Stable isotope profiles of partially migratory salmonid populations in Atlantic rivers of Patagonia

In the present study, profiles of stable isotope composition were characterized for two species with partially migratory populations in rivers along the latitudinal gradient of Patagonia, brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss . The effects of factors ( e.g. ontogeny of fishes, location, species and fasting) that may influence the stable isotope analysis (SIA) were evaluated, as was SIA evaluated as a tool to assign individual fish to their corresponding ecotype. Anadromous fishes exhibited enriched δ¹⁵N (15·2 ± 1·0‰; mean ± s . d .) and δ¹³C (−19·2 ± 1·3‰) relative to resident fishes'δ¹⁵N (8·8 ± 1·1‰) and δ¹³C (−23·2 ± 2·5‰). For both species, the difference in δ¹⁵N was larger between resident (range 6·8–10·7‰) and anadromous (range 14·3–17·8‰) fishes than that in δ¹³C. Values of δ¹³C, while not as dramatically contrasting in rainbow trout, provided a powerful anadromy marker for brown trout in the region. Increases were found in both δ¹⁵N and δ¹³C during the spawning migration of anadromous rainbow trout, most likely due to fasting. Differences in stable isotopes between location, size and species were found, suggesting different stable isotopes base levels in freshwater environments and different trophic levels and feeding location of anadromous populations. The SIA was demonstrated as a powerful tool for ecotype discrimination in Patagonian Rivers, overriding any effect of sampling location, size or species.     

Stable isotopes of carbon and nitrogen as indicators of diet and trophic structure of the fish community in a shallow hypereutrophic lake

Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes were employed to elucidate energy flows and trophic interactions in Lake Apopka, a hypereutrophic lake in central Florida, U.S.A. Isotope compositions of lake biota ranged from −27·1 to −3·0‰ for δ¹³C, and from 3·7 to 13·9‰ for δ¹⁵N. The food web was based primarily on plankton production with diatoms, Microcystis and zooplankton dominating the diet of fish. Carbon isotope evidence showed that pico- and nano-phytoplankton were not a direct carbon source for fish, but were important to zooplankton. δ¹⁵N mass balance estimates indicated that planktivorous fish obtained 48–85% of their diets from zooplankton. The ∼3‰ range of δ¹⁵N in gizzard shad reflected increasing dependence on zooplankton as fish grew whereas the positive relationship between total length and δ¹⁵N of largemouth bass reflected increasing predation on larger planktivorous fish with growth. The broad ranges of δ¹³C (−25·9 to −9·5‰) and δ¹⁵N (5·8 to 14·4‰) of blue tilapia were indicators of diet diversity. Two presumed omnivores (brown bullhead and white catfish) and piscivores (black crappie, largemouth bass and Florida gar) were found to depend on planktivorous fish. However, stable isotope data revealed no trophic links between blue tilapia, an abundant fish in the near-shore area, and piscivores.     

Nitrogen and carbon isotope ratios in termites: an indicator of trophic habit along the gradient from wood-feeding to soil-feeding

1. Nitrogen and carbon stable-isotope ratios (δ¹⁵N and δ¹³C) of body tissues, mound/nest materials and dietary substrates were determined in termite species with differing trophic habits, sampled from the Mbalmayo Forest Reserve, southern Cameroon.
2. δ¹⁵N of termite tissues was enriched gradually along a spectrum of species representing a trophic gradient from wood- to soil-feeding. Species that could be identified from their general biology and from gut content analysis as feeding on well-rotted wood or as wood/soil interface feeders showed δ¹⁵N intermediate between sound-wood-feeders and soil-feeders. It is proposed that δ¹⁵N is therefore a possible indicator of the functional position of species in the humification process. Differences in δ¹³C were also observed between wood-feeding and soil-feeding forms.
3. High values of δ¹⁵N in soil-feeding termites suggest that nitrogen fixation is of little importance in these species.
4. A wide range of isotope effects (the difference in isotope ratios between termites and their diet) was observed for both nitrogen (Δδ¹⁵N = –1.6 to + 8.8‰) and carbon (Δδ¹³C = –2.2 to + 3.0‰), which suggests a diversity of nutrient acquisition mechanisms within termites and diverse relationships between termites and their intestinal micro-organisms.     

