Nitrogen Stable Isotope Ratios Research Articles

Dataset on the isotopic (ẟ13C, ẟ15N) and elemental (C, N) composition of estuarine primary producers in the subtropical Southwestern Atlantic coast

Benthic and pelagic primary producers had their isotopic (ẟ13C, ẟ15N) and elemental (C, N) composition monitored in the Patos Lagoons estuary, in southern Brazil. The present dataset comprises temporal data obtained through seasonal samplings of C3 (Scirpus spp.) and C4 (Spartina densiflora) salt marsh plants, ephemerous bloom-forming drift macroalgae (Ulvophyceae), the widgeon grass Ruppia maritima, particulate (POM) and sedimentary (SOM) organic matter in shallow waters (< 2m) of a subtropical estuary from austral summer 2010 to autumn 2016. POM and SOM were collected as proxies of phytoplankton and microphytobenthos, respectively. Salt marsh plants were randomly sampled (N = 126) at a regularly flooded low marsh area, whereas submerged drift macroalgae (N = 29) and Ruppia plants (N = 14) were collected in adjacent mudflats. POM was collected (N = 33) by filtering water samples using glass fiber filter. SOM was obtained (N = 35) by removing superficial sediment. In laboratory, samples were processed and further analyzed for total organic carbon (TOC), total nitrogen (TN) and carbon (13C/12C) and nitrogen (15N/14N) stable isotopes ratios. With a total of 237 samples analyzed, this dataset provides key information on the isotopic and elemental composition of distinct estuarine primary producers and sources of particulate organic matter (POM and SOM) and their temporal variability in a highly variable aquatic environment. Such knowledge may add to ecological studies investigating food webs, biogeochemical cycles and sources tracking in coastal systems.

Social status and diet. Reconstruction of diet of individuals buried in some early medieval chamber graves from Poland by carbon and nitrogen stable isotopes analysis

The study presents results of the investigations of diet based on carbon and nitrogen stable isotope ratios (δ13C and δ15N) of the bone collagen of individuals buried in medieval elite chamber graves from the territory of the state of the first Piasts, Poland (the second half of the 10th and the first half of the 11th century). The aim of the research was to determine the diet of individuals buried in such funerary structures, to compare this with commoners buried in ordinary graves, and investigate any sex-related patterns. Rib bone samples were taken from individuals buried in chamber graves at Bodzia, Dziekanowice, Pień and Sowinki. Results indicate that the elite male diet was based on C3 plants with possible contribution of some C4 plants (millet) and substantial consumption of animal proteins including fish. The bone collagen δ13C and δ15N of male chamber burials suggested consumption of higher trophic level foodstuffs (meat and fish) whilst the female diet, and that of the juveniles, was similar to the commoners in the rest of the population.

A protocol for nitrogen isotopic measurement of dissolved organic nitrogen with a combination of oxidation-denitrification and gas phase diffusion methods

ABSTRACT Isotopic tracing technique is one of the most effective methods to identify nitrogen source and fate in aquatic environments. Although dissolved organic nitrogen (DON) is a key component in nitrogen cycles, information on nitrogen stable isotope ratios in DON (δ 15N-DON) is limitedly available for its low recovery through a direct measurement. Indirect measurement is based on mass balance calculations and easy to use with high recovery of DON. However, in theory, the result from mass balance calculation is sensitive to the level of DON content, and its applicability remains to be examined for waters containing a variety of DON content in total dissolved nitrogen (TDN). In this study, we established a protocol for indirect measurement of δ 15N-DON values based on the combination of multiple analytical methods. Precision and accuracy in the measurement were assessed by varying the composition of DON and dissolved inorganic nitrogen, and quantitation thresholds were presented at different acceptable levels. The results illustrated an advantage of the developed protocol possibly applicable to water samples particularly with low DON content that is commonly detected in freshwater. This method is expected to expand the use of isotope tracing techniques for understanding the nitrogen cycle in water environments.

Using near-infrared reflectance spectroscopy (NIRS) to estimate carbon and nitrogen stable isotope composition in animal tissues.

