, with organic compounds susceptible to diagenetic alteration or contamination. If molecular isotopic information is well preserved, compound-specific amino acid isotope analysis (CSI-AA) has the potential to provide direct proxies for baseline SI ratios, bypassing many contamination issues, and to allow assessment of the diagenetic state. We collected shell from both archeological middens and nearby littoral zones in coastal Alaska, and used a simple organic extraction approach based on decalcification with sequential weak HCl additions to liberate organic material. We measured CSI-AA patterns, molar AA distributions, and the CSI-AA degradation parameter (ΣV), in the context of bulk SI ratios in fossil shell, modern shell, and soft tissue from five common taxa (urchin, limpet, mussel, periwinkle, chiton). CSI-AA patterns in both soft tissue and shell were consistent with primary consumers, and were indistinguishable in most modern and fossil shell pairs, showing that amino acid δ15 N values can be well preserved in archeological shell. AA molar distributions were also similar, although most fossil shell was enriched in Asx and Gly. Comparison between CSI-AA results from modern specimens confirmed that the source AA group (tracking isotopic baselines) are transferred without substantial modification into the shell record. In contrast, the Trophic AA group had elevated δ15 N values in shell versus soft tissue for all taxa examined, suggesting that a correction factor will be required for any CSI-AA proxies using these AAs. Overall, this new data indicates that the CSI-AA analysis of fossil shell represents a promising new approach to determining isotopic baselines in coastal paleo-ecosystems.

Testing the usefulness of hydrogen and compound-specific stable isotope analyses in seabird feathers: a case study in two sympatric Antarctic storm-petrels

Nitrogen and carbon stable isotopes provide tools to investigate ecological segregation, prey choice and spatial distribution in seabirds. However, the interpretation of stable isotopes is frequently hampered by a lack of isotopic baseline data. In this study, two techniques proposed to overcome such shortages were tested: compound-specific isotope analyses of amino acids (AA-CSIA) and the analysis of hydrogen stable isotope ratios (HSIA). Feathers of two sympatric storm-petrels were compared. The two species, Black-bellied storm-petrels Fregetta tropica and Wilson’s storm-petrels Oceanites oceanicus, moult in oceanic waters and differ in diet composition. For HSIA, a range of species with broad diet and non-breeding distribution was also investigated. Differences in carbon isotope values suggested differences in the spatial distribution and thus, in isotopic baseline values, during moult. Bulk nitrogen analyses of adult feathers did not detect species differences in trophic level. However, AA-CSIA detected clear differences in trophic levels in line with expectations: Black-bellied storm-petrels fed at a higher trophic level than Wilson‘s storm-petrels. Hydrogen values also differed between the species, but contrary to expectations were highly enriched in Black-bellied storm-petrels, but much less enriched in Wilson’s storm-petrels. Hydrogen data of seven petrel species challenge the suggestion that depleted δD values indicate a higher percentage of isosmotic fish. The present results suggest that the difference in hydrogen ratios may be explained by these petrels moulting in different ocean zones. Amino acid-specific stable isotope analyses were useful for estimating isotopic baselines and thus true trophic levels, whereas hydrogen isotopes were not.

Protistan microzooplankton and the trophic position of tuna: quantifying the trophic link between micro- and mesozooplankton in marine foodwebs

Abstract The importance of the trophic link between <200-µm protistan microzooplankton and mesozooplankton is a cornerstone of modern understanding of foodweb structure of marine pelagic ecosystems that is well demonstrated in experimentally measured contributions of protistan consumers to mesozooplankton diets, in constrained global budgets, and in regional studies that show the inadequacies of meeting zooplankton metabolic requirements by herbivory alone. Nonetheless, protistan trophic steps are poorly reflected in stable isotope analyses by standard methods and systematically neglected in fisheries-related trophic research, which focuses instead on interactions that can be measured in stomach contents. Here, we apply recent advances in compound-specific isotope analysis of amino acids (CSIA-AA) based on alanine as a trophic indicator of protistan foodweb steps to evaluate the implications of lower foodweb structure on trophic position (TP) estimates of tunas. CSIA-AA results for mesozooplankton of the subtropical North Pacific suggest that tuna TPs are underestimated by 0.9, which give rise to new TP estimates of 4.7–5.4 for the three main tuna species of that region. These, in turn, are used to compute a trophic enrichment factor (TEF = 4.45) based on alanine that includes 15N enrichments for protistan consumers and can be applied more broadly in the region. Accounting for the magnitude and variability of protistan trophic steps in the foodwebs supporting pelagic fisheries has important implications for understanding regional variability in energy flows and foodweb structure and their temporal responses to climate change.

