Evaluation of commercial and natural food in experimental cultures of white shrimp based on stable carbon isotopes

The farming of shrimp is developing quickly worldwide, and recently, ingredients such as seaweeds in low proportion (25 to 4 %), incorporated into the commercial food, have been shown to improve the shrimp productive variables. The change of commercial foods to commercial feed with a proportion of natural food, and finally, to natural food has been little and simultaneously evaluated. The aim of our study was to determine the relative contribution of dietary carbon to the growth of Litopenaeus vannamei fed with a proportion of 4 % Sargassum (δ¹³C = ?20.9?±?0.05?‰), 4 % Ulva (δ¹³C = ?20.6?±?0.6?‰) meal, and a control diet (δ¹³C?=??22.6?±?0.2?‰) in 60-L tanks for 30 days, and finally, with the green seaweed Ulva spp. (δ¹³C = ?13.2?±?0.25?‰) and Ulva meal (δ¹³C = ?14.5?±?0.6?‰) in open-air ponds for 120 days, by measuring δ¹³C for each of the foods and in the muscle of shrimp. After 15 days, the rates of metabolic turnover (Δ¹³C = δ¹³C_shrimp ? δ¹³C_food) were constant until the end of the experiment in the tanks. The muscle of shrimp assimilated carbon from all diets, which demonstrated the potential use of combined diets and the optimization of their use in diets containing seaweed. Our data will be useful in future interpretations of field and laboratory isotopic values for this species.     

Kangaroo tooth enamel oxygen and carbon isotope variation on a latitudinal transect in southern Australia: implications for palaeoenvironmental reconstruction

Tooth enamel apatite carbonate carbon and oxygen isotope ratios of modern kangaroos (Macropus spp.) collected on a 900-km latitudinal transect spanning a C₃–C₄ transition zone were analysed to create a reference set for palaeoenvironmental reconstruction in southern Australia. The carbon isotope composition of enamel carbonate reflects the proportional intake of C₃ and C₄ vegetation, and its oxygen isotope composition reflects that of ingested water. Tooth enamel forms incrementally, recording dietary and environmental changes during mineralisation. Analyses show only weak correlations between climate records and latitudinal changes in δ¹³C and δ¹⁸O. No species achieved the δ¹³C values (~?1.0 ‰) expected for 100 % C₄ grazing diets; kangaroos at low latitudes that are classified as feeding primarily on C₄ grasses (grazers) have δ¹³C of up to ?3.5 ‰. In these areas, δ¹³C below ?12 ‰ suggests a 100 % C₃ grass and/or leafy plant (browse) diet while animals from higher latitude have lower δ¹³C. Animals from semi-arid areas have δ¹⁸O of 34–40 ‰, while grazers from temperate areas have lower values (~28–30 ‰). Three patterns with implications for palaeoenvironmental reconstruction emerge: (1) all species in semi-arid areas regularly browse to supplement limited grass resources; (2) all species within an environmental zone have similar carbon and oxygen isotope compositions, meaning data from different kangaroo species can be pooled for palaeoenvironmental investigations; (3) relatively small regional environmental differences can be distinguished when δ¹³C and δ¹⁸O data are used together. These data demonstrate that diet–isotope and climate–isotope relationships should be evaluated in modern ecosystems before application to the regional fossil record.     

δ 13C variation of C3 and C4 plants across an Asian monsoon rainfall gradient in arid northwestern China

We have investigated carbon isotopic compositions of four plant genus/species, Bothriochloa ischaemum (C₄), Stipa bungeana (C₃), Lespedeza sp. (C₃) and Heteropappus less (C₃), along a precipitation gradient in northwest China in order to assess the impact of water availability on the carbon isotopic discrimination against ¹³C during carbon assimilation in this area. This information is necessary for reconstruction of paleovegetation, particularly paleo‐C₃/C₄ plant ratios using δ¹³C value of organic matter in loess and paleosols in the Chinese Loess Plateau. The δ¹³C of C₃ plants, as a group, exhibits a negative correlation with the annual precipitation amount with a total change and sensitivity of 5‰ and ?1.1‰/100 mm, respectively, for the precipitation range from 200 to 700 mm. The C₄ grass, B. ischaemum responds to aridity by decreasing 1.7‰ for over the precipitation range from 350 to 700 mm; the plant δ¹³C is significantly correlated with annual precipitation with a slope ?0.61‰/100 mm. This result implies that without considering the effect of water availability on the plant δ¹³C values, reconstruction of percent C₄ vegetation during the last glaciation can be overestimated by about a factor of two.     

