Amino Acid δ15N Research Articles

Lipid extraction alters amino acid composition and bulk, but not amino acid, carbon and nitrogen isotope values.

Concerns exist over observed shifts in value and variance of nitrogen isotopes following physicochemical extraction of lipids from organic matter. The mechanisms behind these apparent changes in bulk tissue δ15N values are not fully understood yet have major implications for analytical costs and integrity of data interpretations. Changes in proximate analysis, amino acid composition, C:N ratios, bulk tissue and amino acid δ13C and δ15N values, and resulting isotope-based food web metrics were compared between lipid-intact and lipid-extracted muscle tissue of fishes spanning <1% to >20% muscle fat content to identify mechanisms of nitrogen isotope fractionation associated with physicochemical lipid extraction. Bulk δ13C and δ15N values increased and %N, C:N ratios and crude protein content decreased following lipid extraction. Resulting bulk isotope niche spacing and overlap varied significantly between lipid-intact and lipid-extracted tissues. While amino acid composition significantly changed during lipid extraction, particularly for lipid-associated amino acids (e.g., Glu, Lys, Ser), individual amino acid δ13C and δ15N values, and their associated compound-specific isotope analysis of amino acids (CSIA-AA)-based food web metrics, did not. Physicochemical lipid extraction caused significant tissue composition changes (e.g., leaching of amino acids and 15N-deplete nitrogenous waste) that affected δ13C and δ15N values and tissue %C and %N beyond simply removing lipids. However, lipid extraction did not alter individual amino acid δ13C or δ15N values or their associated CSIA-AA-based food web metrics.

Omnivorous summer feeding by juvenile Antarctic krill in coastal waters

AbstractThe Antarctic krill Euphausia superba is often considered an herbivore but is notable for its trophic flexibility, which includes feeding on protistan and metazoan zooplankton. Characterizing krill trophic position (TP) is important for understanding carbon and energy flow from phytoplankton to vertebrate predators and to the deep ocean, especially as plankton composition is sensitive to changing climate. We used repeated field sampling and experiments to study feeding by juvenile krill during three austral summers in waters near Palmer Station, Antarctica. Our approach was to combine seasonal carbon budgets, gut fluorescence measurements, imaging flow cytometry, and compound‐specific isotope analysis of amino acids. Field measurements coupled to experimentally derived grazing functional response curves suggest that phytoplankton grazing alone was insufficient to support the growth and basal metabolism of juvenile krill. Phytoplankton consumption by juvenile krill was limited due to inefficient feeding on nanoplankton (2–20 μm), which constituted the majority of autotrophic prey. Mean krill TP and the metazoan dietary fraction increased in years with higher mesozooplankton biomass, which was not coupled to phytoplankton biomass. Comparing TP estimates using δ15N of different amino acids indicated a substantial and consistent food‐web contribution from heterotrophic protists. Phytoplankton, metazoans, and heterotrophic protists all were important contributors to a diverse krill diet that changed substantially among years. Juvenile krill fed mostly on heterotrophic prey during summer near Palmer Station, and this food web complexity should be considered more broadly throughout the changing Southern Ocean.

Seasonal and ocean basin scale variation in the food web baseline and trophic ecology of a wide-ranging Southern Ocean predator derived from amino acid δ15N trends

Seasonal and ocean basin scale variation in the food web baseline and trophic ecology of a wide-ranging Southern Ocean predator derived from amino acid δ15N trends

Holocene climate change shifted Southern Ocean biogeochemical cycling and predator trophic dynamics