Stable isotopes analysis (SIA) of carbon and nitrogen provides valuable information about trophic interactions and animal feeding habits.We used near‐infrared reflectance spectroscopy (NIRS) and support vector machines (SVM) to develop a model for screening isotopic ratios of carbon and nitrogen (δ 13C and δ 15N) in samples from living animals. We applied this method on dried blood samples from birds previously analyzed for δ 13C and δ 15N to test whether NIRS can be applied to accurately estimate isotopic ratios.Our results show a prediction accuracy of NIRS (R 2 > 0.65, RMSEP < 0.28) for both δ 13C and δ 15N, representing a 12% of the measurement range in this study.Our study suggests that NIRS can provide a time‐ and cost‐efficient method to evaluate stable isotope ratios of carbon and nitrogen when substantial differences in δ 13C or δ 15N are expected, such as when discriminating among different trophic levels in diet.

Stable C and N Isotope Composition of European Anchovy, Engraulis encrasicolus, from the Marmara Sea and the Black Sea

The aim of this study is to determine the stable isotope ratios of anchovy caught in the Black Sea and Marmara Sea. Stable carbon and nitrogen isotope ratios (δ13C, δ15N) of European anchovy (Engraulis encrasicolus) were estimated at four sampling sites (İğneada, İstanbul, Trabzon and Hopa) in the Black Sea and Marmara Sea (Turkey). δ13C and δ15N values of European anchovy ranged from -22.31 to -19.19 ‰ and from 3.81 to 12.79 ‰, while C/N ratios ranged from 2.01 to 6.21 in muscle tissue of European anchovy, respectively. İğneada station had more depleted δ13C values and more enriched δ15N values than other stations. This difference might be due to the terrestrial input and agricultural activities in this region.

Unravelling climate change impacts from other anthropogenic influences in a subalpine lake: a multi-proxy sediment study from Oberer Soiernsee (Northern Alps, Germany)

Mountain lakes are increasingly impacted by a series of both local and global disturbances. The present study reveals the eutrophication history of a remote subalpine lake (Oberer Soiernsee, Northern Alps, Germany), triggered by deforestation, alpine pasturing, hut construction, tourism and atmospheric deposition, and identifies the intertwined consequences of on-going global warming on the lake’s ecosystem. The primary objective was to disentangle the various direct and indirect impacts of these multiple stressors via down-core analyses. Our multi-proxy approach included subfossil diatom assemblages, carbon and nitrogen stable isotope ratios and subfossil pigments from dated sediments. Shifts within the diatom assemblages were related to variations in trophic state, lake transparency, water temperature and thermal stratification. The organic carbon isotope (δ13Corg) records, the diatom valve density and the pigment concentrations documented the development of primary production and composition. Total nitrogen isotope values (δ15N) are more likely to reflect the history of atmospheric nitrogen pollution than lake-internal processes, also mirrored by the decoupling of δ15N and δ13Corg trends. The composition of sedimentary pigments allowed a differentiation between planktonic and benthic primary production. Concordant trends of all indicators suggested that the lake ecosystem passed a climatic threshold promoted by local and long-distance atmospheric nutrient loadings.

Stable Isotopes Reveal the Dominant Species to Have the Widest Trophic Niche of Three Syntopic Microtus Voles