Effects of charring on the carbon isotopic composition of grass (Poaceae) epidermis

Charred modern grass epidermis preserves the carbon isotopic composition of the parent plant photosynthetic pathway. Fifty-nine modern grasses and sedges were collected in lowland western Uganda. All charred epidermal samples from C₄ grasses or sedges preserve a carbon isotopic value within the range typical for C₄ plants (−17 to −10‰), and charred epidermal fragments from C₃ plants have carbon isotopic values between −30 and −26‰. The process of charring results in a slightly enriched carbon isotopic signature (−11.9‰ mean charred value as compared to −12.8‰ mean unaltered grass tissue value). δ¹³C measurements of replicate samples from the same plant vary within 1–2‰, yet all values for the same plant stay within the expected values for the photosynthetic pathway of the plant. δ¹³C measurements on >180-μm charred grass epidermal fragments extracted from surface sediment samples from three lakes on the lowland western Ugandan landscape confirm the predominant lowland C₄ grass input (δ¹³C=−16 to −19‰). These results demonstrate the utility of using carbon isotopic analysis of charred grass epidermis to reconstruct C₃ vs. C₄ grassland assemblages on the landscape. Furthermore, such downcore δ¹³C profiles can be used to highlight key zones of C₃ vs. C₄ grass change for which taxonomic analysis of fossil grass epidermis could provide more detailed information regarding grassland community composition.     

Associations between carbon isotope ratios of ecosystem respiration, water availability and canopy conductance

We tested the hypothesis that the stable carbon isotope signature of ecosystem respiration (δ¹³C_R) was regulated by canopy conductance (G_c) using weekly Keeling plots (n=51) from a semiarid old‐growth ponderosa pine (Pinus ponderosa) forest in Oregon, USA. For a comparison of forests in two contrasting climates we also evaluated trends in δ¹³C_R from a wet 20‐year‐old Douglas‐fir (Pseudotsuga menziesii) plantation located near the Pacific Ocean. Intraannual variability in δ¹³C_R was greater than 8.0‰ at both sites, was highest during autumn, winter, and spring when rainfall was abundant, and lowest during summer drought. The δ¹³C_R of the dry pine forest was consistently more positive than the wetter Douglas‐fir forest (mean annual δ¹³C_R: ?25.41‰ vs. ?26.23‰, respectively, P=0.07). At the Douglas‐fir forest, δ¹³C_R–climate relationships were consistent with predictions based on stomatal regulation of carbon isotope discrimination (Δ). Soil water content (SWC) and vapor pressure deficit (vpd) were the most important factors governing δ¹³C_R in this forest throughout the year. In contrast, δ¹³C_R at the pine forest was relatively insensitive to SWC or vpd, and exhibited a smaller drought‐related enrichment (～2‰) than the enrichment observed during drought at the Douglas‐fir forest (～5‰). Groundwater access at the pine forest may buffer canopy–gas exchange from drought. Despite this potential buffering, δ¹³C_R at the pine forest was significantly but weakly related to canopy conductance (G_c), suggesting that δ¹³C_R remains coupled to canopy–gas exchange despite groundwater access. During drought, δ¹³C_R was strongly correlated with soil temperature at both forests. The hypothesis that canopy‐level physiology is a critical regulator of δ¹³C_R was supported; however, belowground respiration may become more important during rain‐free periods.     

