ISOTOPIC ANALYSIS OF δ13C, δ15N, AND δ34S "A FEEDING TALE" IN TEETH OF THE LONGBEAKED COMMON DOLPHIN, DELPHINUS CAPENSIS

Stable isotopic analyses of carbon, nitrogen, and sulfur were performed on teeth of different ages and sexes of the longbeaked common dolphin, Delphinus capensis, from the Gulf of California. Similarities in diet are suggested between the sexes, with no significant differences in isotopic compositions being observed. Differences in the δ¹³C, δ¹⁵N, and δ³⁴S signatures were found among the age groups (nursing calf, juvenile, subadult, and adult). These data suggest that this species is generally a coastal feeder, and that it changes its feeding habits with increasing age, drawing more nutrition from higher trophic level organisms later in life.     

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.     

Correlating δ13C and δ18O in cellulose of trees

We measured the carbon and oxygen isotopic composition of stem cellulose of Pinus sylvestris, Picea abies, Fagus sylvatica and Fraxinus excelsior. Several sites along a transect of a small valley in Switzerland were selected which differ in soil moisture conditions. At every site, six trees per species were sampled, and a sample representing a mean value for the period from 1940 to 1990 was analysed. For all species, the mean site δ¹³C and δ¹⁸O of stem cellulose are related to the soil moisture availability, whereby higher isotope ratios are found at drier sites. This result is consistent with isotope fractionation models when assuming enhanced stomatal resistance (thus higher δ¹³C of incorporated carbon) and increased oxygen isotope enrichment in the leaf water (thus higher δ¹⁸O) at the dry sites. δ¹⁸ O-δ¹³C plots reveal a linear relationship between the carbon and oxygen isotopes in cellulose. To interpret this relationship we developed an equation which combines the above-mentioned fractionation models. An important new parameter is the degree to which the leaf water enrichment is reflected in the stem cellulose. In the combined model the slope of the δ¹⁸O-δ¹³C plot is related to the sensitivity of the p_i/p_a of a plant to changing relative humidity.     

Reach-scale geomorphology affects organic matter and consumer δ13C in a forested Piedmont stream

1. We investigated the spatial (longitudinal position and reach geomorphology) and seasonal (spring and autumn) influences on the variation of δ¹³C among organic matter sources and consumers in a forested Piedmont river, South Carolina, U.S.A. 2. Six sites were sampled along a continuum and varied in basin area from approximately 30 to 300 km². Sites fell into two geomorphic categories (i) high‐gradient, rock bed (‘rock’) or (ii) low‐gradient, sand bed (‘sand’) sites. 3. Variation in δ¹³C was more strongly related to reach geomorphology than longitudinal position. δ¹³C of biofilm and consumers was consistently enriched at rock sites. Leaf litter (i.e. coarse particulate organic matter, CPOM) δ¹³C did not vary with bed type. There was significant δ¹³C enrichment at rock sites for biofilm, seston, fine benthic organic matter (FBOM), and eight of nine consumer trophic guilds (e.g. grazing invertebrates, insectivorous fishes). δ¹³C of biofilm and four trophic guilds was also positively correlated with drainage area, but the magnitude of enrichment was less than between bed types. 4. δ¹³C was generally enriched in spring, but this varied among organic matter types, consumers, and by bed type. CPOM and seston were enriched in spring, FBOM was enriched in autumn, and biofilm showed no trend. Five consumer guilds were enriched in spring, and only one fish guild, generalised carnivores, showed enrichment of muscle tissue in autumn. 5. Consumer δ¹³C enrichment at rock sites suggests greater reliance on algal carbon than for consumers at sand sites, but we also found δ¹³C enrichment of biofilm at rock sites. Thus, differences in consumer δ¹³C between bed types could be related to (i) increased consumption of biofilm at rock compared with sand sites, or (ii) consumption of biofilm at rock sites that is enriched relative to biofilm at sand sites or (iii) both mechanisms. 6. δ¹³C signatures in local food webs appear to respond to processes operating at multiple spatial scales. Overall downstream enrichment of biofilm and consumers was disrupted by strong local effects related to bed morphology. These results suggest that human alteration of channel habitat will have corresponding effects on stream food webs, as assessed by changes in δ¹³C.     

