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.     

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.     

Canopy-scale δ13C of photosynthetic and respiratory CO2 fluxes: observations in forest biomes across the United States

The δ¹³C values of atmospheric carbon dioxide (CO₂) can be used to partition global patterns of CO₂ source/sink relationships among terrestrial and oceanic ecosystems using the inversion technique. This approach is very sensitive to estimates of photosynthetic ¹³C discrimination by terrestrial vegetation (Δ_A), and depends on δ¹³C values of respired CO₂ fluxes (δ¹³C_R). Here we show that by combining two independent data streams – the stable isotope ratios of atmospheric CO₂ and eddy‐covariance CO₂ flux measurements – canopy scale estimates of Δ_A can be successfully derived in terrestrial ecosystems. We also present the first weekly dataset of seasonal variations in δ¹³C_R from dominant forest ecosystems in the United States between 2001 and 2003. Our observations indicate considerable summer‐time variation in the weekly value of δ¹³C_R within coniferous forests (4.0‰ and 5.4‰ at Wind River Canopy Crane Research Facility and Howland Forest, respectively, between May and September). The monthly mean values of δ¹³C_R showed a smaller range (2–3‰), which appeared to significantly correlate with soil water availability. Values of δ¹³C_R were less variable during the growing season at the deciduous forest (Harvard Forest). We suggest that the negative correlation between δ¹³C_R and soil moisture content observed in the two coniferous forests should represent a general ecosystem response to the changes in the distribution of water resources because of climate change. Shifts in δ¹³C_R and Δ_A could be of sufficient magnitude globally to impact partitioning calculations of CO₂ sinks between oceanic and terrestrial compartments.     

Disentangling CO2 fluxes: direct measurements of mesocosm‐scale natural abundance 13CO2/12CO2 gas exchange, 13C discrimination,and labelling of CO2 exchange flux components in controlled environments

A ¹³C/¹²C mass spectrometer was interfaced with a open gas exchange system including four growth chambers to investigate CO₂ exchange components of perennial ryegrass (Lolium perenne L.) stands. Chambers were fed with air containing CO₂ with known δ¹³C (δ_CΟ2?2.6 or ?46.8‰). The system did not fractionate C isotopes and no extraneous CO₂ leaked into chambers. The on‐line ¹³C discrimination (Δ) of ryegrass stands in light was independent of δ_CΟ2 when δ_CΟ2 was constant. The δ of CO₂ exchanged by the stands in light (δ_Nd) and darkness (δ_Rn) differed by 0.7‰, suggesting some Δ in dark respiration at the stand‐level. However, Δ decreased by ～ 10‰ when δ_CΟ2 was switched from ?46.8 to ?2.5‰, and increased by ～ 10‰ following a shift from ?2.6 to ?46.7‰ due to isotopic disequilibria between photosynthetic and respiratory fluxes. Isotopic imbalances were used to assess (non‐photorespiratory) respiration in light and the replacement of the respiratory substrate pool(s) by new photosynthate. Respiration was partially inhibited by light, but increased during the light period and decreased in darkness, in association with temperature changes. The labelling kinetics of respiratory CO₂ indicated the existence of two major respiratory substrate pools: a fast pool which was exchanged within hours, and a slow pool accounting for ～ 60% of total respiration and having a mean residence time of 3.6 d.     

Carbon isotope composition of current-year shoots from Fagus sylvatica in relation to growth, respiration and use of reserves

Temporal variations in the stable carbon isotope composition (δ¹³C) of leaves and current‐year stems were examined in beech trees over one year. The δ¹³C of both tissues were equal in the bud stage and started to diverge during growth, with values decreasing by 2·5 and 4·5‰ for stems and leaves, respectively. The dynamics of the δ¹³C and content of non‐structural sugars were also assessed. The beginning of the growth period was characterized by a decrease in starch content and high starch δ¹³C values. Later in the season, the δ¹³C of leaf soluble sugars progressively decreased from the end of May and the δ¹³C of stem sucrose was at least 1·5‰ higher than that of leaves. The δ¹³C of CO₂ respired by stem tissue increased during stem growth and exhibited large seasonal variations ( from ?22·1 to ?26·3‰). These values generally fell between those of starch and total organic matter. The results of the study showed that the δ¹³C of stems is altered by two apparent fractionation steps: one during sugar transfer from leaves to stems and one during stem respiration. These results may have implications for analysis of isotopic signals in tree rings and forest ecosystems.     

Variation in the carbon and oxygen isotope composition of plant biomass and its relationship to water‐use efficiency at the leaf‐ and ecosystem‐scales in a northern Great Plains grassland

Measurements of the carbon (δ¹³C_m) and oxygen (δ¹⁸O_m) isotope composition of C₃ plant tissue provide important insights into controls on water‐use efficiency. We investigated the causes of seasonal and inter‐annual variability in water‐use efficiency in a grassland near Lethbridge, Canada using stable isotope (leaf‐scale) and eddy covariance measurements (ecosystem‐scale). The positive relationship between δ¹³C_m and δ¹⁸O_m values for samples collected during 1998–2001 indicated that variation in stomatal conductance and water stress‐induced changes in the degree of stomatal limitation of net photosynthesis were the major controls on variation in δ¹³C_m and biomass production during this time. By comparison, the lack of a significant relationship between δ¹³C_m and δ¹⁸O_m values during 2002, 2003 and 2006 demonstrated that water stress was not a significant limitation on photosynthesis and biomass production in these years. Water‐use efficiency was higher in 2000 than 1999, consistent with expectations because of greater stomatal limitation of photosynthesis and lower leaf c_i/ca during the drier conditions of 2000. Calculated values of leaf‐scale water‐use efficiency were 2–3 times higher than ecosystem‐scale water‐use efficiency, a difference that was likely due to carbon lost in root respiration and water lost during soil evaporation that was not accounted for by the stable isotope measurements.     