Extensive carbon isotopic heterogeneity among methane seep microbiota

To assess and study the heterogeneity of δ¹³C values for seep microorganisms of the Eel River Basin, we studied two principally different sample sets: sediments from push cores and artificial surfaces colonized over a 14 month in situ incubation. In a single sediment core, the δ¹³C compositions of methane seep-associated microorganisms were measured and the relative activity of several metabolisms was determined using radiotracers. We observed a large range of archaeal δ¹³C values (> 50‰) in this microbial community. The δ¹³C of ANME-1 rods ranged from −24‰ to −87‰. The δ¹³C of ANME-2 sarcina ranged from −18‰ to −75‰. Initial measurements of shell aggregates were as heavy as −19.5‰ with none observed to be lighter than −57‰. Subsequent measurements on shell aggregates trended lighter reaching values as ¹³C-depleted as −73‰. The observed isotopic trends found for mixed aggregates were similar to those found for shell aggregates in that the initial measurements were often enriched and the subsequent analyses were more ¹³C-depleted (with values as light as −56‰). The isotopic heterogeneity and trends observed within taxonomic groups suggest that ANME-1 and ANME-2 sarcina are capable of both methanogenesis and methanotrophy. In situ microbial growth was investigated by incubating a series of slides and silicon (Si) wafers for 14 months in seep sediment. The experiment showed ubiquitous growth of bacterial filaments (mean δ¹³C = −38 ± 3‰), suggesting that this bacterial morphotype was capable of rapid colonization and growth.     

Stable isotope variability of meso-zooplankton along a gradient of dissolved organic carbon

1.  The δ¹³C and δ¹⁵N signatures of zooplankton vary with dissolved organic carbon (DOC), but inconsistent and limited taxonomic resolution of previous studies have masked differences that may exist among orders, genera or species and are attributable to dietary and/or habitat differences. Here we investigate differences among the isotopic signatures of five zooplankton taxa ( Daphnia , Holopedium , large Calanoida, small Calanoida and Cyclopoida) in Precambrian shield lakes with a sixfold range of DOC concentration.
2.  δ¹³C signatures of Daphnia , small calanoids and large calanoids became more depleted with increasing lake DOC, whereas Holopedium and cyclopoid δ¹³C became enriched with increasing DOC concentration.
3.  The variability of δ¹³C and δ¹⁵N isotopic signatures among zooplankton groups was reduced in high-DOC, compared to low-DOC lakes, especially for δ¹³C. Differences in δ¹³C and POM-corrected δ¹⁵N accounted for up to 33.7% and 19.5% of the variance, respectively, among lakes of varying DOC concentration.
4.  The narrow range of signatures found in higher DOC lakes suggests that different taxa have similar food sources and/or habitats. In contrast, the wide range of signatures in low-DOC lakes suggests that different taxa are exploiting different food sources and/or habitats. Together with the variable trends in zooplankton isotopic signatures along our DOC gradient, these results suggest that food web dynamics within the zooplankton community of temperate lakes will change as climate and lake DOC concentrations change.     

Stable carbon isotope fractionation by marine phytoplankton during photosynthesis

Abstract. In the marine environment, the range of values of carbon isotope fractionation between particulate tissue of phytoplankton and inorganic carbon can be more than 20‰ (− 35‰ < δ¹³C < − 14‰). This review considers the influence of seawater temperature, lipid content of phytoplanktonic cells, kinetic fractionation, and carbon pathway on δ¹³C values observed at sea.
In order to study the contribution of carboxylases (RUBISCO and the β-carboxylases phosphoenolpyruvate carboxylase, phosphoenoplpyruvate carboxykinase and pyruvate carboxylase) to variations of particulate δ¹³C values at sea, we present results obtained simultenously on carboxylase activities and δ¹³C in various environmental conditions. The lowest δ¹³C values are clearly associated with predominance of ribulose-1.5-bisphosphate carboxylase activity, but it was more difficult to explain the high δ¹³C values. Different hypotheses are discussed.     