AbstractStudies of Antarctic paleo‐archives have produced conflicting hypotheses on the relative impact of long‐term climate change and historic exploitation of marine mammals on Southern Ocean krill predator foraging ecology. We disentangle these hypotheses using amino acid stable isotope analysis on a 7000‐yr Holocene archive of Adélie penguin (Pygoscelis adeliae) eggshells to differentiate variation in diet and trophic dynamics from baseline biogeochemical cycling as drivers of the rapid decline in krill predator bulk tissue δ15N values in recent centuries. Contrary to previous hypotheses suggesting solely trophic dynamic mechanisms as drivers of this decline, we identified an abrupt decline in source amino acid δ15N values, indicative of major changes in biogeochemical cycling at the base of the Southern Ocean food web that mirrored the decline in penguin bulk tissue δ15N values. These abrupt shifts in penguin δ15N values and associated biogeochemical cycling aligned with climatic events during the Little Ice Age that decreased surface δ15NNO3−, likely connected to a proposed increase in Ekman upwelling via a southward migration of the Westerlies. This baseline shift was in addition to a long‐term, gradual decline in penguin trophic position over the Holocene that began prior to both recent anthropogenic climate change and a proposed “krill‐surplus” following historic marine mammal exploitation in the 19th and 20th centuries. In resolving these outstanding hypotheses about drivers of Southern Ocean food web dynamics, this study emphasizes the fundamental importance of climate‐induced variability in biogeochemical cycling on ecological processes and improves the ability of paleo‐archives to inform the ecological consequences of future environmental change in the Southern Ocean.

Reconstructions of individual fish trophic geographies using isotopic analysis of eye-lens amino acids.

Fish eye lenses are a proteinaceous structure that grows by accumulating layers in a chronological manner. Each layer becomes metabolically inert, capturing the ratio of heavy/light carbon and nitrogen isotopes at time of formation. Therefore, eye lenses contain chronological isotopic records and can be used to create a temporal isotopic history throughout an individual's lifetime. We analyzed eye lens amino-acid δ15N to address spatio-temporal baseline variability and to reconstruct trophic histories of 10 individual Red Snapper. Proteins from sequential eye lens laminae were derivatized to measure 10 amino acids, from which glutamic acid (trophic) and phenylalanine (source) were used to estimate trophic positions at different points in life. Best-fitting regressions were generated to represent individual (R2 ≥ 0.89) and generalized (R2 = 0.77) trophic trajectory for Red Snapper. The resulting trophic trajectories indicated an increase in trophic position with increasing length. Until recently, there has not been a lifetime isotopic structure with enough organic nitrogen to recreate geographic histories using compound-specific stable isotope analysis of amino acids (CSIA-AA). This study confirms that eye-lens laminae can be used to reconstruct trophogeographic histories via CSIA-AA.

Autochthonous production sustains food webs in large perialpine lakes, independent of trophic status: Evidence from amino acid stable isotopes

Abstract Lakes are recipients of allochthonous organic matter and nutrients. However, the importance of these subsidies for food webs and how they vary with lake trophic status remains unclear, especially for large lakes. We assessed the source and fate of organic matter and nutrients in seven perialpine lakes across a gradient of trophic status. We measured carbon and nitrogen stable isotopes of amino acids of lake‐residing Atlantic trout, Salmo trutta, to determine the source of primary production (i.e., how carbon is fixed in the ecosystem) and how it is transferred through food webs, respectively. Based on essential amino acid carbon fingerprinting, we estimated the probability of organic carbon originating from autochthonous (algal), allochthonous (terrestrial plant), and recycled (bacterial) sources. In addition, we used amino acid δ15N to track how this primary production is transferred to consumers in general, and by using different trophic amino acids (glutamic acid and alanine), identify the trophic pathways involving either metazoan or protozoans. We found a high likelihood of autochthonous origin of organic carbon (86 ± 9%) in trout that contrasted with allochthonous origins of particulate organic matter and some sediments. We showed that those estimates are good proxies of source reliance. Our results also highlighted the importance of bacterial origin of organic carbon in fish (12%). The likely autochthonous origin of this carbon was supported by trophic markers (Ala δ15N) that suggest the role of protists in transferring recycled organic carbon up the food web. While the sources of nitrogen sustaining food webs varied among lakes, we found a conserved carbon fingerprinting of fish. Overall, this suggests an uncoupling between the source of nutrients and organic carbon in large perialpine lakes. Across a wide range of trophic status (c. 2 orders of magnitude range of phosphorus concentration), several lines of evidence suggested that perialpine lake food webs shared a common reliance on autochthonous and bacterial production. Our study is the first to quantify the dependence on allochthonous organic carbon in lake food webs based on new amino acid stable isotope markers (carbon fingerprinting and Ala δ15N) and shows promise for estimating the source of carbon fixation in ecosystems. Our results support previous suggestions that terrestrial organic carbon is a relatively minor source for aquatic consumers despite contributing to the pool of organic matter, and more importantly, its contribution does not vary substantially with trophic status in perialpine lakes.