Comparative feeding ecology of abyssal and hadal fishes through stomach content and amino acid isotope analysis

The snailfishes, family Liparidae (Scorpaeniformes), have found notable success in the hadal zone from ~6000−8200m, comprising the dominant ichthyofauna in at least five trenches worldwide. Little is known about the biology of these deepest-living fishes, nor the factors that drive their success at hadal depths. Using recent collections from the Mariana Trench, Kermadec Trench, and neighboring abyssal plains, this study investigates the potential role of trophic ecology in structuring fish communities at the abyssal-hadal boundary. Stomach contents were analyzed from two species of hadal snailfishes, Notoliparis kermadecensis and a newly-discovered species from the Mariana Trench. Amphipods comprised the majority (Kermadec: 95.2%, Mariana: 97.4% index of relative importance) of stomach contents in both species. Decapod crustaceans, polychaetes (N. kermadecensis only), and remains of carrion (squid and fish) were minor dietary components. Diet analyses of abyssal species (families Macrouridae, Ophidiidae, Zoarcidae) collected from near the trenches and the literature are compared to those of the hadal liparids. Stomachs from abyssal fishes also contained amphipods, however macrourids had a higher trophic plasticity with a greater diversity of prey items, including larger proportions of carrion and fish remains; supporting previous findings. Suction-feeding predatory fishes like hadal liparids may find an advantage to descending into the trench – where amphipods are abundant. More generalist feeders and scavengers relying on carrion, such as macrourids, might not benefit from this nutritional advantage at hadal depths. Compound specific isotope analysis of amino acids was used to estimate trophic level of these species (5.3±0.2 Coryphaenoides armatus, 5.2±0.2 C. yaquinae, 4.6±0.2 Spectrunculus grandis, 4.2±0.2 N. kermadecensis, 4.4±0.2 Mariana snailfish). Source amino acid δ15N values were especially high in hadal liparids (8.0±0.3‰ Kermadec, 6.7±0.2‰ Mariana), suggesting a less surface-derived food source than seen in the scavenging abyssal macrourids, C. armatus (3.5±0.3‰) and C. yaquinae (2.2±0.3‰). These results are compared to bulk muscle tissue isotopic compositions. This study provides the first comprehensive examination of the feeding ecology of the ocean's deepest-living fishes and informs new understanding of trophic interactions and fish community structure in and near the hadal zone.

The effect of trophic level on individual amino acid δ15N values in a terrestrial ruminant food web

ABSTRACTBulk collagen δ15N analysis is widely used to investigate past diet and trophic positions, but these values average the δ15N values of the constituent amino acids. Compound–specific isotope analysis of amino acids (AAs) can help elucidate the complex metabolic effects underpinning bulk δ15N values. Although trophic level effects on individual AA δ15N values have been investigated in aquatic and terrestrial invertebrate food webs, most archaeological applications involve terrestrial herbivores, hence a greater understanding of these effects between diet and consumer in this food chain is required. The North Wyke Farm Platform provided baseline nitrogen isotope information for cattle grazing on a Lolium perenne-dominated pasture. Bulk dentine δ15N values show a shift expected for a one trophic level increase, but obscure insight into the underlying metabolic processes that cause this change in value. However, determination of AA δ15N values of hydrolysable plant protein and cattle tooth dentine clarifies the trophic effect on consumer AA δ15N values. The observed trophic shift in the studied system is different from previously studied food webs, with a trophic enrichment factor, based on the δ15N values of glutamate and phenylalanine, of 4.0‰ compared to 7.6‰ commonly used in ecological and archaeological studies. This emphasises the need to understand the trophic shifts in the particular food web being investigated in order to apply isotopic investigations in archaeological contexts.