δ13C of wood in growth-rings indicates cambial activity of drought-stressed trees of Eucalyptus globulus

1. Growth, density and δ¹³C of wood and leaf area were measured in two adjacent stands of 6 year-old Eucalyptus globulus growing in the 600–700 mm year^–1 rainfall region of south-western Australia. Study sites were identical except for differences in the availability of water owing to physical properties of soil profiles and location of sites within the landscape.
2. Abundance of ¹³C (expressed as δ¹³C) in wood of trees growing on the drought-prone site (– 24·8‰±1·4) was greater than in other trees (– 25·8‰±1·2, P <0·001) throughout the 6 years and, with further development, the δ¹³C signatures of wood may become useful indices of drought-susceptibility in plantations within a few years of establishment. The seasonal pattern of δ¹³C of wood appeared to reflect seasonal variation in water availability and duration of cambial activity.
3. Basic density of wood of trees growing on the more drought-prone site (496±14·0 kg m^–3) was reduced compared to other trees (554±5·3 kg m^–3, P <0·001). δ¹³C of wood across boundaries of growth-rings suggested that drought stopped cambial activity resulting in less production of late wood and less dense wood.
4. The stand growing on the drought-prone site had reduced growth, wood yield and leaf area but identical specific leaf area. Annual growth was correlated with the previous season's rainfall. Together, these results suggested that within the same evaporative climate, drought reduces growth primarily by reducing leaf area and that there is a lag between onset of drought and reduced productivity.     

Natural 13C and 15N abundance of field-collected fungi and their ecological implications

The natural abundance of ¹³C and ¹⁵N was measured in basidiocarps of at least 115 species in 88 genera of ectomycorrhizal, wood-decomposing and litter-decomposing fungi from Japan and Malaysia. The natural abundance of ¹³C and ¹⁵N was also measured in leaves, litter, soil and wood from three different sites. ¹⁵N and ¹³C were enriched in ectomycorrhizal and wood-decomposing fungi, respectively, relative to their substrates. Ectomycorrhizal and wood-decomposing fungi could be distinguished on the basis of their δ¹³C and δ¹⁵N signatures. Although there was high variability in the isotopic composition of fungi, the following isotope- enrichment factors (ε, mean±SD) of the fungi relative to substrates were observed:
ε_{ectomycorrhizal fungi/litter} = 6.1±0.4‰¹⁵N
ε_{ectomycorrhizal fungi/wood} = 1.4±0.8‰¹³C
ε_{wood-decomposing fungi/wood} = −0.6±0.7‰¹⁵N
ε_{wood-decomposing fungi/wood} = 3.5±0.9‰¹³C
The basis of isotope fractionation in C metabolism from wood to wood-decomposing fungus is discussed.     

Absence or masking of metabolic fractionations of 13C in a freshwater benthic food web

1. Although marine research has indicated that metabolic fractionations of ¹³C due to differences in organismal trophic position and proximal composition can complicate the isotopic interpretation of energy flow pathways, such potentially confounding problems have never been examined in freshwater benthic food webs.
2. The δ¹³C values of animals comprising a littoral benthic food web composited from four Canadian Shield lakes showed no relationship with either individual trophic position (δ¹⁵N) or lipid content (C/N ratios).
3. Differences in the relative incorporation of autochthonous and allochthonous energy sources by freshwater benthic organisms will alter their δ¹³C and δ¹⁵N values, thereby masking any possibility of observing ¹³C trophic enrichment.
4. Removal of the possibly confounding influences of lipids through either empirical correction or by analytical extraction may be unnecessary in studies of freshwater benthic food webs. Likewise, a priori adjustments in δ¹³C for freshwater benthic organisms in order to accommodate trophic fractionations which are presumed to occur, based on data from marine offshore food webs, may also be inappropriate.     