Trophic level delineation and resource partitioning in a South African afromontane forest bird community using carbon and nitrogen stable isotopes

Southern African forests are naturally fragmented yet hold a disproportionately high number of bird species. Carbon and nitrogen stable isotopes were measured in feathers from birds captured at Woodbush (n = 27 species), a large afromontane forest in the eastern escarpment of Limpopo province, South Africa. The δ¹³C signatures of a range of forest plants were measured to categorise the food base. Most plants sampled, including two of five grass species, had δ¹³C signatures typical of a C₃ photosynthetic pathway (?29.5 ± 1.9‰). Three grass species had a C₄ signature (?12.0 ± 0.6‰). Most bird species had δ¹³C values representing a predominantly C₃‐based diet (?24.8‰ to ?20.7‰). δ¹⁵N values were as expected, with higher levels of enrichment associated with a greater proportion of dietary animal matter. The cohesive isotopic niche defining most species (n = 22), where the ranges for δ¹³C and δ¹⁵N were 2.4‰ and 3.4‰, respectively, highlight the difficulties in understanding diets of birds in a predominantly C₃‐based ecosystem using carbon and nitrogen stable isotopes. However, variation in isotopic values between and within species provides insight into possible niche width and the use of resources by different birds within a forest environment.     

Australian mulga ecosystems –13C and 15N natural abundances of biota components and their ecophysiological significance

Samples of recently produced shoot material collected in winter/spring from common plant species of mulga vegetation in eastern and Western Australia were assayed for ¹³C and ¹⁵N natural abundance. ¹³C analyses showed only three of the 88 test species to exhibit C₄ metabolism and only one of seven succulent species to be in CAM mode. Non-succulent winter ephemeral C₃ species showed significantly lower mean δ¹³C values (– 28·0‰) than corresponding C₃-type herbaceous perennials, woody shrubs or trees (– 26·9, – 25·7 and – 26·2‰, respectively), suggesting lower water stress and poorer water use efficiency in carbon acquisition by the former than latter groups of taxa. Corresponding values for δ¹⁵N of the above growth and life forms lay within the range 7·5–15·5‰. δ¹⁵N of soil NH₄⁺ (mean 19·6‰) at a soft mulga site in Western Australia was considerably higher than that of NO₃^– (4·3‰). Shoot dry matter of Acacia spp. exhibited mean δ¹⁵N values (9·10 ± 0·6‰) identical to those of 37 companion non-N₂-fixing woody shrubs and trees (9·06 ± 0·5‰). These data, with no evidence of nodulation, suggested little or no input of fixed N₂ by the legumes in question. However, two acacias and two papilionoid legumes from a dune of wind-blown, heavily leached sand bordering a lake in mulga in Western Australia recorded δ¹⁵N values in the range 2·0–3·0‰ versus 6·4–10·7‰ for associated non-N₂-fixing taxa. These differences in δ¹⁵N, and prolific nodulation of the legumes, indicated symbiotic inputs of fixed N in this unusual situation. δ¹⁵N signals of lichens, termites, ants and grasshoppers from mulga of Western Australia provided evidence of N₂ fixation in certain termite colonies and by a cyanobacteria-containing species of lichen. Data are discussed in relation to earlier evidence of nitrophily and water availability constraints on nitrate utilization by mulga vegetation.     

Trophic fractionation of carbon and nitrogen stable isotopes in Chironomus riparius reared on food of aquatic and terrestrial origin

1. Trophic fractionation was studied in short‐term laboratory feeding experiments with larvae of the deposit‐feeding midge Chironomus riparius. Larvae were fed food of terrestrial (oats, peat) and aquatic origin (Spirulina, Tetraphyll^®). 2. By analysing both whole larvae and isolated gut contents we were able to distinguish between the isotopic signature of recently ingested food and that of assimilated carbon and nitrogen in body tissue. Additionally we studied the effects of microbial conditioning, i.e. the colonisation and growth on food particles of microbes, on the isotopic signal of food resources. 3. Nitrogen fractionation for the different food types ranged from 0.67‰ to 2.68‰ between consumer and diet and showed that isotopic fractionation can be much lower than the value of 3.4‰ that is commonly assumed. 4. Microbial degradation of food particles resulted in an approximate doubling of the δ¹⁵N in 8 days, from 6.24 ± 0.05‰ to 11.36 ± 0.56‰. Values for δ¹³C increased only marginally, from ?20.66 ± 0.11‰ to ?20.34 ± 0.12‰. These results show that microbial conditioning of food may affect dietary isotope signatures (in particular N) and, unless accounted for, could introduce an error in measures of trophic fractionation. Microbial conditioning could well account for some of the variation in fractionation reported in the literature.     