Spatial food‐web structure in the eastern tropical Pacific Ocean based on compound‐specific nitrogen isotope analysis of amino acids

AbstractThe effective evaluation of trophic interactions in pelagic food webs is essential for understanding food web ecology, conservation biology, and management. We tested the applicability of compound‐specific isotope analysis of amino acids (CSIA‐AA) for (1) characterizing trophic positions (TPs) of nine species from four trophic groups (tunas, squids, myctophids, and euphausiids) within a pelagic food web in the eastern tropical Pacific (ETP) Ocean, (2) evaluating trophic discrimination factors (TDFs) of each trophic group, and (3) detecting spatial changes in TPs and food chain length across a region with heterogeneous productivity. Although δ15N values of bulk tissues generally increased from south‐to‐north, CSIA‐AA revealed that trophic positions were uniform throughout our study area. These results suggest that variability in δ15N values were largely driven by nitrogen cycling dynamics in the ETP, which highlights the importance of these processes for the interpretation of δ15N values in food web studies. Absolute TP estimates were unrealistic for higher‐level species, and TDFs (tunas: 4.0‰, squids: 4.6‰, myctophids: 5.0‰, and euphausiids: 7.0‰) were lower than a widely used ecosystem TDF. We used remotely sensed oceanographic data to evaluate the physical oceanography and biological productivity throughout our study area and found significant relationships between δ15N values, nitrate concentrations, and SST across our study area. We did not find a gradient in phytoplankton cell size co‐occurring with an expected productivity gradient across our sampling region, which substantiated our isotope results indicating non‐significant spatial changes in TP and food chain length across the ETP.

Mercury levels in largemouth bass (Micropterus salmoides) from regulated and unregulated rivers

Within areas of comparable atmospheric mercury deposition rates methylmercury burden in largemouth bass populations vary significantly between regulated and unregulated rivers. To investigate if trophic dynamics strongly influenced pollutant body load, we sampled largemouth bass from two adjacent rivers, one regulated and one unregulated, and applied a suite of biochemical and stable isotope assays to compare their trophic dynamics. Total mercury burden in the bass from the unregulated Sipsey River (Elrod, AL, USA) and the regulated Black Warrior River (Demopolis, AL, USA) averaged 0.87 mg kg−1 and 0.19 mg kg−1 wet weight, respectively. For both populations, age, weight, and length were positively correlated with muscle mercury concentration. Compound specific stable isotope analysis of amino acids showed the trophic position of both populations was just under four. Quantitative and isotopic analysis of neutral lipid fatty acid of Sipsey River bass indicated a greater reliance upon the detrital component of the food web compared to Demopolis Reservoir bass which fed within the autochthonous, pelagic component of the food web. Since the close proximity of the rivers makes differences in atmospheric deposition unlikely and both populations had similar trophic position, our findings indicate that food web dynamics should be included among the factors that can strongly influence mercury concentration in fish.

A reliable compound-specific nitrogen isotope analysis of amino acids by GC-C-IRMS following derivatisation into N-pivaloyl-iso-propyl (NPIP)esters for high-resolution food webs estimation.

The signatures of natural stable nitrogen isotopic composition (δ(15)N) of individual amino acid (AA) have been confirmed to be a potentially effective tool for elucidating nitrogen cycling and trophic position of various organisms in food webs. In the present study, a two-stage derivatisation approach of esterification followed by acylation was evaluated. The biological samples underwent acid hydrolysis and the released individual AA was derivatived into corresponding N-pivaloyl-isopropyl (NPIP) esters for nitrogen isotopic analysis in gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). Usually, 13 individual AA derivatives were separated with fine baseline resolution based on a nonpolar gas chromatography column (DB-5ms). The minimum sample amount required under the presented conditions is larger than 20ngN on column in order to accurately determine the δ(15)N values. The δ(15)N values determined by GC-C-IRMS with a precision of better than 1‰, were within 1‰ after empirical correction compared to the corresponding measured by element analysis (EA)-IRMS. Bland-Altman plot showed highly consistency of the δ(15)N values determined by the two measurement techniques. Cation-exchange chromatography was applied to remove interfering fraction from the extracts of plant and animal samples and without nitrogen isotope fractionation during the treatment procedure. Moreover, this approach was carried out to estimate the trophic level of various natural organisms in a natural lake environment. Results highly proved that the trophic level estimated via the presented AA method well reflected the actual food web structure in natural environments.