Absence or masking of metabolic fractionations of 13C in a freshwater benthic food web

1. Although marine research has indicated that metabolic fractionations of ¹³C due to differences in organismal trophic position and proximal composition can complicate the isotopic interpretation of energy flow pathways, such potentially confounding problems have never been examined in freshwater benthic food webs.
2. The δ¹³C values of animals comprising a littoral benthic food web composited from four Canadian Shield lakes showed no relationship with either individual trophic position (δ¹⁵N) or lipid content (C/N ratios).
3. Differences in the relative incorporation of autochthonous and allochthonous energy sources by freshwater benthic organisms will alter their δ¹³C and δ¹⁵N values, thereby masking any possibility of observing ¹³C trophic enrichment.
4. Removal of the possibly confounding influences of lipids through either empirical correction or by analytical extraction may be unnecessary in studies of freshwater benthic food webs. Likewise, a priori adjustments in δ¹³C for freshwater benthic organisms in order to accommodate trophic fractionations which are presumed to occur, based on data from marine offshore food webs, may also be inappropriate.     

Stable isotope analysis of macroinvertebrates and their food sources in a glacier stream

1. Food sources and trophic structure of the macroinvertebrate community along a longitudinal gradient were examined in a glacier stream of the Swiss Alps (Val Roseg). Analysis of multiple stable isotopes (δ¹³C and δ¹⁵N) and measurement of C : N ratios were used to differentiate between allochthonous and autochthonous organic matter.
2. Although isotopic signatures of algae varied widely among sites and dates, it was possible to discriminate between allochthonous and autochthonous food sources using a site-specific approach.
3. Dominant food sources of herbivorous invertebrates in all main channel sites were epilithic diatoms and the filamentous gold alga Hydrurus foetidus . Allochthonous organic matter was of some importance only in a groundwater-fed stream close to the floodplain margin.
4. Seasonal changes in the δ¹³C signature of the macroinvertebrates corresponded with seasonal changes in δ¹³C of the gold alga H. foetidus . This indicated that the energy base remains autochthonous throughout the year.
5. Because of limited food sources, feeding plasticity of the invertebrate community was high. Both grazers and shredders fed predominantly on algae, whereas gatherer-collectors seemed to be omnivorous.
6. The overall enrichment of δ¹⁵N was 2.25‰ ( r ²=0.99) per trophic level. On a gradient from the glacier site to a downstream forested site trophic enrichment was constant but variation in δ¹⁵N within trophic levels decreased.     

Does the 13C of foliage‐respired CO2 and biochemical pools reflect the 13C of recently assimilated carbon?

Isotopic labelling experiments were conducted to assess relationships among ¹³C of recently assimilated carbon ( δC _A), foliage respiration ( δC _F), soluble carbohydrate ( δC _SC), leaf waxes ( δC _LW) and bulk organic matter ( δC _OM). Slash pine, sweetgum and maize were grown under ¹³C depleted CO₂ to label biomass and then placed under ambient conditions to monitor the loss of label. In pine and sweetgum, δC _F of labelled plants (∼−44 and −35‰, respectively) rapidly approached control values but remained depleted by ∼4–6‰ after 3–4 months. For these tree species, no or minimal label was lost from δC _SC, δC _LW and δC _OM during the observation periods. δC _F and δC _SC of labelled maize plants rapidly changed and were indistinguishable from controls after 1 month, while δC _LW and δC _OM more slowly approached control values and remained depleted by 2–6‰. Changes in δC _F in slash pine and sweetgum fit a two-pool exponential model, with the fast turnover metabolic pool (∼3–4 d half-life) constituting only 1–2% of the total. In maize, change in δC _F fits a single pool model with a half-life of 6.4 d. The ¹³C of foliage respiration and biochemical pools reflect temporally integrated values of δC _A, with change in isotopic composition dampened by the size of metabolic carbon reserves and turnover rates.     

δ 13C of CO2 respired in the dark in relation to δ13C of leaf carbohydrates in Phaseolus vulgaris L. under progressive drought

The variations in δ ¹³C in both leaf carbohydrates (starch and sucrose) and CO₂ respired in the dark from the cotyledonary leaves of Phaseolus vulgaris L. were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in ¹³C) with decreasing stomatal conductance and decreasing p _i/ p _a during the first half (15 d) of the dehydration cycle. Thereafter, when stomata remained closed and leaf net photosynthesis was near zero, the tendency was reversed: the carbohydrates became lighter (depleted in ¹³C). This may be explained by increased p _i/ p _a but other possible explanations are also discussed. Interestingly, the variations in δ ¹³C of CO₂ respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ ¹³C of CO₂ respired in the dark and sucrose, respired CO₂ always being enriched in ¹³C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in ¹³C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing ¹³C-enriched CO₂; and that (ii) this leads to ¹³C depletion in the remaining leaf material.     