Simple SummaryDiets and the trophic positions of animals are fundamental issues in their ecology. We analysed the isotopic niches (as a proxy for trophic niches) of common (Microtus arvalis), field (M. agrestis), and root (M. oeconomus) voles co-occurring in orchards, berry plantations, and nearby meadows using isotopic (δ15N and δ13C) compositions from hair samples. We tested if the niche of the dominant common vole was widest, whether its width was related to the presence of other Microtus species, and whether there were intraspecific differences in average δ13C and δ15N stable isotope values. The obtained results showed relative stability in the trophic niche across the vegetative period. The isotopic niche of the common vole was the widest, exceeding the other two Microtus species by 1.6–3 times. Co-occurring vole species were separated according to δ13C (i.e., used different plants as main food), but they maintained similarity according to δ15N distribution. The effect of animal age and gender on the width of the trophic niche was strongest in root vole, which is a species that has spread across the country in the last 70 years. These results give new insights into the trophic ecology small herbivores, showing the impact of species co-occurrence.Diets and trophic positions of co-occurring animals are fundamental issues in their ecology, and these issues in syntopic rodents have been studied insufficiently. Using carbon (δ13C) and nitrogen (δ15N) stable isotope ratios from hair samples, we analysed the trophic niches of common (Microtus arvalis), field (M. agrestis), and root (M. oeconomus) voles co-occurring in orchards, berry plantations, and nearby meadows (as control habitat to orchards and plantations). We tested if the niche of the dominant common vole was the widest, whether its width depended on the presence of other vole species, and whether there were intraspecific differences. Results suggest stability in the trophic niches of all three Microtus species, as season explained only 2% of the variance. The widest trophic niche was a characteristic of the dominant common vole, the range of δ13C values exceeding the other two species by 1.6, the range of δ15N values exceeding the other two species by 1.9, and the total area of niche exceeding that of the other voles by 2.3–3 times. In the meadows and apple orchards, co-occurring vole species were separated according to δ13C (highest values in the dominant common vole), but they maintained similar δ15N values. Results give new insights into the trophic ecology small herbivores, showing the impact of species co-occurrence.

Soil carbon and nitrogen under different land-use and landscape loca-tions in central Brazil

In Central Brazil, plateaus, the most common geomorphologic form, have been undergoing intense conversion from native vegetation to pasture and agriculture in recent decades. We used carbon stable isotope ratios (δ13C) and nitrogen stable isotope ratios (δ15N) to assess possible changes in soil organic matter dynamics under such land use modifications. This study aimed to evaluate the differences in soil δ13C and δ15N and C and N stocks between native vegetation and agricultural or pasture areas in different locations of a plateau in the savannas of Central Brazil. We sampled soil up to 100 cm depth in pasture areas in the summit of the plateau and no-tillage and conventional tillage on the border of a plateau, as well as soils under native vegetation in both landscape locations. Both soil δ13C and δ15N, and C and N stocks showed no differences between land uses. The different relationships between δ15N and C/N ratio at different locations indicated distinct behavior of the soil organic matter between the summit and border of the plateau. Therefore, in addition to land-use, landscape location contributes to both δ13C and δ15N, and C and N stocks in the soil of the plateau.

Legacy and emerging contaminants in common guillemot Uria aalge eggs in Ireland and Wales

Guillemot eggs from multiple Irish colonies and one Welsh colony were analysed for legacy pollutants such as polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs) and other organochlorine compounds (OCs), as well as metals. Stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) were measured in eggs to understand the influence of diet on contaminant levels detected. Wide-scope target and suspect screening techniques were used on a single guillemot egg, providing novel information on contaminants of emerging concern. Stable isotope ratio analysis showed that guillemots from Great Saltee Island and Lambay Island (Ireland's east coast) had a similar carbon source (δ13C) and fed at similar trophic levels (δ15N), pollutant levels were higher in eggs from Lambay Island near Dublin, Ireland's industrialised capital city. Guillemot eggs from Aughris Head (Atlantic west coast of Ireland), and Skomer Island (Wales) had differing isotopic niches to other colonies. Egg samples from Aughris Head had the lowest levels of pollutants in this study (with the exception of mercury) and amongst the lowest levels reported worldwide. In contrast, Skomer Island had the highest level of pollutants with higher concentrations of Σ16PCB, Σ6PBDE and HCB than Irish colonies, most likely a result of its proximity to historically industrial areas. Levels of PCBs, p,p’ –DDE and mercury in guillemot eggs have decreased over time according to this study, in concurrence with worldwide trends. Levels of pollutants in guillemot eggs, in this study, fall below existing thresholds for adverse effects in other species, with the exception of mercury.