Fractionation of δ15N and δ13C for Atlantic salmon and its intestinal cestode Eubothrium crassum

Stable isotopes of nitrogen (δ¹⁵N) and carbon (δ¹³C) were measured for Atlantic salmon Salmo salar and their intestinal cestode, Eubothrium crassum , sharing the same diet. Atlantic salmon muscle tissues were enriched in ¹⁵N and depleted in ¹³C compared to their prey (sprat Sprattus sprattus sprattus ) and their intestinal cestode. There was no significant difference in δ¹⁵N or δ¹³C between E. crassum and the sprat. Differences in nutrient uptake and intestine physiology between Atlantic salmon and E. crassum are discussed, as well as how these may give rise to different fractionations of stable isotopes between a host and its parasites. Furthermore, Atlantic salmon contained a significantly higher lipid content than their prey, which may partly explain differences in δ¹³C values between the host and its cestode. In addition, cestodes inhabiting lipid-rich hosts were also lipid rich. Larger Atlantic salmon were enriched in ¹⁵N compared to smaller fish. Cestodes inhabiting large hosts were also enriched in ¹⁵N compared to parasites living in smaller hosts. The last two results were explained by larger fish possibly feeding from a higher trophic level, or from larger and older prey, that resulted in both a higher lipid content and an enrichment in ¹⁵N.     

Using the δ13C and δ15N of whitefish scales for retrospective ecological studies: changes in isotope signatures during the restoration of Lake Geneva, 1980–2001

The scales of whitefish Coregonus lavaretus were used in place of dorsal muscle, which necessitates killing the fish, to study food webs from the δ¹³C and δ¹⁵N isotopic ratios in the organic fraction. As scales are composed of both organic and calcified fractions, a protocol for scale decalcification was first devised. The δ¹³C and δ¹⁵N values of the decalcified scales were then shown to be closely correlated to those of the dorsal muscle, demonstrating that scales could be used in place of muscle to study food webs. Changes in the δ¹³C of whitefish were determined from a scale collection that extended over the period during which the trophic state of Lake Geneva was recovering.     

Effect of digestion on the δ13C and δ15N of fish-gut contents

Gut contents of sand goby Pomatoschistus minutus showed higher C and N isotope values than the food before consumption. This enrichment was more pronounced in the hindgut than in the foregut, probably because of preferential assimilation of ¹²C and ¹⁴N along the gastro-intestinal tract. The results indicated that the shift towards higher values in the alimentary canal occurs in the first 2 h after feeding.     

Relationships of grain δ13C and δ18O with wheat phenology and yield under water-limited conditions

Stable carbon isotope composition (δ¹³C) of dry matter has been widely investigated as a selection tool in cereal breeding programmes. However, reports on the possibilities of using stable oxygen isotope composition (δ¹⁸O) as a yield predictor are very scarce and only in the absence of water stress. Indeed, it remains to be tested whether changes in phenology and stomatal conductance in response to water stress overrule the use of either δ¹³C or δ¹⁸O when water is limited. To answer this question, a set of 24 genotypes of bread wheat ( Triticum aestivum ) were assayed in two trials with different levels of deficit irrigation and a third trial under rainfed conditions in a Mediterranean climate (northwest Syria). Grain yield (GY) and phenology (duration from planting to anthesis and from anthesis to maturity) were recorded, and the δ¹³C and δ¹⁸O of grains were analysed to assess their suitability as GY predictors. Both δ¹³C and δ¹⁸O showed higher broad-sense heritabilities ( H ²) than GY. Genotype means of GY across trials were negatively correlated with δ¹³C, as previously reported, but not with δ¹⁸O. Both isotopes were correlated with grain filling duration, whereas δ¹⁸O was also strongly affected by crop duration from planting to anthesis. We concluded that δ¹⁸O of grains is not a proper physiological trait to breed for suboptimal water conditions, as its variability is almost entirely determined by crop phenology. In contrast, δ¹³C of grains, despite being also affected by phenology, still provides complementary information associated with GY.     