Reconstructing herbivore diets: a multivariate statistical approach to interpreting amino acid nitrogen isotope values.

Recent studies have demonstrated that there exists significant variability in amino acid (AA) δ15N values of terrestrial plants, discriminating among plant types (i.e., legume seeds, grasses, tree leaves) as well as tissues of the same plant. For the first time, we investigate the potential of the spacing between the δ15N values of different AAs to differentiate between plant types and thus elucidate their relative importance in herbivore diet. Using principal component analysis, we show that it is possible to distinguish among five plant categories-cereal grains, rachis, legume seeds, herbaceous plants, and woody plants-whose consumption has different implications for understanding herbivore ecology and management practices. Furthermore, we were able to correctly classify the herbaceous plant diet of modern cattle using AA δ15N values of their tooth dentine adjusted for trophic enrichment. The AA δ15N patterns of wild and domestic herbivores from archaeological sites seem to be consistent with diets comprised predominantly of herbaceous plants, but there is variation in AA δ15N values among individuals that may reflect differing inputs of other plant types. The variation in AA δ15N values does not necessarily reflect the variation in herbivore bulk collagen δ13C and δ15N values, indicating that AA δ15N values have the potential to provide additional insights into plant dietary sources compared to bulk tissue isotope values alone. Future work should focus on defining trophic enrichment factors for a wider range of terrestrial herbivores and expanding libraries of primary producer AA δ15N values.

Using geostatistical analysis for simultaneous estimation of isoscapes and ontogenetic shifts in isotope ratios of highly migratory marine fish

Tracking migration of highly migratory marine fish using isotope analysis (iso-logging) has become a promising tool in recent years. However, application of this method is often hampered by the lack of essential information such as spatial variations in isotope ratios across habitats (isoscapes) and ontogenetic shifts of isotope ratios of target animals. Here, we test the utility of geostatistical analysis to generate isoscapes of δ13C and δ15N in the western Pacific and estimate the ontogenetic shifts in δ13C and δ15N values of a target species. We first measured δ13C and δ15N in the white muscle of juvenile (n = 210) and adult (n = 884) skipjack tuna Katsuwonus pelamis sampled across the northwest Pacific. Next we fitted a geostatistical model to account for the observed spatial variations in δ13C and δ15N of skipjack by fork length and other environmental variables with spatial random effects. We then used the best-fit models to predict the isoscapes of δ13C and δ15N in 2021. Furthermore, we measured δ15N of amino acids (δ15NAAs) of skipjack (n = 5) to determine whether the observed spatial variation of isotope ratios resulted from baseline shifts or differences in trophic position. The geostatistical model reasonably estimated both isoscapes and ontogenetic shifts from isotope ratios of skipjack, and the isoscapes showed that δ13C and δ15N can clearly distinguish the latitudinal migration of skipjack in the western Pacific. The δ15NAAs supported the results of the geostatistical model, that is, observed variations in skipjack δ15N were largely derived from a baseline shift rather than regional differences in trophic position. Thus, we showed that geostatistical analysis can provide essential basic information required for iso-logging without compound-specific isotope analysis.