Stable isotope ratio characteristics and origin tracing of Portunus trituberculatus

A method for geographical discrimination of Portunus trituberculatus was explored to provide technical support for the protection of geographical indication products and for tracing the origin of seafood. P. trituberculatus were collected from three major production areas, including the Yellow Sea, the Bohai Sea, and the East China Sea. The variations of carbon and nitrogen stable isotope values of origins and the correlation of stable isotope ratios in different tissues were analyzed. The results showed that there were significant differences in carbon and nitrogen stable isotope ratio among different origins. Significant isotope fractionation effects were observed among different tissues. The discriminant model was developed and the origin discriminant analysis was performed by the stable isotope ratios of different tissues in P. trituberculatus. The correct rate of origin diffe-rentiationf using carbon and nitrogen stable isotopes in muscle and gills (>95%) was significantly higher than that of hepatopancreas and gonad, indicating that stable isotope ratios of muscle and gills could effectively differentiate P. trituberculatus in different sea areas. This study filled the gap of stable isotope tracing technology for P. trituberculatus.

Trophic niche partitioning of four pelagic shark species in the tropical Atlantic based on multi-tissue stable isotopes ratios

Stable carbon and nitrogen isotope ratios (δ13C and δ15N, respectively) of multiple tissues with different turnover rates can provide trophic information at different timescales, and thus play an important role in tracing the changes of feeding, habitat utilization and trophic niche of consumers. The δ13C and δ15N contents of muscle, liver and blood of blue sharks (Prionace glauca), longfin mako sharks (Isurus paucus), crocodile sharks (Pseudocarcharias kamoharai) and shortfin mako sharks (I. oxyrinchus) from tropical Atlantic were measured, and the trophic niche was evalua-ted. The results showed that I. oxyrinchus, P. kamoharai, and P. glauca had similar δ15N values, higher than that of I. paucus. Feeding segregation was found between P. glauca and other species, showing unique trophic niche. The largest trophic niche width was observed in I. oxyrinchus, indicating the high diversity of prey and (or) feeding habitats. High trophic niche overlap was found in P. kamoharai and I. oxyrinchus, implying their potential competition for resources. There was no correlation between the differences among tissues in the δ13C or δ15N values and the body size of I. oxyrinchus, P. kamoharai and P. glauca, indicating no recent diet shifts for those species. I. paucus showed significant correlation between δ15N differences in liver, blood and muscle with the shark fork length, indicating its short-term diet shift. The similarity of δ13C and δ15N values and the higher metabolic rates of liver and blood were found in all four shark species, implying the similar incorporation rates of both tissues, which were considered as the short-term indicator of diet.

Geographical differences in the stable isotope ratios of Pacific saury in the North Pacific Ocean

We collected samples of Pacific saury Cololabis saira (249–331 mm in knob length; ages 0 and 1) in the North Pacific Ocean from 154°E to 165°W during their northward migration in the early summers of 2013, 2014, and 2015, and measured the stable nitrogen and carbon isotope ratios (δ15N and δ13C) of their muscle tissues. A hierarchical cluster analysis based on δ15N and δ13C yielded three groups (G1–G3). G1 (mean δ15N: 12.9%; mean δ13C: –20.3%) had the highest δ15N and occurred only in the eastern area of 170° W in 2013 and 2015. G2 (δ15N: 9.3%; δ13C: −20.1%) and G3 (δ15N: 7.3%, δ13C: −20.9%) occurred in all years mainly in eastern and western areas of 170° W, respectively. The latter two groups presented reasonable δ15N and δ13C considering the trophic enrichment and potential prey such as Neocalanus copepods that exist in each area. In contrast, the substantially enriched δ15N of G1 was characteristic of organisms at higher trophic levels in the same area. Thus, this group is most likely an immigrant from outside the survey area. This study showed that Pacific saury utilize different ecosystems east and west of 170° W during their northward migration.

Mercury in Barents Sea fish in the Arctic polar night: Species and spatial comparison

Although mercury (Hg) in polar ecosystems has been well-studied, there is little information on Hg in the Arctic during low-productivity seasons like the polar night. We quantified Hg concentrations, carbon, and nitrogen stable isotope ratios (δ13C and δ15N) in the muscle of polar cod (Boreogadus saida), Atlantic cod (Gadus morhua), and capelin (Mallotus villosus) sampled from the North-West and North-East Barents Sea during November–December 2019. Hg concentrations varied between species (14–175 ng/g dw), dependent on region, but were well below the toxicity threshold for fish health and the EU-accepted threshold for human consumption. Interspecific differences were observed only in the North-East region, with Atlantic cod having highest Hg concentrations, explained by its larger size, higher trophic position and benthopelagic feeding. Spatial differences in polar cod with higher Hg concentrations in the North-East than the North-West were likely due to a combination of differences in food web structure and Hg exposure.

