Differences In δ15N Research Articles

A pilot study on the isotopic characterization of feeding habits of Diptera in a tropical rain forest

Flies (Diptera) are among the most diverse groups of insects and are known to utilize various food resources, including plants, detritus, microbial tissues, and fresh and dead animal tissues. However, their feeding habits in the field remain poorly understood. We conducted a pilot study to apply stable nitrogen (N) and carbon (C) isotope techniques to examine the feeding habits of flies in a tropical rain forest in Sarawak, Malaysia. The fly samples comprised 13 families and >18 species. The results showed significant differences in nitrogen and carbon isotope ratios (δ15N and δ13C) among families and species within a family. The observed pattern is largely consistent with their known feeding habits; flies that use carcasses and carrion as diets (e.g., Sarcophagidae and Calliphoridae) have significantly higher δ15N values than those likely utilizing plant-based diets (e.g., Cecidomyiidae). There were significant differences in δ15N values among the six species of Calliphoridae, which is consistent with insect succession on carcasses. The differences in δ15N may be explained by the use of carrion at different stages of decomposition, because microbial decomposition can lead to the 15N enrichment. Tachinid flies had relatively low δ13C values, reflecting the use of lepidopterans as a host. This pilot study shows that the δ15N and δ13C values of flies provide insights into the diversity of feeding habits of fly communities, which could also serve as an indicator of resource availability in an entire ecosystem.

High-Throughput Sequencing Analysis Revealed a Preference for Animal-Based Food in Purple Sea Urchins.

Sea urchins play an important role in marine ecosystems. Owing to limitations in previous research methods, there has been insufficient understanding of the food sources and ecological functional value of purple sea urchins, leading to considerable controversy regarding their functional positioning. We focused on Daya Bay as the research area, utilizing stable isotope technology and high-throughput sequencing of 16S rDNA and 18S rDNA to analyze sea urchins and their potential food sources in stone and algae areas. The results showed that the δ13C range of purple sea urchins in the stone area is -11.42~-8.17‱, and the δ15N range is 9.15~10.31‱. However, in the algal area, the δ13C range is -13.97~-12.44‱, and the δ15N range is 8.75~10.14‱. There was a significant difference in δ13C between the two areas (p < 0.05), but there was no significant difference in δ15N (p > 0.05). The main food source for purple sea urchins in both areas is sediment. The sequencing results of 18S rDNA revealed that, in the algal area, the highest proportion in the sea urchin gut was Molluska (57.37%). In the stone area, the highest proportion was Arthropoda (76.71%). The sequencing results of 16S rDNA revealed that, in the algal area, Bacteroidetes was the dominant group in the sea urchin gut (28.87%), whereas, in the stone area, Proteobacteria was the dominant group (37.83%). Diversity detection revealed a significant difference in the number of gut microbes and eukaryotes between the stone and algal areas (p < 0.05). The results revealed that the main food source of purple sea urchins in both areas is sediment, but the organic nutritional value is greater in the algal area, and the richness of microbiota and eukaryotes in the gut of purple sea urchins in the stone area is greater. These results indicated that purple sea urchins are likely omnivores and that the area where they occur impacts their growth and development. The results of this study provide a theoretical basis for the restoration of wild purple sea urchin resources and the selection of areas for restocking and release.

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

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

Variability in resource use of sympatric cowfish Aracana aurita and A. ornata in southeastern Australian waters

Sympatric species that share morphological similarities tend to diversify their prey resources to coexist. The striped cowfish (Aracana aurita) and ornate cowfish (A. ornata) live together on the continental shelf from western to southern Australia including Tasmanian waters. The present study analyzed stomach contents and stable isotope ratios (δ13C and δ15N) to investigate the use of food resource between the two species collected in Tasmanian waters, Australia. In total, 139 A. aurita and 46 A. ornata were collected from 2014 to 2018. The small size class of A. aurita (<140 mm SL) mainly consumed polychaetes and amphipods, while its large size class (≥140 mm SL) preyed more on bivalves and crustacean decapods. The main prey items of the small size class of A. ornata (<140 mm SL) were amphipods and crabs. No differences in diet were observed between the sexes of either species. Stable isotope analyses showed significant differences in δ15N and δ13C between the two species, but not between the two size classes of A. aurita (ANOVA, P < 0.05). Integrating analyses of stomach contents and stable isotopes indicated that the two species preferred different food resources based on different trophic sources. Consequently, although the habitats of the two species overlap, both species can co-exist through dietary segregation.