A novel trophic archive: Practical considerations of compound-specific amino acid δ15N analysis of carbonate-bound organic matter in bivalve shells (Arctica islandica)

Compound-specific stable nitrogen isotope (δ15N) analysis of amino acids (CSIA-AA) can be used to refine the trophic position (TP) of organisms and determine the ecosystem δ15N baseline. It serves as a powerful tool to understand trophic dynamics and the nitrogen biogeochemistry of ecosystems. Extending this information back in time could be achieved by applying CSIA-AA to bivalve shell carbonate-bound organics (CBOM) in conjunction with growth pattern analysis (bivalve sclerochronology). However, due to the low content of AA in shells, the measurement of δ15N values of individual AA (δ15NAA values) is extremely challenging. Furthermore, a standardized pretreatment procedure is currently not available. In this study, we firstly present a refined analytical technique for calibrating δ15NAA data obtained by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS), considering (1) the effect of low signals on δ15NAA values, (2) AA-specific drift in δ15NAA values during analytical sequences, (3) the use of internal standard material(s), and (4) scale-normalization. Using the new approach, we achieve an analytical precision of 0.21 and 0.51 ‰ for six derivatized AA standards and 0.31 to 1.06 ‰ for 13 derivatized AA from shell powder. Secondly, a critical step of sample pretreatment, i.e., demineralization method, is addressed by comparing three demineralization methods. The cation-exchange resin (CEX resin) method showed minimal methodologically induced fractionation for 10 AA of the shell and is hence considered suitable for CSIA-AA. Lastly, using the refined analytical technique and sample pretreatment, we report the first high-resolution sclerochronologal δ15NAA series of an Arctica islandica specimen. Relatively small sample sizes (50–100 mg) from the inner portion of outer shell layer (iOSL) allow for a temporal resolution of 1–3 years per sample. The δ15NAA data from the iOSL show a lower TP of the studied bivalve in the ontogenetic older portion (age 8–14: 1.5 ± 0.1) than the younger portion (age 1–7: 2.1 ± 0.1), suggesting an ontogenetic change of its food sources from benthic particles to pelagic phytoplankton. Our findings also propose a novel, high-resolution proxy (i.e., δ15NPhe in CBOM from the ontogenetic older shell portion) for the ecosystem nitrogen isotopic baseline. This study opens promising potentials to reconstruct high-resolution changes of the origin and fate of nitrogen in past aquatic food webs.

Using MixSIAR to quantify mixed contributions of primary producers from amino acid δ15N of marine consumers

Estimations of the trophic position and the food web nitrogen baseline from compound-specific isotope analysis of individual amino acids (CSIA-AA) are challenged when the diet of consumer organisms relies on different proportions of vascular and non-vascular primary producers. Here we propose a method to infer such proportions using mixing models and the δ15N CSIA-AA values from marine herbivores. Combining published and new data, we first characterized CSIA-AA values in phytoplankton, macroalgae and vascular plants, and determined their characteristic β values (i.e. the isotopic difference between trophic and source AA). Then, we applied MixSIAR Bayesian isotope mixing models to investigate the transfer of these isotopic signals to marine herbivores (molluscs, green turtles, zooplankton and fish), and their utility to quantify autotrophic sources. We demonstrated that primary producer groups have distinct δ15NAA fingerprints that can be tracked into their primary consumers, thus offering a rapid solution to quantify resource utilization and estimate βmix values in mixed-sourced environments.

Amino acid δ13C and δ15N fingerprinting of sea ice and pelagic algae in Canadian Arctic and Subarctic Seas

The on-going decline in Arctic sea ice represents a significant loss of habitat for sea ice algae, which are a major contributor to primary production in the Arctic. Data on sea ice algal production is limited due to difficulties in both accessing sea-ice and sampling under-ice algae. Compound-specific isotope analysis (CSIA) of amino acids (AAs) is emerging as a powerful tool to trace element origins and biogeochemical processes in marine food webs and may address the knowledge gaps in sea ice algal productivity dynamics. Here we measured δ13C-AA and δ15N-AA in natural communities of sea ice and pelagic algae collected from regions offshore Labrador and Nunavut, Canada. Significant difference in δ13C-AA patterns between sea ice and pelagic algae was observed in different size classes. This difference was further supported by multivariate analyses based on normalized δ13C of essential amino acids (EAAs), which demonstrated a clear separation between sea ice and pelagic algae. Beta (β) values and trophic position (TP) calculated from δ15N of Glutamic Acid and Phenylalanine (Phe) and ∑V parameter for microbial resynthesis indicated a slightly higher heterotrophic biomass in pelagic and sea ice samples as compared to cultured samples. This finding is consistent with the Phe-normalized δ15N of Alanine and Threonine, which provided better separations between sea ice/pelagic algae and other end-member groups. Overall, our study provides first insights into the potential differences in δ13C-AA and δ15N-AA patterns between sea ice and pelagic algae and suggests carbon of sea ice origins may be distinguished from pelagic sources using CSIA-AA approach. These observations highlight the potential of CSIA-AA to estimate proportional contributions of sea ice and pelagic algae to export production and efficiency of benthic-pelagic coupling in polar marine environments.