Carbon and nitrogen isotope fractionation of amino acids in an avian marine predator,the gentoo penguin (Pygoscelis papua)

Compound‐specific stable isotope analysis (CSIA) of amino acids (AA) has rapidly become a powerful tool in studies of food web architecture, resource use, and biogeochemical cycling. However, applications to avian ecology have been limited because no controlled studies have examined the patterns in AA isotope fractionation in birds. We conducted a controlled CSIA feeding experiment on an avian species, the gentoo penguin (Pygoscelis papua), to examine patterns in individual AA carbon and nitrogen stable isotope fractionation between diet (D) and consumer (C) (Δ¹³C_C‐D and Δ¹⁵N_C‐D, respectively). We found that essential AA δ¹³C values and source AA δ¹⁵N values in feathers showed minimal trophic fractionation between diet and consumer, providing independent but complimentary archival proxies for primary producers and nitrogen sources respectively, at the base of food webs supporting penguins. Variations in nonessential AA Δ¹³C_C‐D values reflected differences in macromolecule sources used for biosynthesis (e.g., protein vs. lipids) and provided a metric to assess resource utilization. The avian‐specific nitrogen trophic discrimination factor (TDF_Glu‐Phe = 3.5 ± 0.4‰) that we calculated from the difference in trophic fractionation (Δ¹⁵N_C‐D) of glutamic acid and phenylalanine was significantly lower than the conventional literature value of 7.6‰. Trophic positions of five species of wild penguins calculated using a multi‐TDF_Glu‐Phe equation with the avian‐specific TDF_Glu‐Phe value from our experiment provided estimates that were more ecologically realistic than estimates using a single TDF_Glu‐Phe of 7.6‰ from the previous literature. Our results provide a quantitative, mechanistic framework for the use of CSIA in nonlethal, archival feathers to study the movement and foraging ecology of avian consumers.     

Stable Isotope Evidence of Puma concolor (Felidae) Feeding Patterns in Agricultural Landscapes in Southeastern Brazil

We evaluated puma (Puma concolor) feeding patterns in southeastern Brazilian agricultural landscapes using carbon and nitrogen stable isotope analyses of hair collected from fecal samples (N = 64). We classified the samples into three groups: feeding patterns based on forest remnants, on the agricultural matrix or both. We observed a predominance of consumption of C₃ prey (~47% of individuals) in the area with the highest proportion of forest coverage. Conversely, C₄ prey were highly consumed (~40% of individuals) where the agricultural matrix was predominant. The δ¹³C values for pumas in both areas indicated that their food resources come from both forest remnants and the agricultural matrix and that some individuals preferentially consumed C₄ prey, indicating that food resources from the agricultural matrix make up most of their diet (~46% of prey individuals). The wide range of puma's δ¹⁵N values in both areas indicated a diet based on different types of prey. However, the C₄ group had higher values, indicating that both pumas and their prey feed on enriched resources from the agricultural matrix. The results confirm the high behavioral plasticity of pumas in using highly anthropogenic habitats. The stable isotope analyses conducted in this study yielded new information on large carnivore trophic ecology that might be useful in the development of new conservation strategies in disturbed areas.     