Comparison of δ13C and δ15N of ecologically relevant amino acids among beluga whale tissues

Ecological applications of compound-specific stable isotope analysis (CSIA) of amino acids (AAs) include 1) tracking carbon pathways in food webs using essential AA (AAESS) δ13C values, and 2) estimating consumer trophic position (TP) by comparing relative differences of ‘trophic’ and ‘source’ AA δ15N values. Despite the significance of these applications, few studies have examined AA-specific SI patterns among tissues with different AA compositions and metabolism/turnover rates, which could cause differential drawdown of body AA pools and impart tissue-specific isotopic fractionation. To address this knowledge gap, especially in the absence of controlled diet studies examining this issue in captive marine mammals, we used a paired-sample design to compare δ13C and δ15N values of 11 AAs in commonly sampled tissues (skin, muscle, and dentine) from wild beluga whales (Delphinapterus leucas). δ13C of two AAs, glutamic acid/glutamine (Glx, a non-essential AA) and, notably, threonine (an essential AA), differed between skin and muscle. Furthermore, δ15N of three AAs (alanine, glycine, and proline) differed significantly among the three tissues, with glycine δ15N differences of approximately 10 ‰ among tissues supporting recent findings it is unsuitable as a source AA. Significant δ15N differences in AAs such as proline, a trophic AA used as an alternative to Glx in TP estimation, highlight tissue selection as a potential source of error in ecological applications of CSIA-AA. Amino acids that differed among tissues play key roles in metabolic pathways (e.g., ketogenic and gluconeogenic AAs), pointing to potential physiological applications of CSIA-AA in studies of free-ranging animals. These findings underscore the complexity of isotopic dynamics within tissues and emphasize the need for a nuanced approach when applying CSIA-AA in ecological research.

Differences and drivers of leaf stable carbon and nitrogen isotope in herbs under different vegetation types on the eastern Qinghai-Tibet Plateau

The natural abundance of stable carbon and nitrogen isotopes (δ13C and δ15N) in leaves can provide comprehensive information on the physiological and ecological processes of plants and has been widely used in ecological research. However, recent studies on leaf δ13C and δ15N have focused mainly on woody species, few studies have been conducted on herbs in different vegetation types, and their differences and driving factors are still unclear. In this study, we focused on the herbs in subalpine coniferous forests, alpine shrublands, and alpine mea-dows on the eastern Qinghai-Tibet Plateau, and investigated the differences in leaf δ13C and δ15N of herbs and the driving factors. The results showed that there were significant differences in leaf δ13C and δ15N values of herbs among different vegetation types, with the highest δ13C and δ15N values in alpine meadows, followed by alpine shrublands, and the lowest in subalpine coniferous forests. Using variation partitioning analysis, we revealed that differences in leaf δ13C and δ15N of herbs among various vegetation types were driven by both leaf functional traits and climate factors, with the contribution of leaf functional traits being relatively higher than that of climate factors. Hierarchical partitioning results indicated that mean annual temperature (MAT), chlorophyll content index, leaf nitrogen content per unit area (Narea), and leaf mass per area were the main drivers of leaf δ13C variations of herbs across different vegetation types, while the relative importance of Narea and MAT for variation in leaf δ15N of herbs was much higher than those other variables. There was a strong coupling relationship between leaf δ13C and δ15N as indicated by the result of the ordinary least squares regression. Our findings could provide new insights into understanding the key drivers of leaf δ13C and δ15N variations in herbs across different vegetation types.