Integrative assessment of amino acid nitrogen isotopic composition in biological tissue samples determined by GC/C/IRMS, LC × EA/IRMS, and LC × GC/C/IRMS

AbstractCompound‐specific isotope analysis of nitrogen (δ15N) in amino acids (CSIA‐AA) has significantly contributed to environmental sciences such as anthropology, biogeochemistry, and ecology. Several methods exist for determining δ15N of amino acids (AAs). Although these methods have their own strength and weakness, they have not been intercalibrated yet, especially for biological samples with matrices. To address this issue, we systematically compared AA δ15N values among three methods using gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS), preparative liquid chromatography (LC) separation followed by elemental analyzer/IRMS (LC × EA/IRMS), and LC separation followed by GC/C/IRMS (LC × GC/C/IRMS). The δ15N values of glutamic acid (δ15NGlu) and phenylalanine (δ15NPhe) in fish muscle, two crucial AAs for estimating the trophic positions (TPs) of organisms, were compared among methods. Although a significant difference in fish muscle δ15NGlu values was found among the three analytical methods, their δ15NGlu and δ15NPhe values were fairly consistent between all pairs of methods (n = 8, R2 = 0.9968 for GC/C/IRMS vs. LC × GC/C/IRMS; 0.9936 for LC × EA/IRMS vs. LC × GC/C/IRMS; and 0.9912 for GC/C/IRMS vs. LC × EA/IRMS), which resulted in similar TP estimates among the methods. Thus, the results provide empirical validation that the CSIA‐AA is comparable among different methods in interdisciplinary research fields. We also highlighted some critical features of each of the three analytical methods that can be used as a guideline for future CSIA‐AA research.

Calibrating bulk and amino acid δ13C and δ15N isotope ratios between bivalve soft tissue and shell for paleoecological reconstructions

Ecological isotope proxies measured in ancient bivalve shell matrix protein have great promise for paleoecological reconstruction. Compound-specific isotopes of amino acids (CSI-AA) may be an ideal tool for developing paleoecological proxies, as initial work has indicated that CSI-AA is less subject to alteration under geologic conditions relative to bulk isotope values. While CSI-AA proxies have been applied in modern bivalve soft tissues and a few shell studies, they have yet to be systematically investigated in shell matrix protein. Here, we measured stable isotope values of carbon (δ13C) and nitrogen (δ15N) in both bulk and individual amino acids (AA), comparing soft tissue and shell organic fractions to test the fidelity of a suite of ecological isotope proxies in shell. We sampled three ubiquitous bivalve species in four seasons for one year from two common coastal environments: littoral rocky intertidal and estuarine delta ecosystems. Particulate organic matter (POM) was simultaneously collected to investigate relationships between tissue types and local POM isotope signatures. The ecological proxies tested include trophic niche breadth from bulk isotopes, and baseline δ13C and δ15N values, resource contribution, and trophic level from CSI-AA data. We found niche breadth corresponded well between tissue types, but that bulk isotope values were significantly higher in shell compared to soft tissue. In the CSI-AA record, there were no differences in baseline δ13C and δ15N proxies between tissue types. While no consistent seasonal trends were observed in the δ13C record, summer bulk and baseline δ15N values were lowest in both ecosystems in both tissues. Food resource contribution estimates from shell also closely matched soft tissue, however trophic level estimates were consistently higher in shell, attributed to a systematic offset in Glutamic acid δ15N values. We therefore propose here a new mollusk-specific trophic discrimination factor, and corresponding shell isotope enrichment factor to correct for isotopic routing, both required for accurate paleoecological reconstructions.