Lower–middle Frasnian organic carbon isotope record of conodonts in East European Platform

The early–middle Frasnian boundary interval of the northern part of the East European Platform (northwest of Russia) corresponding to the transitans-punctata isotope event is revealed by biostratigraphically constrained conodont carbon stable isotope data (δ¹³C_con). The dynamics of δ¹³C_con demonstrate a three-fold pattern with positive peaks at the onset of the main phase of the transitans-punctata isotope event (upper part of Montagne Noire 4 conodont Zone, MN4; up to -22.5‰ VPDB), and during the late part of the event (lower and middle parts of MN6 Zone; up to -24.0‰ VPDB and -22.1‰ VPDB). The stratigraphic level near the MN5 and MN6 boundary is marked by a prominent negative excursion in δ¹³C_con (down to -31.8‰ VPDB) that resembles the negative excursion in the terminal phase of the transitans-punctata isotope event worldwide. The δ¹³C_con variations in different taxa are generally consistent but demonstrate some differences in values and amplitudes. It is assumed that variations in the carbon isotopic compositions of conodonts were mainly controlled by changes in the isotope composition of the planktons as the main food source for conodonts.     

Environmental and physiological controls on the carbon isotope composition of CO2 respired by leaves and roots of a C3 woody legume (Prosopis velutina) and a C4 perennial grass (Sporobolus wrightii)

Accurate estimates of the δ¹³C value of CO₂ respired from roots (δ¹³C_{R_root}) and leaves (δ¹³C_{R_leaf}) are important for tracing and understanding changes in C fluxes at the ecosystem scale. Yet the mechanisms underlying temporal variation in these isotopic signals are not fully resolved. We measured δ¹³C_{R_leaf}, δ¹³C_{R_root}, and the δ¹³C values and concentrations of glucose and sucrose in leaves and roots in the C₄ grass Sporobolus wrightii and the C₃ tree Prosopis velutina in a savanna ecosystem in southeastern Arizona, USA. Night‐time variation in δ¹³C_{R_leaf} of up to 4.6 ± 0.6‰ in S. wrightii and 3.0 ± 0.6‰ in P. velutina were correlated with shifts in leaf sucrose concentration, but not with changes in δ¹³C values of these respiratory substrates. Strong positive correlations between δ¹³C_{R_root} and root glucose δ¹³C values in P. velutina suggest large diel changes in δ¹³C_{R_root} (were up to 3.9‰) influenced by short‐term changes in δ¹³C of leaf‐derived phloem C. No diel variation in δ¹³C_{R_root} was observed in S. wrightii. Our findings show that short‐term changes in δ¹³C_{R_leaf} and δ¹³C_{R_root} were both related to substrate isotope composition and concentration. Changes in substrate limitation or demand for biosynthesis may largely control short‐term variation in the δ¹³C of respired CO₂ in these species.     

The intramolecular ¹³C-distribution in ethanol reveals the influence of the CO₂ -fixation pathway and environmental conditions on the site-specific ¹³C variation in glucose

Efforts to understand the cause of ¹²C versus ¹³C isotope fractionation in plants during photosynthesis and post‐photosynthetic metabolism are frustrated by the lack of data on the intramolecular ¹³C‐distribution in metabolites and its variation with environmental conditions. We have exploited isotopic carbon‐13 nuclear magnetic resonance (¹³C NMR) spectrometry to measure the positional isotope composition (δ¹³C_i, ‰) in ethanol samples from different origins: European wines, liquors and sugars from C₃, C₄ and crassulacean acid metabolism (CAM) plants. In C₃‐ethanol samples, the methylene group was always ¹³C‐enriched (～2‰) relative to the methyl group. In wines, this pattern was correlated with both air temperature and δ¹⁸O of wine water, indicating that water vapour deficit may be a critical defining factor. Furthermore, in C₄‐ethanol, the reverse relationship was observed (methylene‐C relatively ¹³C‐depleted), supporting the concept that photorespiration is the key metabolic process leading to the ¹³C distribution in C₃‐ethanol. By contrast, in CAM‐ethanol, the isotopic pattern was similar to but stronger than C₃‐ethanol, with a relative ¹³C‐enrichment in the methylene‐C of up to 13‰. Plausible causes of this ¹³C‐pattern are briefly discussed. As the intramolecular δ¹³C_i‐values in ethanol reflect that in source glucose, our data point out the crucial impact on the ratio of metabolic pathways sustaining glucose synthesis.     