Legacy impacts and recovery of δ15N, δ13C and C/N storage in soils due to historic land use

Physical and chemical modifications within soils have been proposed as a marker of the Anthropocene, as soils can preserve modifications due to past land use for long periods of time. Soils are the primary terrestrial reservoir of C and N and are especially important for sequestration and emission of C related to land use changes. The northeast US has a well-documented sequence of deforestation and reforestation related to land use changes following European settlement, yet the impact of land use and recovery on C and N stocks and isotopes is still poorly constrained. We analyze δ15N, δ13C, and C/N to evaluate changes to soil C and N related to historical land use across an Anthropocene chronosequence comprised of four land use classes that vary in terms of duration of disturbance and recovery time from past agricultural activity. Reforested soils show minimal difference in δ15N and δ13C and display no overall statistical relationship with abandonment length, while modern agricultural soils have higher δ15N values. Differences in total C and N between land use classes are more distinct, as SOC decreases and total N increases with longer land use duration. Historic agriculture increased C and N storage, and recently abandoned land still has the potential to act as a sink to store more organic C. In total, land use imparts clear changes to SOC and N stocks that persist long after abandonment, providing a distinct marker of anthropogenic activities. However, stable C and N isotopes of soils within reforested classes show only slight differences between land use classes, indicating shorter timescales of isotopic resetting of C and N signatures following abandonment.

Stocks and Sources of Soil Carbon and Nitrogen in Non-Native Kandelia obovata Afforestation and Spartina alterniflora Invasion: A Case Study on Northern Margin Mangroves in the Subtropical Coastal Wetlands of China

For decades in China, carbon neutrality policies have spurred the establishment of northern margin mangroves as artificial blue carbon ecosystems. However, there has been limited research on the impact of plantation and invasion on the stocks and sources of soil carbon and nitrogen in rehabilitated coastal wetlands. Non-native Kandelia obovata afforestation began on Ximen Island, Zhejiang, China, where Spartina alterniflora invasion had also occurred decades ago. Soil cores were collected from both mangrove and salt marsh habitats with depths from 0 to 50 cm and were analyzed for total carbon (TC), soil organic carbon (SOC), total nitrogen (TN), and the isotope of carbon and nitrogen in sediments. The results indicated that there were no significant differences in the TC, SOC, and C/N ratio between the K. obovata and the S. alterniflora, but there were significant differences in TN, isotope δ13C, and δ15N. The SOC content of both ecosystems in the 0–20 cm layer was significantly higher than that in the 30–50 cm layer. Our study has shown that the main sources of carbon and nitrogen for mangroves and salt marshes are different, especially under the impact of external factors, such as tidal waves and aquaculture. These findings provide insight into the ecological functioning of subtropical coastal wetlands and an understanding of the biogeochemical cycles of northern margin mangrove ecosystems.

Dietary 14C reservoir effects and the chronology of prehistoric burials at Sakhtysh, central European Russia.

We present a robust radiocarbon (14C) chronology for burials at Sakhtysh, in European Russia, where nearly 180 inhumations of Lyalovo and Volosovo pottery-using hunter-gatherer-fishers represent the largest known populations of both groups. Past dating attempts were restricted by poor understanding of dietary 14C reservoir effects (DREs). We developed a DRE correction approach that uses multiple linear regression of differences in 14C, δ13C, and δ15N between bones and teeth of the same individuals to predict DREs of up to approximately 900 years. Our chronological model dates Lyalovo burials to the early fifth millennium BCE, and Volosovo burials to the mid-fourth to early third millennium. It reveals a change in the subsistence economy at approximately 3300 BCE, coinciding with a reorientation of trade networks, and dates the final burial to the early Fatyanovo period, the regional expression of the Yamnaya/Corded Ware expansion. Our approach is applicable when freshwater 14C reservoir effects are poorly constrained and grave goods cannot be dated directly.

Changes in urinary stable nitrogen isotope ratios during controlled short-term energy deficit: a proof-of-principle analysis