Trophic level and overlap of sea lions (Zalophus californianus) in the Gulf of California, Mexico

Stable isotope and scat analyses were used in concert to determine trophic level and dietary overlap among California sea lions from different rookeries in the Gulf of California. Isotopic analysis of the fur of sea lion pups revealed differences in δ¹⁵N and δ¹³C values among rookeries during the breeding season. Mean δ¹⁵N and δ¹³C values varied from 20.2‰ to 22.4‰ and from −15.4‰ to −14.0‰, respectively. The pattern of differences among rookeries was similar between years in most cases. Isotopic variations among rookeries were associated with differences in prey consumption. There was a significant correlation between δ¹⁵N value and trophic level, as determined by scat analysis. Joint application of isotopic and scat analyses allowed us to identify how the feeding habits of sea lions vary with location. Our results suggest the presence of spatial structure in available prey as well as the localized use of prey by sea lions across the Gulf of California.     

Different carbon support for respiration and secondary production in unproductive lakes

This study investigates the allocation of allochthonous organic carbon (AlloOC) to pelagic respiration and biomass production in unproductive lakes. Metabolic process rates and stable isotopic composition (δ¹³C) of crustacean zooplankton and respired CO₂ were measured in the epilimnion of 13 forest lakes in northern Sweden. The δ¹³C of zooplankton was low (−31.2 to −38.0‰) compared to that of respired CO₂ (−28.4 to −30.6‰), implying that the relative importance of AlloOC was lower for zooplankton (ca 40%) than for respiration (ca 80%). Combining δ¹³C and carbon flux data revealed that a large amount of metabolized AlloOC was lost in respiration, compared to the amount transferred to zooplankton (<3%). Thus, despite large respiratory losses, AlloOC was still important for zooplankton growth, implying a high supply of AlloOC in comparison to phytoplankton generated organic carbon in the lakes.     

Stable isotopes indicate the extent of freshwater feeding by cormorants Phalacrocorax carbo shot at inland fisheries in England

1. The numbers of cormorants Phalacrocorax carbo feeding at English freshwater fisheries during winter have increased rapidly over the last 20 years, causing concern among fishery managers and anglers.
2. In order to assess the extent of freshwater feeding, stable isotope ratios of carbon and nitrogen (δ¹³C and δ¹⁵N) in feathers of wild cormorants from inland freshwater fisheries were compared with those in the feathers of piscivorous birds with marine diets (captive 'marine-fed' cormorants, free-ranging shags Phalacrocorax aristotelis ) and freshwater diets (juvenile goosanders Mergus merganser ).
3. Isotope signatures of feathers represent the diet at the time of growth. Feathers grown at different times of the year were taken from wild cormorants; each feather type therefore represented the diet over a different temporal scale.
4. Isotopic analyses of feathers indicated that, when shot, nearly all of the cormorants had been feeding entirely on freshwater prey. The mean δ¹³C value of primary feathers growing when birds were shot was –22·2‰, indicative of an entirely freshwater diet.
5. The move to freshwater habitats from coastal breeding grounds occurred over several months, but once established cormorants appear to have fed at freshwater sites throughout the autumn and winter.
6. The suitability of using a two-source isotopic mixing model in order to quantify the extent of freshwater feeding in piscivorous birds is discussed.
7. Although the results indicate long-term residency and feeding in freshwater systems, they do not indicate whether birds were feeding regularly at the sites at which they were shot, or the composition of the diet. It is recommended that further studies using telemetry and multiple isotope analyses be carried out in order to address these issues.     