Trophic structure and major trophic links in conventional versus organic farming systems as indicated by carbon stable isotope ratios of fatty acids

Using bulk tissue and fatty acid ¹³C analysis we investigated major trophic pathways from soil microorganisms to microbial consumers to predators in conventional versus organic farming systems planted for the first time with maize. Organic farming led to an increase in microbial biomass in particular that of fungi as indicated by phospholipid fatty acids (PLFAs). Microbial PLFAs reflected the conversion from C₃ to C₄ plants by a shift in δ¹³C of 2‰, whereas the isotopic signal in fatty acids (FAs) of Collembola was much more pronounced. In the euedaphic Protaphorura fimata the δ¹³C values in maize fields exceeded that in C₃ (soybean) fields by up to 10‰, indicating a close relationship between diet and vegetation cover. In the epedaphic Orchesella villosaδ¹³C values shifted by 4‰, suggesting a wider food spectrum including carbon of former C₃ crop residues. Differences in δ¹³C of corresponding FAs in consumers and resources were assessed to assign food web links. P. fimata was suggested as root and fungal feeder in soybean fields, fungal feeder in conventional and leaf consumer in organically managed maize fields. O. villosa likely fed on root and bacteria under soybean, and bacteria and fungi under maize. Comparison of δ¹³C values in FAs of the cursorial spider Pardosaagrestis and O. villosa implied the latter as important prey species in soybean fields. In contrast, the web‐building spider Mangora acalypha showed no predator–prey relationship with Collembola. The determination of δ¹³C values in trophic biomarker FAs allowed detailed insight into the structure of the decomposer food web and identified diet‐shifts in both consumers at the base of the food web and in top predators in organic versus conventional agricultural systems. The results indicate changes in major trophic links and therefore carbon flux through the food web by conversion of conventional into organic farming systems.     

Use of carbon and nitrogen isotope ratios in termite research

In this paper, I review carbon and nitrogen isotopic (natural abundance levels) studies of termites. The carbon isotope ratio of CH₄ emitted from termites, together with the emission rates of CO₂, CH₄ and H₂, showed several trends corresponding to the kinds of symbiotic microbes and feeding habits. The fraction of methane oxidized in the nest structure was estimated by comparing carbon isotope ratio of CH₄ emitted from the nest with that produced by termites in the nest. Symbiotic nitrogen fixation in the gut of termites has been shown to have a significant contribution to the nitrogen economy in some wood-feeding termites. The carbon isotope ratio distinguishes between C4 from C3 plants, and the fractional contribution of grass in the diet can thereby be estimated. The carbon and nitrogen isotope ratios in termites are discernible among soil-feeders, fungus cultivators and wood-feeders. Wood/soil-interface feeders have intermediate values between wood- and soil-feeders, and thus carbon and nitrogen stable isotope ratios are assumed to characterize the degree of humification of the material consumed by termites. It is suggested that carbon and nitrogen isotope ratios are useful indicators of the functional position of termites in the decomposition process. A similar isotope pattern has been obtained in earthworms, suggesting that isotope signatures might be useful parameters in investigating detritivorous animals in general.     

Estimating stable carbon isotope values of microphytobenthos in the Arctic for application to food web studies