PurposeStable isotope ratios of nitrogen (δ15N) have previously been shown to increase in human hair during periods of catabolism. The goal of this study was to assess changes in δ15N in urinary urea (δ15Nurea) and Δ15N during a short-term controlled energy deficit.MethodsWe analyzed samples from 6 recreationally active men (25 ± 1 years, BMI: 23.5 ± 0.6 kg/m2) who participated in a repeated measures cross-over study involving 4 days of energy deficit (ED, ~ 15 kcal/kg FFM) without and with exercise (ED-EX, ED + EX) and control conditions in energy balance (CON-EX, CON + EX). δ15Nurea was analyzed from urine samples, and Δ15N was calculated as δ15Nurea–δ15Ndiet, with δ15Ndiet obtained from diet prescriptions.Resultsδ15Nurea was significantly elevated in ED-EX (4.4 ± 0.2‰) when compared to CON-EX (3.7 ± 0.1‰; p = 0.026) and CON + EX (3.34 ± 0.13‰, p = 0.001). As a consequence, Δ15N was positive in ED-EX (0.2 ± 0.2‰) and remained negative in ED + EX (− 0.6 ± 0.5‰), CON-EX (− 1.0 ± 0.2) and CON + EX (− 1.1 ± 0.2). Differences in Δ15N were significant between ED-EX and CON-EX (p = 0.005) and ED-EX and CON + EX (p = 0.006).ConclusionOur results suggest that δ15Nurea and subsequently Δ15N are responsive to a short-term energy deficit, likely due to increased amino acid oxidation to meet energy demands and preferable elimination of 14N.

Early share of 15N-labelled fertilizer between trees and crop in young temperate alley-cropping system

Nitrogen (N) partitioning between trees and crops in alley-cropping systems (AC) may increase the N fertilizer use, but more investigation is needed to elucidate it for young trees. The aim of this study was to explore the share of N between crops and three tree species after a few years of tree establishment in AC. Using 15N labeling (10 atm% as 15NH4,15NO3 form), we traced the fate of N fertilizer in wheat associated with four-year-old trees at Ramecourt alley-cropping experimental site. The 15N abundance of tree leaves and crop whole-plant was compared between samples collected in AC, in sole-crop control (CC) and in unlabeled and unfertilized pure-forest control plots (FC). The wheat δ15N increased from 0.7 ± 0.6‰ to 994 ± 474‰ before and 12 days after labeling and remained stable until harvest. For natural abundance, no significant difference in wheat δ15N was observed between AC and CC. The same result was observed for labeled treatment. The percentage of wheat N derived from fertilizer was on average 27 ± 17% and no effect of type of system or associated tree species was observed. Tree foliar δ15N was multiplied by 1.5, 1.8 and 4 for willow, hornbeam and wild cherry respectively compared to unlabeled trees in AC. The percentage of tree foliar N derived from fertilizer was on average 0.04 ± 0.04% and no significant difference was observed between tree species. The wheat associated with trees allocated significantly more tracer N to ears than wheat in monocrop. From this study, we succeeded in quantifying the beginning of tree/crop sharing of N fertilizer using labeling method and evidenced tree species effect. Young trees in AC contributed to valorize the N fertilizer without significant effect on global wheat uptake and favored its valorization in wheat grain.

Feeding ecology based on stable carbon and nitrogen isotopes: A comparative study on different Guiana dolphin tissues

Different tissues are used for stable isotope analysis in cetacean investigations. However, variation in the isotopic composition of tissues with different turnover rates has been reported for cetaceans. To better understand stable carbon and nitrogen isotopes (δ13C and δ15N) in skin compared to other tissues, this study assessed the isotopic variation among the liver, muscle, and skin of Guiana dolphins (Sotalia guianensis), as well as the influence of sex on these variations. No differences were found in δ13C among male tissues, but females showed lower values in the liver compared to muscle and skin. Differences in δ15N were observed among all tissues, with different variation patterns for males and females. Four females were distinguished from males and other females by their 13C depletion in all tissues and δ15N variation pattern. We conclude that skin and muscle may be equivalent in δ13C values for Guiana dolphins. The multiple-tissue analysis brings new insights into their feeding ecology and provides background for stable isotope analysis using non-destructive sampling techniques in small cetaceans.

Effects of lipid extraction on human bone collagen: Comparing stable carbon and nitrogen isotope values with and without lipid extraction