Lateral and longitudinal flight dispersals of a stonefly, Alloperla ishikariana (Plecoptera, Chloroperlidae), from the hyporheic zone in a gravel-bed river in Japan

The life-history traits of amphibitic insects are not well understood. These insects inhabit saturated interstitial areas below the riverbed (hyporheic zone) at the larval stage, mate in terrestrial habitats, and return to rivers for oviposition, but there is no knowledge concerning their dispersal characteristics. We sought to address this by examining how far amphibitic insects disperse away from the channel (laterally) and along upstream or downstream (longitudinally) in a gravel-bed river. Alloperla ishikariana was selected as the focal species because it numerically dominates other amphibites in an 18-km study segment of a 4th-order gravel-bed river in Hokkaido, Northern Japan. Malaise traps were set at various distances from the channel towards the riparian forest to estimate lateral dispersal distances. An elevated stable nitrogen isotope ratio in downstream larvae, caused by the influence of effluent from a wastewater treatment plant, was used to assess longitudinal dispersal by identifying and tracking adult movements. Laterally, 50th and 90th percentile dispersal distances were 11.66 and 35.09 m for female A. ishikariana and 20.59 and 59.20 m for male, respectively; this overlapped with distances previously estimated for other aquatic benthic taxa. Longitudinally, 50th and 90th percentile dispersal distances were 0.74 and 1.43 km for female and 3.11 and 7.87 km for male, respectively. Alloperla ishikariana had one of the longest upstream traveling distances compared with other aquatic insects, and the longest among Plecoptera taxa known thus far where male exhibited a greater dispersal distance. A higher number of adults demonstrated upstream movement, suggesting an upstream bias in the longitudinal dispersal of A. ishikariana. Overall, amphibitic stoneflies did not exhibit distinct dispersal characteristics compared with the results of previous reports on presumably benthic taxa. Our findings support an improved visualization of a multi-dimensionally connected river ecosystem in terms of material flow, including vertical connectivity.

Nutritional contribution of seaweed Ulva lactuca single-cell detritus and microalgae Chaetoceros calcitrans to the growth of the Pacific oyster Crassostrea gigas

The Pacific oyster Crassostrea gigas (Thunberg, 1793) is the most cultivated bivalve in the world. Nonetheless, the massive production of microalgae as feed represents a substantial cost during laboratory production stages. The use of single cell detritus (SCD) from seaweed Ulva lactuca, cultivated in fish farm effluents, has been proposed as an alternative to microalgae Chaetoceros calcitrans in oyster culture, with the aim of reducing microalgae production costs. Seaweed meal was subjected to an acid and enzymatic digestion process to obtain SCD particles smaller than 20 μm to feed the oysters. Five levels of SCD (w:w) replacing microalgae were evaluated: 0, 25, 50, 75, and 100% in a feeding assay of 5 weeks. At the end of the experiment, growth parameters, condition index, enzymatic activity in the digestive gland (amylase, protease, lipase, and aminopeptidase) were analysed. In addition, during the course of the experiment, the stable isotope ratio of nitrogen (δ15N) was analysed at natural abundance levels in both feed sources and in the mantle tissue of oysters reared on different feeding regimes. Contribution to growth was estimated using an isotope mixing model. Better growth (74 to 94% dry weight gain) and condition indexes (81–75) were observed in oysters fed on experimental regimes having from 0 to 50% substitution of microalgae with SCD, showing no significant differences among them. Oysters under the latter treatments also showed similar enzymatic activity for amylase, alkaline-protease, and lipase. At higher substitution levels of microalgae (75–100%), oysters presented lower growth (13 to 34% dry weight gain) and poor condition indexes (<60); 100% of SCD also elicited higher amylase, alkaline-protease, and lipase activities, whereas (Leu) aminopeptidase N activity was lower. The digestive capacity of lipase was improved in oysters fed with 50 and 75% levels of SCD. Isotopic equilibrium for nitrogen (δ15N) was reached by day 14 in the 50% substitution treatment. Metabolic turnover rates of nitrogen decreased (0.15 to 0.08 day−1) whereas elemental half times in tissue increased (4.1 to 8.2 days) with higher microalgae substitution. Oysters under treatment with 50% microalgae substituted by SCD incorporated similar amounts of dietary nitrogen and dry matter from the microalgal biomass as from SCD to meet the nitrogen requirement for oyster growth. In conclusion, results suggest that SCD from U. lactuca can substitute up to 50% of microalgae C. calcitrans without modifying C. gigas productivity.