Stipa tenacissima Does not Affect the Foliar δ13C and δ15N of Introduced Shrub Seedlings in a Mediterranean Semi-arid Steppe

Recent studies have shown that the tussock grass Stipa tenacissima L. facilitates the establishment of late-successional shrubs, in what constitutes the first documented case of facilitation of woody plants by grasses. With the aim of increasing our knowledge of this interaction, in the present study we investigated the effects of S. tenacissima on the foliar δ13C, δ15N, nitrogen concentration, and carbon ： nitrogen ratio of introduced seedlings of Pistacia lentiscus L., Quercus coccifera L., and Medicago arborea L. in a semi-arid Mediterranean steppe. Six months after planting, the values of δ13C ranged between -26.9‰ and -29.6‰, whereas those of δ15N ranged between -1.9‰ and 2.7‰. The foliar C ： N ratio ranged between 10.7 and 53.5, and the nitrogen concentration ranged between 1.0% and 4.4%. We found no significant effect of the microsite provided by S. tenacissima on these variables in any of the species evaluated. The values of δ13C were negatively correlated with predawn water potentials in M. arborea and were positively correlated with relative growth rate in Q. coccifera. The values of δ15N were positively correlated with the biomass allocation to roots in the latter species. The present results suggest that the modification of environmental conditions in the are surrounding S. tenacissima was not strong enough to modify the foliar isotopic and nitrogen concentration of shrubs during the early stages after planting.     

Seasonal variation in δ13C and δ18O of cellulose from growth rings of Pinus radiata

Seasonal variation in δ¹³C and δ¹⁸O of cellulose (δ¹³C_c and δ¹⁸O_c) was measured within two annual rings of Pinus radiata growing at three sites in New Zealand. In general, both δ¹³C_c and δ¹⁸O_c increased to a peak over summer. The three sites differed markedly in annual water balance, and these differences were reflected in δ¹³C_c and δ¹⁸O_c. Average δ¹³C_c and δ¹⁸O_c from each site were positively related, so that the driest site had the most enriched cellulose. δ¹³C_c and δ¹⁸O_c were also related within each site, although both the slope and the closeness of fit of the relationship varied between sites. Supporting the theory, the site with the lowest average relative humidity also had the greatest change in δ¹⁸O_c‰ change in δ¹³C_c. Specific climatic events, such as drought or high rainfall, were recorded as a peak or a trough in enrichment, respectively. These results suggest that seasonal and between‐site variation in δ¹³C_c and δ¹⁸O_c are driven by the interaction between variation in climatic conditions and soil water availability, and plant response to this variation.     

Bulk leaf δ18O and δ13C reflect the intensity of intraspecific competition for water in a semi-arid tussock grassland

We investigated the extent to which plant water and nutrient status are affected by intraspecific competition intensity and microsite quality in a monodominant tussock grassland. Leaf gas exchange and stable isotope measurements were used to assess the water relations of Stipa tenacissima tussocks growing along a gradient of plant cover and soil depth in a semi-arid catchment of Southeast Spain. Stomatal conductance and photosynthetic rate decreased with increasing intensity of competition during the wet growing season, leading to foliar δ ¹⁸O and δ ¹³C enrichment. A high potential for runoff interception by upslope neighbours exerted strong detrimental effects on the water and phosphorus status of downslope S. tenacissima tussocks. Foliar δ ¹⁵N values became more enriched with increasing soil depth. Multiple stepwise regression showed that competition potential and/or rhizosphere soil depth accounted for large proportions of variance in foliar δ ¹³C, δ ¹⁸O and δ ¹⁵N among target tussocks (57, 37 and 64%, respectively). The results presented here highlight the key role that spatial redistribution of resources (water and nutrients) by runoff plays in semi-arid ecosystems. It is concluded that combined measurement of δ ¹³C, δ ¹⁸O and nutrient concentrations in bulk leaf tissue can provide insight into the intensity of competitive interactions occurring in natural plant communities.     

Effects of the ecto- and VA-mycorrhizal fungi Hydnagium carneum and Glomus clarum on the δ15N and δ13C values of Eucalyptus globulus and Ricinus communis