This study analyzes 24 human bone fragments to assess how lipid extraction affects stable carbon and nitrogen isotope ratios. Eighteen samples were archaeological bone and six were modern bone. Each bone was split into two pieces, producing 48 bone samples, and half underwent lipid extraction and half did not. This was done to assess whether the two techniques yielded statistically significant differences in δ13C and δ15N. The results are an important contribution for establishing standard stable isotope preparation protocols, which affect time and cost of laboratory procedures. Further, given that many dietary stable isotope studies compare original results to published material, this study addresses whether those comparisons are suitable if some samples undergo lipid extraction while others do not.We also examine 144 articles that report stable carbon and nitrogen isotope results to evaluate the proportion of studies that conduct lipid extraction versusthose that do not. We explore those outcomes by archaeological geographic region and by stable isotope laboratories. Findings show that 15% of the published studies do lipid extraction, and they tend to be grouped by researchers and laboratories who conduct research in specific global regions. The meta-analysis of the published articles and the findings that some laboratories conduct lipid extraction, while others do not, prompted the experimental portion of this study.Results of the experimental study show no significant differences in δ13C between samples prepared with and without lipid extraction and no significant differences in δ15N between samples prepared with and without lipid extraction. These results suggest that stable carbon and nitrogen isotope analysis of human bone collagen may not require lipid extraction for certain dietary reconstruction studies, specifically for non-coastal contexts (i.e., non-marine based diets). The isotope data was analyzed overall and among subgroups (e.g., archaeological bone, modern bone, mid-valley Peruvian ecological zones, and highland Peruvian ecological zones). In all cases, stable isotope ratios from samples prepared with and without lipid extraction were not significantly different.

Changing food webs before and during the Last Glacial Maximum based on stable isotopes of animal bone collagen from Lower Austria

ABSTRACTWe investigated palaeofood web structures using stable isotope analyses on animal bone collagen from four Upper Palaeolithic sites dated to the Early Gravettian (Krems‐Hundssteig and Krems‐Wachtberg: 33–31k cal a bp, Langenlois: 31–29k cal a bp) and to the Early Epigravettian (Kammern‐Grubgraben: 24–20k cal a bp). In both periods, δ13C values show niche partitioning between hare, horse and mammoth on one side, and reindeer and ibex on the other, indicating different diets and habitats between both herbivore groups. The δ15N differences between carnivores and herbivores suggest a difference of one trophic level during the pre‐Last Glacial Maximum (pre‐LGM) period at the Early Gravettian sites and a tendency towards secondary carnivores during the LGM at Kammern‐Grubgraben. δ15N values of pre‐LGM mammoths are elevated in relation to other herbivores but shifted to the level of other herbivores in the LGM. A general δ15N value shift in herbivores of 3.3‰ from the pre‐LGM to the LGM is related to climatic deterioration. This may have led to the disappearance of certain ecological niches and to a shift from broader to overlapping ecological herbivore niches shortly before the LGM, as demonstrated by SIBER analyses.

Contrasting intra-individual variation in size-based trophic and habitat shifts for two coastal Arctic fish species

Within and among species variation in trophic and habitat shifts with body size can indicate the potential adaptive capacity of species to ecosystem change. In Arctic coastal ecosystems, which experience dramatic seasonal shifts and are undergoing rapid change, quantifying the trophic flexibility of coastal fishes with different migratory tactics has received limited attention. We examined the relationships among body length and condition (Fulton’s K, phase angle from Bioelectrical Impedance Analysis) with trophic and habitat shifts (differences in δ15N and δ13C between blood tissues with different turnover rates) of two abundant and culturally important species, anadromous Arctic char (Salvelinus alpinus, n = 38) and sedentary Greenland cod (Gadus ogac, n = 65) during summer in coastal marine waters near Ulukhaktok, Northwest Territories, Canada. Habitat shifts (δ13C) increased with length (i.e., pelagic to benthic-littoral) and crossed-equilibrium (zero) at mid-sizes for both species. Seasonal trophic shifts (δ15N) were generally positive (i.e., increasing trophic level) for Arctic char and negative for Greenland cod. As hypothesised, intra-individual variation in size-based trophic shifts (δ15N-length residuals) increased with length for Arctic char. However, there were no trends with length in Greenland cod. Our findings highlight the importance of flexibility through ontogeny and mobility for Arctic char, whereas Greenland cod were generalist to localized prey and habitat across all sizes. The significant effect of body condition (phase angle) on size-based trophic shifts in Arctic char, and size-based habitat shifts in Greenland cod, highlight the potential trade-offs of contrasting life history strategies and capacity for ontogenetic niche plasticity.