Trophic ecology of young of the year Alboran Sea bluefin tuna (Thunnus thynnus) and Atlantic bonito (Sarda sarda) by stable isotope analyses

Trophic ecology of young of the year Alboran Sea bluefin tuna (Thunnus thynnus) and Atlantic bonito (Sarda sarda) by stable isotope analyses

Compound-specific isotope analysis of amino acids: A tool to unravel complex symbiotic trophic relationships

Carbon and nitrogen stable isotope ratios of amino acids (δ13CAA and δ15NAA) have been recently used to unravel trophic relationships in aquatic and terrestrial environments. However, none have studied the specific case of a symbiotic relationship. Here we use the stable isotope ratios of amino acids (AAs) to investigate the link between a scarab larva (Pericoptustruncatus) and its mite guest (Mumulaelaps, Mesostigmata: Laelapidae: Hypoaspidini). Five scenarios for the relationship between larva and mite were proposed and δ13CAA and δ15NAA respective data and patterns helped eliminate those that were inconsistent. The calculated gap of two trophic levels ruled out a parasitic trophic relationship scenario. The trophic relationship between P. truncatus was shown to most likely be commensalistic with the mites feeding on the larva's castings. Alongside this study, a comparison with the stable isotope bulk analysis method was made and demonstrated that the AA method brings a significant refinement to the results by providing a means of determining absolute tropic level without the need for prior knowledge of the isotopic composition of primary source material.

Short-term variability of human diet at Basketmaker II Turkey Pen Ruins, Utah: Insights from bulk and single amino acid isotope analysis of hair

Strands of human hair excavated from Basketmaker II Turkey Pen Ruins in Utah were examined using stable carbon and nitrogen isotope analysis and show that these individuals were heavily reliant on maize but their diet fluctuated over a period of months. Through serial bulk carbon and nitrogen analysis and serial single amino acid carbon isotope analysis of individual strands of hair, this study addresses the questions of how human diets at the site varied over a period of months and if domesticated turkeys (Meleagris gallopavo) were part of the diet during any point of the year. Hair carbon isotope values range from −9.2‰ to −12.4‰ with an average of −10.8±1.2, and nitrogen isotope values range from 6.0‰ to 7.3‰ with an average of 6.6‰±0.5. There was also variability in the single amino acid δ13C values, with an overall pattern of terrestrial C4 signatures of individual amino acids (δ13C for leucine averaged −17.8‰±1.7). Δ13Cvaline-phenylalanine values ranging from −1.6‰ to 2.6‰ also point strongly to a terrestrial diet. From the stable isotope data presented here, we suggest that BMII diet, while being heavily maize based, did vary during the year, and that turkeys were never a diet staple.

Approach to determine individual trophic level and the difference in food sources of Japanese anchovy Engraulis japonicus in Sagami Bay, based on compound-specific nitrogen stable isotope analysis of amino acids

Compound-specific stable isotope analysis of amino acids (AA method) has recently been used to estimate the trophic level (TL) of animals. The individual TL (TLGlu/Phe) of Japanese anchovy Engraulis japonicus was precisely determined using the AA method without sampling or analysis of primary producers. Previously, TLs of organisms were determined using bulk nitrogen stable isotopic composition (δ15Nbulk) of tissues (bulk method). The δ15Nbulk for Japanese anchovies varied widely because of their migratory behavior, and the TL of anchovies in Sagami Bay remained ambiguous. We showed that the TLGlu/Phe of anchovy was almost 3 and that food-web structures differed between inshore and offshore waters. Furthermore, there was little difference in TLGlu/Phe for representative Japanese anchovies in Sagami Bay, despite the large intra-sample variation in δ15Nbulk for anchovies ranging over a trophic shift of 3–4 ‰ in Sagami Bay. We speculated that this variation in δ15Nbulk resulted from differences in food sources among the different food webs. Estimation of individual TLs in some simple food chains using the AA method may enable the determination of the food-web structure of migratory fishes, which is difficult to determine using the traditional bulk method.