Environmental fate and trophic transfer of synthetic musk compounds and siloxanes in Geum River, Korea: Compound-specific nitrogen isotope analysis of amino acids for accurate trophic position estimation.

Despite the extensive usage of synthetic musk compounds (SMCs) and siloxanes in various personal care products (PCPs), trophic magnification of such chemicals in aquatic environments remains unexplored. In June and September 2020, eleven SMCs and nineteen siloxanes were measured in water, sediments, and biota. Samples were collected from two sites where levels were expected to be influenced by the distance from the wastewater treatment plant (WWTP) in the Geum River, Republic of Korea, were expected. High concentrations of SMCs and siloxanes entered through WWTP were measured in water, sediment, and biota at the both sites and both seasons. The δ15N of amino acids provided a high-resolution food web and accurate trophic position (TP), which is an important factor for determining the trophic magnification factor (TMF). Among 24 TMFs, 19 of them were <1, ranging 0.7-0.8 for 1,3,4,6,7,8‑hexahydro‑4,6,6,7,8,8‑hexamethyl‑cyclopenta‑γ‑2‑benzopyran (HHCB), 0.6-0.8 for 6-Acetyl-1,1,2,4,4,7-hexamethyltetralin (AHTN), 0.7-0.8 for 4-tert-Butyl-3,5-dinitro-2,6-dimethylacetophenone (MK), 0.7-0.9 for octamethylcyclotetrasiloxane (D4), 0.1-0.4 for decamethylcyclopentasiloxane (D5), and 0.04-0.8 for dodecamethylcyclohexasiloxane (D6), and the remaining ones including HHCB, AHTN, MK, and D4 showed values close to 1 or slightly higher (TMF range: 1.0-2.3) indicating no or a little trophic magnification. The TMFs of these compounds were constant across sites and seasons. The TMF values of PCPs might be affected by species specificity and food web structure rather than by chemical properties such as log Kow, which describes a wide range of TMF values in various environments. This study presents valuable implications for assessing risk and managing environmental fate and trophic transfer of SMCs and siloxanes in freshwater environments.

Assessing the potential of amino acid δ13C and δ15N analysis in terrestrial and freshwater ecosystems

Abstract Understanding the structure and dynamics of food webs requires accurate estimates of energy flow among organisms. Bulk tissue carbon (δ13C) and nitrogen (δ15N) isotope analysis is often used to this end; however, the limitations of this technique can outweigh the benefits. The isotope analysis of individual amino acids is being increasingly employed to trace energy flow and estimate consumer trophic level. Central to this compound‐specific approach are the concepts of essential amino acid (AAESS) δ13C fingerprinting and amino acid (AA) δ15N β‐values, both of which have been understudied and are poorly constrained in terrestrial and freshwater producers. We present AAESS δ13C data for 112 terrestrial and freshwater producers collected from two aridland habitats in the northern Chihuahuan Desert (New Mexico, USA) and AA δ15N data for a subset (n = 28) of these samples. We characterized AAESS δ13C fingerprints by performing linear discriminant analysis on the δ13C values of isoleucine, leucine, lysine, phenylalanine, threonine and valine for four producer groups—C3 plants, C4 plants, CAM plants and filamentous green algae. We explored potential biochemical mechanisms underlying these AAESS δ13C fingerprints by calculating differences between the δ13C values of AAESS products and their AA precursors. This allowed us to estimate and compare isotopic discrimination for specific AAESS synthesis pathways across producer groups. We found a near‐perfect separation of AAESS δ13C fingerprints among producer groups; all groups reclassified with &gt;95% success within our multivariate framework. We also found varied isotopic discrimination for specific AAESS synthesis pathways among producer groups. Contrary to previous studies, we found no differences in β‐values between terrestrial C3 and C4 plants for any trophic‐source AA pairing. Furthermore, we found that Lys δ15N values were less variable and more closely related to bulk tissue δ15N values than Phe δ15N values in terrestrial and freshwater producers. Synthesis. We conclude that AAESS δ13C fingerprints are a higher resolution tracer for freshwater food webs, where instream algae have overlapping bulk tissue δ13C values with terrestrial C3 plants. Additionally, βGlx‐Lys and βPro‐Lys are the best for AA δ15N‐based consumer trophic‐level estimates in freshwater food webs containing both terrestrial and aquatic resources.