Nitrogen supply in the upwelling of mid-west region of the South China Sea: The case of mesopelagic fishes

Coastal upwelling zone hosts most fishing grounds worldwide and nitrogen dynamics in those systems mainly have been addressed through the nutrients replenished the ecosystem. Compared with coastal and upper-layer ecosystems, relatively limited knowledge has been explored by nitrogen isotope imprints through the food webs of mesopelagic fishes and their links to upwelling characteristics. Here, we collected several species of mesopelagic fishes in the mid-west region of the South China Sea (SCS) and analyzed them by biochemical methods (fatty acids, δ13C, δ15N, and δ13C of fatty acids). Due to the occurrence of upwelling, we want to evaluate the variation of nitrogen isotope compositions among these mesopelagic species and their links to the upwelling nitrogen supply. The δ15N of mesopelagic fishes in the non-upwelling area was depleted by 2‰ when compared with those in upwelling area, which indicated different nitrogen sources impacting for fishes in the two areas. A combination of multi-biochemical proxies was used to divide mesopelagic fishes into six groups and indicated that the feeding behaviors and upwelling were the main factors to affect the biological composition of mesopelagic fishes, even in such small regions. The differences in δ15N values between the upwelling and non-upwelling areas allowed us to estimate that N2 fixation supported about 37% of the N-demand of food sources in the non-upwelling area, which was larger than the 10% in the upwelling area. These results illustrate that nutrients from the deep layers in upwelling systems might be the main factors that cause differences in δ15N in mesopelagic fishes between upwelling and non-upwelling areas. Thus, more studies of N dynamics in mesopelagic fishes should be conducted in the upwelling systems to illustrate the impact of climate changes on fish biomass in the future.

Regional Variation in Feeding Patterns of Sheepshead (Archosargus probatocephalus) in the Northwest Gulf of Mexico

Feeding patterns of sheepshead, Archosargus probatocephalus, in the northwest Gulf of Mexico were examined from samples collected at two locations in Texas, USA; Galveston and Port Aransas. A total of 53 sheepshead stomachs (Galveston, n = 35; Port Aransas, n = 18) had their contents analyzed along with tissue samples from the muscle and liver for a stable isotope analysis (δ13C, δ15N, and δ34S) because they provide a contrast between short-term (liver) and long-term (muscle) measurements. Multiple species of amphipods made up the majority of the prey items in sheepshead from Galveston (%IRI = 61.79), whereas barnacles were the primary diet item for sheepshead collected in Port Aransas (%IRI = 39.53). We observed diet shifts prior to and during the spawning season. MANOVA revealed significant differences in δ13C, δ15N, and δ34S stable isotope values from the muscle and liver tissue of sheepshead based on the location. In both muscle and liver samples, δ13C values were lower in Galveston than Port Aransas, but δ15N and δ34S values were higher in Galveston than Port Aransas. Niche size and overlap also differed between sheepshead from both locations and tissue types. Sheepshead collected in Galveston had a larger niche size in the muscle and liver samples ((mean ± SD) 479.3 ± 131.2, muscle; 433.3 ± 120.3, liver) than Port Aransas (178.8 ± 54.3, muscle; 270.0 ± 80.9, liver). The trophic niche of sheepshead from Galveston overlapped Port Aransas in muscle samples by 16.47% and 18.56% in liver samples. The trophic niche overlap of sheepshead from Port Aransas measured in muscle samples overlapped with sheepshead from Galveston by 18.49% and by 39.17% in liver samples. This study shows that there are subtle but significant differences between the diets of sheepshead along the Texas coast, with the majority of the differences potentially attributable to differences in the prey field along the natural salinity gradient of the Texas coast. This work provides an insight into the widely generalist grazing behavior of sheepshead, which potentially changes during the springtime spawning season.

Carbon and nitrogen stable isotope compositions provide new insights into the phenotypic plasticity of the invasive species Carpobrotus sp. pl. in different coastal habitats