A new approach to δ15N compound‐specific amino acid trophic position measurements: preparative high pressure liquid chromatography technique for purifying underivatized amino acids for stable isotope analysis

Compound specific isotope analysis of amino acids (CSI‐AA) has emerged as an important new method for investigating trophic dynamics in both aquatic and terrestrial systems. Multiple studies have shown that δ15N values of glutamic acid (Glu) and phenylalanine (Phe) can be coupled to provide precise estimates of trophic position (TP), while simultaneously decoupling baseline δ15N values from the effects of trophic transfer. However, the current standard gas chromatography‐combustion‐isotope ratio mass spectrometry (GC‐C‐IRMS) approach is limited by high expense, limited availability, and relatively low precision. We present a new method for making TP estimates in biological samples by CSI‐AA (TPCSIA), based on a high‐pressure liquid chromatography (HPLC) purification of underivatized amino acids, followed by offline elemental analysis‐isotope ratio mass spectrometry (EA‐IRMS). We compare results from our new HPLC/EA‐IRMS method versus GC‐C‐IRMS in both standard and natural materials. Nitrogen isotopic values of purified Glu and Phe standards were identical within error for both methods. In five widely different marine organisms, the δ15N values of Glu and Phe were also indistinguishable within error between the two approaches; however, the δ15N values produced by the HPLC/EA‐IRMS approach had higher average precision (average SD = 0.3 ± 0.2 ‰) than the GC‐C‐IRMS measurements (average SD = 0.45 ± 0.15 ‰). The resulting TPCSIA estimates were statistically indistinguishable (t < 1.2, df = 6, P > 0.3) between the two methods for all organisms examined. Our HPLC/EA‐IRMS method may therefore allow significant expansion of TPCSIA applications, requiring only commonly available instrumentation to produce high precision TPCSIA values.

Reconstructing δ13C isoscapes of phytoplankton production in a coastal upwelling system with amino acid isotope values of littoral mussels

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 504:59-72 (2014) - DOI: https://doi.org/10.3354/meps10746 Reconstructing δ13C isoscapes of phytoplankton production in a coastal upwelling system with amino acid isotope values of littoral mussels Natasha L. Vokhshoori1,*, Thomas Larsen2, Matthew D. McCarthy1,* 1University of California, Santa Cruz, Ocean Sciences Department, 1156 High Street, Santa Cruz, California 95060, USA 2Christian-Albrechts University of Kiel, Leibniz-Laboratory for Radiometric Dating and Stable Isotope Research, Max-Eyth-Str. 11-13, 24118 Kiel, Germany *Corresponding authors: nvokhsho@ucsc.edu;mdmccar@ucsc.edu ABSTRACT: Compound-specific isotope analysis of amino acids (CSI-AA) is increasingly used to decouple trophic isotopic effects from isotopic composition at the base of food webs. The δ13C values of essential amino acids (EAAs) are particularly useful as recorders of primary production, because animals cannot synthesize EAAs de novo, so diagnostic biosynthetic δ13CEAA patterns remain unchanged up food chains. To test the potential for δ13CAA values to identify C source and resource flow in complex littoral ecosystems, we measured bulk and δ13CAA values of Mytilus californianus adductor muscle tissue in samples spanning >1700 km of coastline from San Diego, California, to southern Oregon, USA. The average bulk δ13C value for the entire region clustered around a relatively constant value (-15.7 ± 0.9‰), with no latitudinal trend (R2 = 0.022). δ13CAA patterns were highly consistent in all mussels, and δ13CEAA patterns closely matched expectations for marine phytoplankton, supporting the hypothesis that consumer EAA isotope values are unchanged from primary production. While bulk δ13C values were ambiguous in terms of source, application of a multivariate δ13CEAA ‘fingerprinting’ approach clearly indicated microalgae as the major source for all mussel EAAs. We hypothesize that integrated δ13C values of coastal phytoplankton production can therefore be directly derived from δ13CEAA of filter feeders in coastal regions. A correction derived from literature data for phytoplankton δ13CEAA values yielded reasonable δ13C values for coastal production, supporting this idea. Together, our results suggest δ13CEAA as a new approach for constructing baseline δ13C isoscapes and may have important implications for ecosystem studies in both modern and paleo-environments. KEY WORDS: Mytilus californianus · Carbon isotopes · Compound-specific isotope analysis · California · Isoscape Full text in pdf format Supplementary material PreviousNextCite this article as: Vokhshoori NL, Larsen T, McCarthy MD (2014) Reconstructing δ13C isoscapes of phytoplankton production in a coastal upwelling system with amino acid isotope values of littoral mussels. Mar Ecol Prog Ser 504:59-72. https://doi.org/10.3354/meps10746 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 504. Online publication date: May 14, 2014 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2014 Inter-Research.