Environmental Regulation of the Nitrogen Supply, Mean Trophic Position, and Trophic Enrichment of Mesozooplankton in the Mekong River Plume and Southern South China Sea

AbstractThe mean trophic position (TP) of mesozooplankton largely determines how much mass and energy is available for higher trophic levels like fish. Unfortunately, the ratio of herbivores to carnivores in mesozooplankton is difficult to identify in field samples. Here, we investigated changes in the mean TP of mesozooplankton in a highly dynamic environment encompassing four distinct habitats in the southern South China Sea: the Mekong River plume, coastal upwelling region, shelf waters, and offshore oceanic waters. We used a set of variables derived from bulk and amino acid nitrogen stable isotopes from particulate organic matter and four mesozooplankton size fractions to identify changes in the nitrogen source and TP of mesozooplankton across these habitats. We found clear indications of a shift in N sources for biological production from nitrate in near‐coastal waters with shallow mixed layer depths toward an increase in diazotroph‐N inputs in oceanic waters with deep mixed layer depths where diazotrophs shaped the phytoplankton community. The N source shift was accompanied by a lengthening of the food chain (increase in the TP). This may provide further support for the connection between diazotrophy and the indirect routing of N through the marine food web. Our combined bulk and amino acid δ15N approach also allowed us to estimate the trophic enrichment (TE) of mesozooplankton across the entire regional ecosystem. When put in the context of literature values, a high TE of 5.1‰ suggested a link between ecosystem heterogeneity and the less efficient transfer of mass and energy across trophic levels.

Long term natural and anthropogenic forcing on aquatic system - evidence based on biogeochemical and pollen proxies from lake sediments in Kashmir Himalaya, India

Freshwater aquatic systems are subjected to rapid deterioration driven by multiple stressors such as climate change and human activity. The understanding of the long-term history of eutrophication and their trends provides an opportunity for developing relevant management strategies. In this study, we examine the natural versus anthropogenic impacts on Ahansar Lake using a multiproxy approach (total organic carbon (TOC), total nitrogen (TN), amino acid composition, δ15N, grain size and pollen data) on a210Pb/137Cs dated sediment core spanning the last 200 years. The amino acid datasets clearly show that the organic matter in Ahansar sediment core is less degraded and can be utilized to understand the paleoproductivity changes. Organic matter (OM) within this core is mostly derived from aquatic sources as deduced from C/N (6–11) and δ15N (0–3.2‰) values. The aquatic productivity gradually increases from 1880s, becoming accelerated after the 1930s, and peaked between 1970 and 2016 AD. This enhancement of primary productivity in the lake indicating the increased eutrophication through time due to anthropogenic activities in the recent decades. The results provide baseline information for policymakers and environmentalists to develop strategical framework for future environmental changes in an aquatic system subjected to anthropogenic stressors.