The genus Carpobrotus N.E.Br. comprises several aggressive invasive species that threaten biodiversity in coastal areas worldwide. We studied the phenotypic plasticity of Carpobrotus sp. pl. invading four coastal habitats in the north-western Iberian Peninsula (coastal cliffs, disturbed areas, dunes and coastal forests). We measured morphological traits and carbon (δ13C) and nitrogen (δ15N) stable isotope compositions of Carpobrotus sp. pl. individuals collected in each habitat. Our results indicated that leaf carbon content (% C) and dry shoot weight were higher on cliffs and lower in mixed forests. In contrast, leaf hydration was higher in mixed forests and lower on cliffs. Leaf nitrogen content (% N) was higher in forests, which might be due to the presence of Acacia longifolia, an alien tree that accumulates N in the soil through symbiotic associations with N fixing bacteria. Differences in δ15N showed the use of different N sources in each habitat. Values were higher in disturbed areas with greater human activity and lower on cliffs and forests. δ13C was higher in cliffs and dunes, suggesting CAM activity where drought and salinity are more intense. Water use efficiency (iWUE) and δ13C were higher on cliffs and dunes, suggesting an adaptation and high tolerance of Carpobrotus sp. pl. to unfavourable conditions such as drought or salinity in the invaded areas.

The Impact of Incomplete Nutrient Consumption in the Southern Ocean on Global Mean Ocean Nitrate δ15N

AbstractIt is understood that the global mean ocean nitrate δ15N is set by the δ15N of the input of fixed nitrogen (N) to the ocean (mostly N2 fixation) and the net isotopic discrimination of fixed N loss (mostly denitrification). Here, we demonstrate that, in addition to the fixed nitrogen input/output budget, the isotopic discrimination of nitrate assimilation in the Southern Ocean also plays a role in setting the δ15N of both deep ocean nitrate and global mean ocean nitrate. A prognostic model is used to simulate the global overturning circulation, focusing on the Southern Ocean overturning cell that ventilates the global pycnocline. N2 fixation and denitrification occur mainly in the model's surface and pycnocline boxes, as is appropriate given the observations. Experiments with this model indicate that, at a steady state, pycnocline nitrate δ15N is mostly controlled by the ocean's fixed N budget. Simultaneously, partial nitrate assimilation in the Southern Ocean sets the nitrate δ15N difference between the pycnocline and the deep ocean, lowering the δ15N of deep ocean nitrate and thus also of global mean ocean nitrate. The Southern Ocean's impact on deep and mean ocean nitrate δ15N depends on (a) the degree of nitrate consumption in the Southern Ocean surface waters and (b) the proportion of water that enters the pycnocline by subduction from the Southern Ocean surface. Including the effect of the Southern Ocean on mean ocean nitrate δ15N modestly reduces a previously calculated imbalance between fixed N inputs and outputs. Moreover, this effect has implications for paleoceanographic N isotope records.

Isotopic Composition (δ15N and δ18O) of Urban Forests in Different Climate Types Indicates the Potential Influences of Traffic Exhaust and Relative Humidity

Previous studies have shown that climatic and anthropogenic factors influence the variations in the stable isotope signature of tree materials in an urban ecosystem. However, the responses of urban trees to various anthropogenic and precipitation interventions are unclear. To fill the research gap, the isotopic composition (δ15N and δ18O) and total nitrogen content of the soil-plant system of urban forests were measured, focusing on typical urban tree species (Populus tomentosa and Ficus virens). The research was done under various land uses in China’s Beijing and Shenzhen megacities (with different humidity conditions). The linear mixed-effect model revealed that the δ15N values in the leaves of roadside Ficus virens were significantly higher than those in urban park sites, away from the road (δ15N difference = 7.2‰). In contrast, Populus tomentosa leaves exhibited a non-significant difference (δ15N difference = −1.8‰). Further regression analysis revealed that the variations in δ15N in tree leaves could be attributed to the proximity to highways, suggesting the influence of vehicle exhaust on δ15N values in roadside trees. The linear mixed-effect model also revealed δ18O values significantly higher in Populus tomentosa leaves than in Ficus virens leaves. To assess the influence of humidity conditions on δ18O values in the leaves, the ratio of the atmospheric and intercellular vapor pressures (ea/ei) of tree canopies was estimated. The result revealed that the ea/ei of Ficus virens was significantly higher than for Populus tomentosa, ascribed to the variation in the humidity conditions of the two megacities. These results highlight that urban trees can potentially serve as bioindicators for atmospheric pollution and humidity. Our findings highlight the ecophysiological responses of urban forests related to N and O as they vary according to traffic exhaust and relative humidity. Consequently, they are potentially valuable indicators of urban atmospheric contamination, forming a nature-based solution for citizen welfare improvement.