Stable Nitrogen Isotope Analysis of Amino Acids by Using Gas Chromatography/Isotope Ratio Mass Spectrometry (GC/IRMS) System with High-temperature Combustion Interface

Stable Nitrogen Isotope Analysis of Amino Acids by Using Gas Chromatography/Isotope Ratio Mass Spectrometry (GC/IRMS) System with High-temperature Combustion Interface

Compound‐specific δ13C and δ15N analysis of amino acids: a rapid, chloroformate‐based method for ecological studies

Compound-specific stable isotope analysis of amino acids has proven informative to many ecological systems, but only a handful of analytical methods are routinely employed. We evaluated a simple, rapid procedure in which biological samples undergo short-duration acid hydrolysis and the resulting amino acids are derivatized with methyl chloroformate for gas chromatography/combustion/isotope-ratio mass spectrometry (GC/C/IRMS). Amino acid derivatives were separated on a polar gas chromatography column, combusted, and δ(13)C and δ(15)N values were measured. Tests of reproducibility and accuracy were conducted for amino acid reference mixtures and biological samples. A brief case study of turtles was used to assess whether isotopic data were consistent with a priori ecological expectations. The methyl chloroformate based reaction successfully converted 15 amino acids from acid hydrolysates of biological materials into separable derivatives. The δ(13)C and δ(15)N values had high average measurement precision (σ <1‰). Reference materials were measured accurately, with good agreement between EA/IRMS and GC/C/IRMS determinations. Analysis of turtle blood samples yielded data consistent with their trophic ecology. This derivatization method is a rapid means of determining carbon and nitrogen isotopic ratios of amino acids present in the biological materials often sampled for ecological studies. While amino acids with charged or polar side chains do not have uniformly high recoveries, the average precision of measurements is comparable with that of other, more established methods. Batches of samples may be prepared from many raw materials in less than a day, representing a significant reduction in preparation time over prevailing methods.

Portuguese

The stable isotope analysis (SIA) has been employed in several areas in Ecology and with different taxonomic groups. This tool has been very important in trophic ecology studies. The basic principle of the SIA is that the proportion of the stable isotopes of an element in an animal tissue could be related with its diet, and it is described by the discrimination factor (DF). The DF is one of the most important parameters in stable isotope analysis. Accurate estimates of the DF are extremely important because they are used in mixture models and to determine the species trophic level. The goals of this study were to quantify, by a scientometric analysis, the research that investigates the sources of variability of the discrimination factor, focusing in fishes. The results obtained were discussed and synthesized, as well the methods employed to estimate the DF. A survey in the ISI Web of Science was performed to find articles related to the theme. Most researches were led by the United States, the United Kingdom and Germany. Muscle was the tissue more commonly utilized. Recently, fin and scale have been used because both represent non-lethal samples. In addition to the use of ? 13 C and ? 15 N, interest in using ?D (deuterium) as a food web tracer has been applied in studies with fish. The type of diet, trophic group, metabolic and growth rate appeared to be important sources of variability. The compound-specific isotope analysis of amino acids offers an opportunity to reduce the variation related to the protein content. We believed that there is a need to investigate (i) DF in tropical areas, (ii) the effect of temperature in the DF, (iii) the variation of the DF in different trophic groups. It is recommended greater emphasis in the use of non-lethal samples.