Glasshouse experiments with Ricinus communis showed that the presence/absence of a VA mycorrhizal fungus (Glomus clarum) changed the δ¹⁵N value of the host by as much as 2‰ when the plants were given urea (released as NH₄⁺) as their only N-source. This small change in Δ¹⁵N would create a large error in calculating sources of plant N. In particular, these results throw into doubt any models of N-cycling which assume that soil N can be treated as a single source. The correct N-source value for VAM-infected NH₄^? -using plants may be the δ¹⁵N of soil NH₄⁺+ 2‰. Treatment effects were also found in the distribution of δ¹⁵N and % N among plant organs. Plants with VAM had a lower N:P atom ratio and were larger in total biomass. Carbon discrimination (δ¹³C) was greater in the VA-infected plants. The measured effects of VAM infection suggest that for some plants the fungus may be the primary site of N assimilation. A parallel experiment with Eucalyptus globulus and the ectomycorrhizal fungus Hydnangium carneum resulted in no significant differences in any of the variables measured for this host-fungus pair when the sole N-sources were inorganic (NO₃^? and NH₄⁺ released from urea). Ectomycorrhizal fungi are diverse in their physiological behaviour, and these data should not be taken as being representative of the whole group. More work is required with other types of mycorrhiza and more complex sources of N. Future work will include a water balance to partition the effects of water use and nutrient supply in determining δ¹³C. An on-line combustion-ANCA-MS method is described for fully automated measurement of natural abundance levels of ^15/14N and ^13/12C for plant materials. This method achieves the required precision while dramatically increasing sample throughout.     

Stable isotope evidence (δ13C, δ18O) for winter feeding on seaweed by Neolithic sheep of Scotland

The antiquity of the use of seaweed to feed domestic animals was investigated through carbon ( δ ¹³C) and oxygen ( δ ¹⁸O) isotope analysis of tooth enamel bioapatite. The analysis was performed on sheep and cattle teeth from two Neolithic sites in Orkney (Scotland). At the Knap of Howar, c . 3600 bc , carbon isotopes reflect grazing on terrestrial plants throughout the year for both sheep and cattle, with no contribution of seaweed to their diet. At the Holm of Papa Westray North (HPWN), c . 3000 bc , significant contribution of seaweed to the sheep diet during winter is indicated by bioapatite δ ¹³C values as high as −5.7‰, far outside of the range of values expected for the feeding on terrestrial C₃ plants, and δ ¹⁸O values higher than expected during winter, possibly caused by ingestion of oceanic water with seaweed. Ingestion of seaweed by sheep at HPWN might have been necessitated by severe reduction of pastures during winter. Results suggest that sheep ingested fresh seaweed rather than dry fodder, perhaps directly on the shore as sheep do nowadays on North Ronaldsay. A significant difference between the two populations is the exclusive reliance on seaweed by the North Ronaldsay sheep, which have developed physiological adaptations to this diet. Contribution of seaweed to the sheep winter diet at HPWN might have been a first step towards this adaptation.     

Physiological and isotopic (δ13C and δ18O) responses of three tropical tree species to water and nutrient availability

Water-use efficiency and stable isotope composition were studied in three tropical tree species. Seedlings of Tectona grandis , Swietenia macrophylla and Platymiscium pinnatum were grown at either high or low water supply, and with or without added fertilizer. These three species previously exhibited low, intermediate and high whole-plant water-use efficiency ( TE ) when grown at high water supply in unfertilized soil. Responses of TE to water and nutrient availability varied among species. The TE was calculated as experiment-long dry matter production divided by cumulative water use. Species-specific offsets were observed in relationships between TE and whole-plant ¹³C discrimination (Δ¹³C_p). These offsets could be attributed to a breakdown in the relationship between Δ¹³C_p and the ratio of intercellular to ambient CO₂ partial pressures ( c _i/ c _a) in P. pinnatum , and to variation among species in the leaf-to-air vapour pressure difference ( v ). Thus, a plot of v · TE against c _i/ c _a showed a general relationship among species. Relationships between δ ¹⁸O of stem dry matter and stomatal conductance ranged from strongly negative for S. macrophylla to no relationship for T. grandis . Results suggest inter-specific variation among tropical tree species in relationships between stable isotope ratios ( δ ¹³C and δ ¹⁸O) and the gas exchange processes thought to affect them.     