Most studies on Arctic food webs have neglected microphytobenthos as a potential food source because we currently lack robust measurements of δ¹³C values for microphytobenthos from this environment. As a result, the role of microphytobenthos in high latitude marine food webs is not well understood. We combined field measurements of the concentration of aqueous carbon dioxide and the stable carbon isotopic composition of dissolved inorganic carbon (δ¹³C_DIC) from bottom water in the Beaufort and Chukchi seas with a set of stable carbon isotopic fractionation factors reflecting differences in algal taxonomy and physiology to estimate the stable carbon isotope composition of microphytobenthos-derived total organic carbon (δ¹³C_p). The δ¹³C_p for Phaeodactylum tricornutum, a pennate diatom likely to be a dominant microphytobenthos taxon, was estimated to be ?23.9 ± 0.4 ‰ as compared to a centric diatom (Porosira glacialis, δ¹³C_p = ?20.0 ± 1.6 ‰) and a marine haptophyte (Emiliana huxleyi, δ¹³C_p = ?22.7 ± 0.5 ‰) at a growth rate (µ) of 0.1 divisions per day (d^?1). δ¹³C_p values increased by ~2.5 ‰ when µ increased from 0.1 to a maximum growth rate of 1.4 d^?1. We compared our estimates of δ¹³C_p values for microphytobenthos with published measurements for other carbon sources in the Arctic and sub-Arctic. We found that microphytobenthos values overlapped with pelagic sources, yet differed from riverine and ice-derived carbon sources. These model results provide valuable insight into the range of possible isotopic values for microphytobenthos from this region, but we remain cautious in regard to the conclusiveness of these findings given the paucity of field measurements currently available for model validation.     

Taxon-specific variation in the stable isotopic signatures (δ13C and δ15N) of lake phytoplankton

1. The variability in the stable isotope signatures of carbon and nitrogen (δ¹³C and δ¹⁵N) in different phytoplankton taxa was studied in one mesotrophic and three eutrophic lakes in south‐west Finland. The lakes were sampled on nine to 16 occasions over 2–4 years and most of the time were dominated by cyanobacteria and diatoms. A total of 151 taxon‐specific subsamples covering 18 different phytoplankton taxa could be isolated by filtration through a series of sieves and by flotation/sedimentation, followed by microscopical identification and screening for purity. 2. Substantial and systematic differences between phytoplankton taxa, seasons and lakes were observed for both δ¹³C and δ¹⁵N. The values of δ¹³C ranged from ?34.4‰ to ?5.9‰ and were lowest in chrysophytes (?34.4‰ to ?31.3‰) and diatoms (?30.6‰ to ?26.6‰). Cyanobacteria were most variable (?32.4‰ to ?5.9‰), including particularly high values in the nostocalean cyanobacterium Gloeotrichia echinulata (?14.4‰ to ?5.9‰). For δ¹³C, the taxon‐specific amplitude of temporal changes within a lake was usually <1–8‰ (<1–4‰ for microalgae alone and <1–8‰ for cyanobacteria alone), whereas the amplitude among taxa within a water sample was up to 31‰. 3. The values of δ¹⁵N ranged from ?2.1‰ to 12.8‰ and were high in chrysophytes, dinophytes and diatoms, but low in the nitrogen‐fixing cyanobacteria Anabaena spp., Aphanizomenon spp. and G. echinulata (?2.1‰ to 1.6‰). Chroococcalean cyanobacteria ranged from ?1.4‰ to 8.9‰. For δ¹⁵N, the taxon‐specific amplitude of temporal changes within a lake was 2–6‰, (2–6‰ for microalgae alone and 2–4‰ for cyanobacteria alone) and the amplitude among taxa within a water sample was up to 11‰. 4. The isotopic signatures of phytoplankton changed systematically with their physical and chemical environment, most notably with the concentrations of nutrients, but correlations were non‐systematic and site‐specific. 5. The substantial variability in the isotopic signatures of phytoplankton among taxa, seasons and lakes complicates the interpretation of isotopic signatures in lacustrine food webs. However, taxon‐specific values and seasonal patterns showed some consistency among years and may eventually be predictable.     

Seasonal dynamics in the stable carbon isotope composition δ¹³C from non-leafy branch, trunk and coarse root CO₂ efflux of adult deciduous (Fagus sylvatica) and evergreen (Picea abies) trees