Stable Isotope Ratios of Herbs and Spices Commonly Used as Herbal Infusions in the Italian Market.

Stable isotope ratio analysis has been widely used for traceability and authenticity purposes in relation to various food commodities, but only in a limited number of herb and spice species. This study explored the stable isotope ratios of carbon, nitrogen, sulfur, oxygen, and hydrogen (δ13C, δ15N, δ34S, δ18O, and δ2H) of 119 herbs and spices belonging to 116 plant species and 57 plant families collected from the Italian market for the first time. The characteristic value ranges of δ13C, δ15N, δ34S, δ18O, and δ2H of the herbs and spices went from −31.0 to −11.6, −4.7 to 12.0, −5.0 to 22.0, 14.7 to 46.0, and −158 to −12‰, respectively. The isotopic profiles within and between common botanical families and their similarity/dissimilarity between herbs and spices belonging to the common botanical families are also discussed here. The results of this exploratory work highlight the possibility of characterizing herbs and spices and suggest widening the scope of the survey through more extensive sampling and focusing on specific plant species.

Fish communities critically depend on forest subsidies in small neotropical streams with high biodiversity value

AbstractTrophic support of species‐rich tropical fish communities remains uncertain in small forest streams where dense and constant canopy cover strongly limits the solar radiation available to aquatic producers. We sampled &gt;1300 fish from 80 species in 14 remote headwater streams during the dry season in French Guiana. Carbon and nitrogen stable isotope analyses and mixing models were run to estimate the trophic position (TP) of each species and its dependence on terrestrial organic carbon sources. Matching patterns were found between nitrogen stable isotope ratios and the literature on diets of species at the base and top of the food web. Nearly 60% of the species fed exclusively on animal prey (mean TP &gt;3), including piscivorous and parasitic fish species. Phylogenetic analysis revealed that TP were distributed broadly along the phylogeny and changed gradually within lineages. Carbon stable isotope ratios differed strongly between herbivores that fed on aquatic primary producers and those that fed on terrestrial plants along banksides. Terrestrial carbon was dominant in nearly 90% of species, and most of them depended more or less exclusively on terrestrial carbon. Ancestral state reconstructions indicated that only a few lineages showed local changes toward the consumption of algal carbon. We conclude that fish in neotropical headwater streams shaded by a dense tree canopy depend strongly on terrestrial subsidies. Changes in this aquatic‐terrestrial linkage would have major consequences for the remarkable fish biodiversity in these streams.Abstract in French is available with online material.

Natural abundance of 13C and 15N provides evidence for plant–soil carbon and nitrogen dynamics in a N‐fertilized meadow