DISCRIMINATION FACTOR IN THE TROPHIC ECOLOGY OF FISHES: A REVIEW ABOUT SOURCES OF VARIATION AND METHODS TO OBTAIN IT

The stable isotope analysis (SIA) has been employed in several areas in Ecology and with different taxonomic groups. This tool has been very important in trophic ecology studies. The basic principle of the SIA is that the proportion of the stable isotopes of an element in an animal tissue could be related with its diet, and it is described by the discrimination factor (DF). The DF is one of the most important parameters in stable isotope analysis. Accurate estimates of the DF are extremely important because they are used in mixture models and to determine the species trophic level. The goals of this study were to quantify, by a scientometric analysis, the research that investigates the sources of variability of the discrimination factor, focusing in fishes. The results obtained were discussed and synthesized, as well the methods employed to estimate the DF. A survey in the ISI Web of Science was performed to find articles related to the theme. Most researches were led by the United States, the United Kingdom and Germany. Muscle was the tissue more commonly utilized. Recently, fin and scale have been used because both represent non-lethal samples. In addition to the use of ? 13 C and ? 15 N, interest in using ?D (deuterium) as a food web tracer has been applied in studies with fish. The type of diet, trophic group, metabolic and growth rate appeared to be important sources of variability. The compound-specific isotope analysis of amino acids offers an opportunity to reduce the variation related to the protein content. We believed that there is a need to investigate (i) DF in tropical areas, (ii) the effect of temperature in the DF, (iii) the variation of the DF in different trophic groups. It is recommended greater emphasis in the use of non-lethal samples.

Environmental perturbation effects on baseline δ15N values and zooplankton trophic flexibility in the southern California Current Ecosystem

Nitrogen isotopic compositions of zooplankton in the California Current Ecosystem (CCE) are known to vary over inter‐annual scales of climate variability, but the extent to which those changes are driven by variations in baseline phytoplankton δ15N values vs. zooplankton trophic position (TP) is poorly resolved. We use field samples collected during a large natural environmental perturbation, the 1998–1999 alternation between El Niño and La Niña states, to test the ability of large dominant CCE zooplankton Euphausia pacifica and Calanus pacificus to alter their TPs in response to environmental variability. To distinguish trophic changes from variations of δ15N values at the base of the food web, the zooplankton were assayed by Compound Specific Isotope Analysis of Amino Acids (CSIA‐AA). Linear mixed‐effect models were developed to utilize data from all amino acids (AAs), providing greater statistical power than the typical CSIA‐AA approach of using only phenylalanine and glutamic acid. We confirm a significant 15N enrichment of ∼ 2‰ at the base of the food web for all AAs and all zooplankton groups during the 1998 El Niño. This baseline enrichment in 15N has been speculated to occur during El Niño events but never conclusively shown. We also demonstrate a significantly elevated TP, implying increased carnivory during 1998, for E. pacifica while C. pacificus did not alter their TP between years. Lastly, TPs calculated from the standard CSIA‐AA equation with laboratory‐derived constants gave unrealistically low estimates, suggesting an assessment of these variables in situ is needed for an accurate application in natural systems.

Trophic ecology of a green turtle breeding population

While many migratory marine organisms converge at breeding areas, identifying for- aging strategies away from these reproductive sites can be challenging. Adult female green tur- tles Chelonia mydas regularly migrate thousands of kilometers between nesting and foraging areas, making it difficult to identify foraging habitats that support nesting populations and to understand their feeding strategies. In this study, we use stable isotope analysis to investigate the trophic ecology and spatial distribution of foraging green turtles in the Greater Caribbean. Fur- ther, we explore the possibility that adult green turtles, originally considered to be herbivores, may, like their counterparts in the Pacific Ocean, display carnivorous feeding strategies. The wide range of carbon and nitrogen isotope values in bulk epidermis observed in the nesting population at Tortuguero, Costa Rica, could indicate that these turtles feed over several trophic levels. Iso- topic niches — or the range of δ 13 C and δ 15 N values, which can be used as a proxy for ecological niche — varied among the 5 green turtle foraging aggregations sampled. Similarly, the isotopic composition of the primary producer Thalassia testudinum also varied substantially with geo- graphic location. However, compound-specific stable isotope analysis of amino acids (AA-CSIA) indicated that individuals in the nesting population with different bulk δ 15 N values feed at the same trophic position. The combined results suggest that spatial differences in the isotopic com- position of seagrass at the base of the food web, rather than differences in turtle foraging strategy, contribute to the isotopic variation in the nesting population. This study improves understanding of the foraging ecology of a highly dispersed and migratory species.