Respiration is a substantial driver of carbon (C) flux in forest ecosystems and stable C isotopes provide an excellent tool for its investigation. We studied seasonal dynamics in δ¹³C of CO₂ efflux (δ¹³C_E) from non‐leafy branches, upper and lower trunks and coarse roots of adult trees, comparing deciduous Fagus sylvatica (European beech) with evergreen Picea abies (Norway spruce). In both species, we observed strong and similar seasonal dynamics in the δ¹³C_E of above‐ground plant components, whereas δ¹³C_E of coarse roots was rather stable. During summer, δ¹³C_E of trunks was about ?28.2‰ (Beech) and ?26.8‰ (Spruce). During winter dormancy, δ¹³C_E increased by 5.6–9.1‰. The observed dynamics are likely related to a switch from growth to starch accumulation during fall and remobilization of starch, low TCA cycle activity and accumulation of malate by PEPc during winter. The seasonal δ¹³C_E pattern of branches of Beech and upper trunks of Spruce was less variable, probably because these organs were additionally supplied by winter photosynthesis. In view of our results and pervious studies, we conclude that the pronounced increases in δ¹³C_E of trunks during the winter results from interrupted access to recent photosynthates.     

Estimating the contribution of leaf litter decomposition to soil CO2 efflux in a beech forest using 13C-depleted litter

The contribution of leaf litter decomposition to total soil CO₂ efflux (F_L/F) was evaluated in a beech (Fagus sylvatica L.) forest in eastern France. The Keeling‐plot approach was applied to estimate the isotopic composition of respired soil CO₂ from soil covered with either control (?30.32‰) or ¹³C‐depleted leaf litter (?49.96‰). The δ¹³C of respired soil CO₂ ranged from ?25.50‰ to ?22.60‰ and from ?24.95‰ to ?20.77‰, respectively, with depleted or control litter above the soil. The F_L/F ratio was calculated by a single isotope linear mixing model based on mass conservation equations. It showed seasonal variations, increasing from 2.8% in early spring to about 11.4% in mid summer, and decreasing to 4.2% just after leaf fall. Between December 2001 and December 2002, cumulated F and F_L reached 0.98 and 0.08 kg_C m^?2, respectively. On an annual basis, decomposition of fresh leaf litter accounted for 8% of soil respiration and 80% of total C loss from fresh leaf litter. The other fraction of carbon loss during leaf litter decomposition that is assumed to have entered the soil organic matter pool (i.e. 20%) represents only 0.02 kg_C m^?2.     

Altitudinal gradients of grassland carbon and nitrogen isotope composition are recorded in the hair of grazers

Aim The hair of grazers provides an isotopic record of environmental and nutritional signals. Here, we assess the effect of altitude on the carbon and nitrogen isotope composition of the hair of ruminant grazers and its relation to grassland vegetation, to evaluate the use of hair isotope data for ecosystem reconstruction, animal nutritional ecology and biogeochemical studies in montane environments. Location European Alps. Methods We sampled grassland vegetation (pure C₃) and the hair of ruminants along an altitudinal gradient (400–2500 m), and analysed their isotope composition (δ¹³C and δ¹⁵N). Results were compared with published effects of altitude on ¹³C in C₃ plants at the species level and on ¹⁵N at the community level. The study was complemented with a comparison of diet and hair isotope composition in ruminants held in confinement. Results δ¹³C of hair increased (c. 1.1‰ km⁻¹) and δ¹⁵N decreased (c. 1.1‰ km⁻¹) with altitude. The same changes occurred in local grassland vegetation, and in regional to global grassland data sets. Offsets between hair and vegetation ¹³C or ¹⁵N (‘diet–hair shift’) were independent of altitude. Sheep (Ovis aries) and cattle (Bos taurus) exhibited a ¹³C shift near +3‰, but that of goats (Capra hircus) was larger (+4.2‰) in alpine environments and in confinement. The diet–hair shift for ¹⁵N was more variable (+2.1 to +3.6‰). Main conclusions Grazer hair provides a faithful spatially and temporally integrated record of grassland isotope composition, useful for ecosystem and environment reconstruction. The effect of altitude on hair ¹⁵N is important for studies of trophic relationships: an altitude shift of 2000 m produced the same effect in hair ¹⁵N as would a shift from an animal tissue‐based to a plant‐based diet. The similarity of altitude effects on δ¹³C of individual plant species, vegetation and hair indicates that the effect of altitude on species‐level ‘intrinsic water use efficiency’ scales up linearly to the community and landscape level.