Trophic structure of a boreal forest arthropod community revealed by stable isotope (δ13C, δ15N) analyses

Terrestrial arthropods are important components of boreal ecosystems but relatively little is known about their trophic structure within communities. We measured δ¹³C and δ¹⁵N values in a broad range of arthropod taxa (Coleoptera, Diptera, Ephemeroptera, Homoptera, Hymenoptera, Lepidoptera, Odonata, Orthoptera, Araneae) from boreal forest in Prince Albert National Park, Saskatchewan, Canada. Isotopic measurements supported previous conventional investigations on foraging niches based on stomach content analysis and direct feeding observations but additional, new information was also obtained using the stable isotope approach. Significant differences were observed in both δ¹⁵N and δ¹³C values between various orders and families or superfamilies within orders. Higher variance in stable isotope values for scavengers (e.g. carrion beetles; Coleoptera, Silphidae) and generalists (e.g. ground beetles; Coleoptera, Carabidae) was found compared to specialists (e.g. grasshoppers; Orthoptera). Consistent isotopic differences between terrestrial and aquatic species were not found. However, aquatic insect δ¹³C values tended to be lower than those of their terrestrial counterparts. We discuss the potential for using stable isotope methods to reconstruct trophic linkages and interaction involving arthropods.     

Combined monitoring of changes in δ13CH4 and archaeal community structure during mesophilic methanization of municipal solid waste

Reconstituted municipal solid waste (MSW) with varying contents of putrescible and cellulosic waste was incubated anaerobically under mesophilic conditions. Standard physicochemical parameters were monitored, together with stable isotopic signatures of produced CH₄ and CO₂. δ¹³C values for CH₄ indicated a change of methanogenic metabolism with time. CH₄ was predominantly produced from H₂/CO₂ at the beginning of the incubations. This period was associated with important shifts in archaeal communities monitored by automated ribosomal intergenic spacer analysis (ARISA) and FISH of oligonucleotidic probes targeting specifically 16S rRNA gene of various methanogenic groups. The onset of the active methane generation phase was characterized by an increase of CH₄δ¹³C, indicating a progressive shift toward an aceticlastic metabolism. When the methane production levelled off, a decrease in the isotopic signature was observed toward values characteristics of hydrogenotrophic metabolism. ARISA profiles were, however, found to be stable from the beginning of the active methane generation phase until the end of the experiment. FISH observation indicated that members of the family Methanosarcinaceae were predominant in the archaeal community during this period, suggesting that these methanogens might exhibit a high metabolic versatility during methanization of waste.     

Arctic and North Atlantic Oscillation phase changes are recorded in the isotopes (δ18O and δ13C) of Cassiope tetragona plants

The Arctic and North Atlantic Oscillations (AO/NAO) are large‐scale annual modes of atmospheric circulation that have shifted in the last 30 years. Recent changes in arctic climate, including increasing surface air temperature, declining sea ice extent, and shifts in the amounts seasonality of precipitation are linked to the strong positive phase of the AO/NAO. Here, we show that phase changes in the AO/NAO are recorded in the isotopic (δ¹⁸O and Δ‐carbon isotope discrimination) characteristics of the long‐lived circum‐arctic plant, Cassiope tetragona, as summer rain has become a more important water source than snowmelt water which in turn has lead to decreases in Δ and reductions in plant stem growth. These isotopic records in C. tetragona may facilitate reconstructions of climate, plant–soil water relations, plant gas exchange attributes and a mechanistic understanding of growth responses to shifts in atmospheric circulation. If plant specimens were available for populations across the arctic as part of the International Polar Year, these archives could provide a circum‐arctic record of historical climate change and associated shifts in physiological plant performance and growth.     

Genetic parameters and QTL analysis of δ13C and ring width in maritime pine

Classical quantitative genetics and quantitative trait dissection analysis (QTL) approaches were used in order to investigate the genetic determinism of wood cellulose carbon isotope composition (δ¹³C, a time integrated estimate of water use efficiency) and of diameter growth and their relationship on adult trees (15 years) of a forest tree species (maritime pine). A half diallel experimental set‐up was used to (1) estimate heritabilities for δ¹³C and ring width and (2) to decompose the phenotypic δ¹³C/growth correlation into its genetic and environmental components. Considerable variation was found for δ¹³C (range of over 3‰) and for ring width (range of over 5 mm) and significant heritabilities (narrow sense 0·17/0·19 for δ¹³C and ring width, respectively, 100% additivity). The significant phenotypic correlation between δ¹³C and ring width was not determined by the genetic component, but was attributable to environmental components. Using a genetic linkage map of a full‐sib family, four significant and four suggestive QTLs were detected for δ¹³C, the first for δ¹³C in a forest tree species, as far as known to the authors. Two significant and four suggestive QTLs were found for ring width. No co‐location of QTLs was found between δ¹³C and growth.