Stable isotope evidence of trophic interactions between introduced brook trout Salvelinus fontinalis and native brown trout Salmo trutta in a mountain stream of south-west France

The potential trophic impact of introduced brook trout Salvelinus fontinalis on native brown trout Salmo trutta in a mountain stream (south-west France) was investigated using stable isotope analysis (SIA). The isotopic signatures (δ¹³C and δ¹⁵N) of S. fontinalis were similar regardless of the absence or presence of S. trutta , and SIA mixing models revealed that S. fontinalis diet consisted mainly of terrestrial invertebrates. Conversely, a significant shift in S. trutta isotopic signatures (depletion of 1·6‰δ¹³C and enrichment of 0·6‰δ¹⁵N) was observed in sympatry with S. fontinalis ; this may be due to a dietary shift towards terrestrial invertebrates. Contrary to an expected dietary divergence in sympatry, an elevated level of dietary overlap was observed between the non-native and native salmonids when in co-occurrence. This dietary convergence is more likely to be due to behavioural interactions than to variations in food availability or fish displacements.     

Population differences in water-use efficiency of Eucalyptus microtheca seedlings under different watering regimes

Gas exchange, water-use efficiency (WUE), carbon isotope composition ( Δ ¹³C) and growth traits were compared among 5 populations of Eucalyptus microtheca F. Muell. Seedlings grown from seed collected across the natural distribution of the species were maintained under water-stressed and well-watered conditions. Gas exchange was measured in terms of net photosynthesis (A) and transpiration (E); WUE was measured in terms of instantaneous water-use efficiency (WUE_i) and transpiration efficiency (WUE_T); growth traits were measured in terms of total biomass (TB), root/shoot ratio (RS), and specific leaf area density (DEN). Significant differences in all traits were detected among the populations. Overall population variation was 1.68–2.50 and 1.48–2.26 μmol CO₂ uptake per mmol H₂O transpired (WUE_i), 1.97–3.04 and 1.64–2.36 g dry matter accumulation per kg water transpired (WUE_T), and Δ ¹³C was −28.81 to −26.75‰ and −30.56 to −30.04‰ under the water-stressed and well-watered conditions, respectively. In addition, WUE_i, WUE_T and Δ ¹³C were significantly correlated with A, E, RS, DEN and TB. The study indicated that measurement of WUE may be a useful trait for selecting genotypes with improved drought adaptation and biomass productivity under different environmental conditions.     

Contribution of current carbon assimilation in supplying root exudates of Lolium perenne measured using steady-state C labelling

Coupling growth of Lolium perenne L. in sterile solution culture with steady-state ¹³CO₂ labelling allowed quantification of the contribution of C, assimilated either before or after a specific time point, both to plant compartments and root exudates. Plants were grown for 27 days in atmospheres containing CO₂ with δ ¹³C signatures of either −13.5 or −36.1‰. Air supplies to plants were then reciprocally switched to the opposing signature (day 0), plants were destructively harvested and root exudates collected over the next 8 days. Following the switch, C assimilated after day 0 and transported to the roots initially only appeared in root tips, later appearing in both tip and non-tip material. The δ ¹³C signature of the root exudate changed exponentially with time. Assimilation pre- and post-day 0 contributed equally to exudate C at 4.5 days. Beyond day 8, assimilation pre-day 0 still contributed 41.7% of exudate C. Over all 8 days, a linear relationship existed between the δ ¹³C signatures of root tips and exudate, suggesting that all newly assimilated C in the exudate was from root tips. Results imply pulse-labelling approaches to study root exudates are discriminative in the sources of exudates labelled and in the sites from which exudation occurs.     

Seasonal and annual changes in leaf δ13C in two co-occurring Mediterranean oaks: relations to leaf growth and drought progression

1. Changes of δ¹³C and its relation to leaf development, biochemical content and water stress were monitored over a 2 year period in two co-occurring Mediterranean oak species: the deciduous Quercus pubescens and the evergreen Quercus ilex .
2. The time course of leaf δ¹³C showed different patterns in the two species. Young Q. pubescens leaves had a high δ¹³C and a marked decrease occurred during leaf growth. In contrast, leaves at budburst and maturity did not differ significantly in the case of Q. ilex . We suggest that the difference between δ¹³C of young leaves was linked to differential use of reserves of carbon compounds in the two species.
3. δ¹³C values of mature leaves were negatively correlated with minimum seasonal values of predawn water potential, suggesting that a functional adjustment to water resources occurred.
4. There was a significant correlation between individual δ¹³C values for two successive years. This interannual dependence showed that δ¹³C rankings between trees were constant through time.