Natural abundance of carbon (C) and nitrogen (N) stable isotope ratios (δ13 C and δ15 N) has been used to indicate ecosystem C and N status and cycling; however, use of this approach to infer plant and microbial N preference under projected ecosystem N enrichment is limited. Here, we investigated natural abundance δ13 C and δ15 N of five dominant plant species, and soil δ15 N of microbial biomass and available N forms under N addition in a meadow steppe. Additional N, applied as urea, led to decreases in δ15 N of soil NO3 - (δ15 Nnitrate , from 3.0 to 0.4‰) and increases in δ15 N of soil NH4 + (δ15 Nammonium , from -1.3 to 11‰) and dissolved organic N (δ15 NDON , from 8.5 to 15‰) that reflected increased net nitrification rates, a possible increase in NH3 volatilization, and greater availability of the three N forms. An overall increase in δ15 N of soil total N (δ15 NTN ) from 7.1 to 7.9‰ indicated accelerated and greater openness of soil N cycling that was also partially revealed by enhanced net N mineralization rates. Plant δ15 N, which ranged from -1.8 to 2.1‰, generally decreased with N addition, indicating a greater reliance on soil NO3 - under N-enrichment conditions. Nitrogen addition decreased δ15 N of microbial biomass N (from 14 to 2.8‰), possibly due to a shift in preferential N form (DON to NO3 - ), that indicated a convergence of plant and microbial preferential N forms and an increase in plant-microbial N competition. Microbes were thus more flexible than plants in the use of different forms of N. Addition of N decreased plant litter δ13 C, while plant species δ13 C remained unaffected, likely due to a shift in the abundance of dominant species with a greater proportion of biomass coming from δ13 C-depleted species. Enrichment factor (the difference in plant δ15 N relative to δ15 NTN ) of four non-legume species was negatively related to soil inorganic N availability, net nitrification rate, and net N mineralization rate, and was proven to be a good indicator of ecosystem N status. Our study highlights the importance of natural abundance of 15 N as an indicator of plant-microbial N competition and ecosystem N cycling in meadow steppe grasslands under projected ecosystem N enrichment.

Assessment of restoration success in a transplanted seagrass bed based on isotopic niche metrics

A major highlight of restoration efforts is to improve the ecological structure and function of the natural ecosystem in the restored habitat. Assessment of restoration success is a crucial component of an optimal ecological management strategy. In studies to determine the restoration success of a transplanted seagrass habitat by assessing trophic recovery, we examined carbon and nitrogen stable isotope ratios of organic matter sources and macrobenthic assemblages in a transplanted eelgrass Zostera marina bed. The eelgrass bed was restored about 2 years after transplantation in a southern coastal bay of Korea, and consequently, the food web structure in the bed was compared with that in a natural reference site. Our results revealed no significant differences in isotopic values of both macrobenthic consumers and their putative food sources between the transplanted and natural seagrass beds. These isotopic similarities in florae and faunae in the two beds suggest a uniformity in food web structure formed by the diversity and availability of resources, and thereby suggest similarities in the resource–consumer relationship. Isotopic niche indices and high dietary overlaps of feeding guilds in the transplanted and natural beds further suggest the transplanted habitat provides similar ecological functions and ecosystem services to its natural counterpart. Collectively, our results suggest the eelgrass transplantation led to successful restoration of a common seagrass bed, with recovery of the functional properties of the food web structure. Finally, our findings support the idea that stable isotope measures can provide a better understanding of the functioning of restored ecosystems, and improve post-transplantation monitoring efforts for the future planning and managing of successful habitat restoration.

Spatial and Seasonal Variations of Stable Isotope Ratios of Particulate Organic Carbon and Nitrogen in the Surface Water of the Kuroshio

AbstractSeasonal stable isotope ratios of carbon and nitrogen (δ13C and δ15N) in the particulate organic matter (POM) of the euphotic layer were investigated around the Kuroshio from 2008 to 2019 (n = 474). Significant seasonality in POM δ13C and δ15N were observed in the coastal and offshore areas divided by the northern edge of the Kuroshio. Seasonal mean δ13C was highest during the summer (−23.2 ± 1.1‰ and −23.7 ± 1.1‰ in coastal and offshore areas, respectively) and lowest during the winter (−24.6 ± 0.8‰ and −25.0 ± 0.9‰ in coastal and offshore areas, respectively) in both areas. Seasonal mean δ15N exhibited different spatial variations. The mean δ15N value in the coastal area was the lowest during the winter (1.0 ± 1.9‰) and increased to a similar level (∼3‰) during the other three seasons. In contrast, δ15N in the offshore area near the surface decreased from the spring (2.5 ± 1.5‰) to the summer (1.2 ± 1.7‰). These spatial and seasonal differences in δ15N are significant in generalized linear models and the generalized additive models, and suggest that the nitrogen sources used in biological production differ between coastal and offshore areas and between seasons. Nitrate originating in deeper water and rivers is the main sources used for new production throughout the year in the coastal area. Contributions from atmospheric deposition and nitrogen fixation are significant in the offshore area during the summer, while nitrate originating in deeper water is main during winter, spring, and fall in the offshore area.