Large‐scale patterns of green turtle trophic ecology in the eastern Pacific Ocean

AbstractTrophic position and niche width are fundamental components of a species’ ecology, reflecting resource use, and influencing key demographic parameters such as somatic growth, maturation, and survival. Concepts about a species’ trophic niche space have important implications for local management and habitat protection, and can shed light about resilience to changing climate for species occurring over broad spatial scales. For elusive marine animals such as sea turtles, trophic niche is challenging to study, and researchers often rely on other metrics, such as isotopic niche, as a proxy. Here, stable isotope analysis (δ13C and δ15N values) was conducted on bulk skin tissue of 718 green turtles (Chelonia mydas) distributed among 16 foraging areas in the eastern Pacific from the USA to Chile, a range spanning ~10,000 km. Compound‐specific nitrogen isotope analysis of amino acids (CSIA‐AA) was applied to 21 turtles among seven sites. Isotopic niche space was determined via Bayesian ellipse area (BEA) and convex hull area (CHA) analyses of bulk isotope values, which were also used along with amino acid δ15N values to determine trophic position (TP). Substantial variability in bulk tissue δ13C and δ15N values was found within and among sites, and amino acid δ15N values confirmed this was largely due to spatial differences in baseline nitrogen isotopic compositions, but also to a lesser extent from TP differences among the green turtle foraging populations. Isotope niche space varied among sites, influenced by the diversity of prey types and relative input of terrestrial‐ vs. marine‐derived nutrients; BEAs were the most suitable measurement of isotopic niche space due to the larger influence of outlying values with the CHA approach. Amino acid isotope‐derived TP estimates that accounted for local habitat conditions (e.g., mixed seagrass/macroalgae diet) performed the best among several approaches; TP ranged from 2.3 to 3.6, which indicates an omnivorous diet for most populations. In addition to providing additional spatial resolution for δ13C and δ15N isoscapes in the eastern Pacific, especially in coastal habitats, this study further establishes CSIA‐AA as an effective tool to study the trophic ecology of sea turtles across a variety of food webs and habitats.

Amino Acid δ15N Can Detect Diet Effects on Pollution Risks for Yellow-Legged Gulls Overlooked by Trophic Position

The use of top-consumers as bioindicators of the health of food webs is hampered by uncertainties in their effective use of resources. In this study, the abundance of stable nitrogen isotopes in amino acids from homogenised eggs of the Yellow-legged Gull (Larus michahellis) allowed to identify variations in trophic resource exploitation between geographically adjacent nesting colonies in the Ria de Vigo (NW Spain) that exhibited marked differences in pollutants. Eggs from nests in the Cíes Islands (located in a National Park) showed a large variability in stable carbon and nitrogen isotopes in bulk egg content encompassing that of eggs from Vigo city (a major fishing harbour). However, both colonies differed in the relative concentration and abundance of nitrogen isotopes of lysine, an essential amino acid present in marine prey, but also extensively used in feed stocks for poultry and swine. Notwithstanding the similarity in trophic position for both colonies, gulls from Cíes Islands may have acquired a substantial fraction of lysine from garbage dump sites, while those of the urban colony relied on fish discards. This unexpected conclusion is partly supported by the large variability reported for gull’s diet in this region and calls for detailed estimations of diet when assessing the conservation status and pollution risks of marine ecosystems.

Influences of Ocean Currents on the Diets of Demersal Fish Communities in the Western North Pacific Revealed by Their Muscle Carbon and Nitrogen Isotopic Compositions

To study the influence of different ocean currents on the trophic spectra found in a sympatric fish community, we analyzed the radiocarbon contents (Δ14C) and stable carbon (δ13C) and nitrogen (δ15N) isotopic compositions in their bulk muscle tissues and the δ15N of individual amino acids in 26 species of demersal fish collected from off Tohoku (Pacific coast), northeastern Japan. The Δ14C values of the fish varied from –42 to +41‰, consistent with the Δ14C of the dissolved inorganic carbon in the cold Oyashio Current (typically about –50‰) and the warm Kuroshio Current (typically about +50‰). The trophic positions (TPs) of the fish estimated from the δ15N values of amino acids increased from 3.1 to 4.5 with increasing snout length. A negative correlation was observed between Δ14C and TP in gadiform fish and flatfish, suggesting that the Oyashio Current delivers a high TP diet to these fish groups. These results suggest that the trophic ecology of marine fish in the coastal western North Pacific is primarily controlled by the two major surface water currents, but is also significantly influenced by a combination of species-specific feeding and migration strategies.