δ13C of CO2 respired in the dark in relation to δ13C of leaf metabolites: comparison between Nicotiana sylvestris and Helianthus annuus under drought

The variations of δ¹³C in leaf metabolites (lipids, organic acids, starch and soluble sugars), leaf organic matter and CO₂ respired in the dark from leaves of Nicotiana sylvestris and Helianthus annuus were investigated during a progressive drought. Under well‐watered conditions, CO₂ respired in the dark was ¹³C‐enriched compared to sucrose by about 4‰ in N. sylvestris and by about 3‰ and 6‰ in two different sets of experiments in H. annuus plants. In a previous work on cotyledonary leaves of Phaseolus vulgaris, we observed a constant ¹³C‐enrichment by about 6‰ in respired CO₂ compared to sucrose, suggesting a constant fractionation during dark respiration, whatever the leaf age and relative water content. In contrast, the ¹³C‐enrichment in respired CO₂ increased in dehydrated N. sylvestris and decreased in dehydrated H. annuus in comparison with control plants. We conclude that (i) carbon isotope fractionation during dark respiration is a widespread phenomenon occurring in C₃ plants, but that (ii) this fractionation is not constant and varies among species and (iii) it also varies with environmental conditions (water deficit in the present work) but differently among species. We also conclude that (iv) a discrimination during dark respiration processes occurred, releasing CO₂ enriched in ¹³C compared to several major leaf reserves (carbohydrates, lipids and organic acids) and whole leaf organic matter.     

Diffusive fractionation complicates isotopic partitioning of autotrophic and heterotrophic sources of soil respiration

Carbon isotope ratios (δ¹³C) of heterotrophic and rhizospheric sources of soil respiration under deciduous trees were evaluated over two growing seasons. Fluxes and δ¹³C of soil respiratory CO₂ on trenched and untrenched plots were calculated from closed chambers, profiles of soil CO₂ mole fraction and δ¹³C and continuous open chambers. δ¹³C of respired CO₂ and bulk carbon were measured from excised leaves and roots and sieved soil cores. Large diel variations (>5‰) in δ¹³C of soil respiration were observed when diel flux variability was large relative to average daily fluxes, independent of trenching. Soil gas transport modelling supported the conclusion that diel surface flux δ¹³C variation was driven by non‐steady state gas transport effects. Active roots were associated with high summertime soil respiration rates and around 1‰ enrichment in the daily average δ¹³C of the soil surface CO₂ flux. Seasonal δ¹³C variability of about 4‰ (most enriched in summer) was observed on all plots and attributed to the heterotrophic CO₂ source.     

Short-term variation in the isotopic composition of organic matter allocated from the leaves to the stem of Pinus sylvestris: effects of photosynthetic and postphotosynthetic carbon isotope fractionation

We aimed to quantify the separate effects of photosynthetic and postphotosynthetic carbon isotope discrimination on δ¹³C of the fast‐turn‐over carbon pool (water soluble organic carbon and CO₂ emitted from heterotrophic tissues), including their diel variation, along the pathway of carbon transport from the foliage to the base of the stem. For that purpose, we determined δ¹³C in total and water‐soluble organic matter of the foliage plus δ¹³C and δ¹⁸O in phloem organic matter of twigs and at three heights along the stem of Pinus sylvestris over a nine‐day period, including four measurements per day. These data were related to meteorological and photosynthesis parameters and to the δ¹³C of stem‐emitted CO₂. In the canopy (foliage and twigs), the δ¹³C of soluble organic matter varied diurnally with amplitudes of up to 1.9‰. The greatest ¹³C enrichment was recorded during the night/early morning, indicating a strong influence of starch storage and remobilization on the carbon isotope signatures of sugars exported from the leaves. ¹³C enrichment of soluble organic matter from the leaves to the twig phloem and further on to the phloem of the stem was supposed to be a result of carbon isotope fractionation associated with metabolic processes in the source and sink tissues. CO₂ emitted from the stem was enriched by 2.3–5.2‰ compared with phloem organic matter. When day‐to‐day variation was addressed, water‐soluble leaf δ¹³C and twig phloem δ¹⁸O were strongly influenced by c_i/c_a and stomatal conductance (G_s), respectively. These results show that both photosynthetic and postphotosynthetic carbon isotope fractionation influence δ¹³C of organic matter over time, and over the length of the basipetal transport pathway. Clearly, these influences on the δ¹³C of respired CO₂ must be considered when using the latter for partitioning of ecosystem CO₂ fluxes or when the assessment of δ¹³C in organic matter is applied to estimate environmental effects in c_i/c_a.     

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.     

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.     

The growth respiration component in eddy CO2 flux from a Quercus ilex mediterranean forest

Ecosystem respiration, arising from soil decomposition as well as from plant maintenance and growth, has been shown to be the most important component of carbon exchange in most terrestrial ecosystems. The goal of this study was to estimate the growth component of whole‐ecosystem respiration in a Mediterranean evergreen oak (Quercus ilex) forest over the course of 3 years. Ecosystem respiration (R_eco) was determined from night‐time carbon dioxide flux (F_c) using eddy correlation when friction velocity (u_*) was greater than 0.35 m s^?1 We postulated that growth respiration could be evaluated as a residual after removing modeled base R_eco from whole‐ecosystem R_eco during periods when growth was most likely occurring. We observed that the model deviated from the night‐time F_c‐based R_eco during the period from early February to early July with the largest discrepancies occurring at the end of May, coinciding with budburst when active aboveground growth and radial growth increment are greatest. The highest growth respiration rates were observed in 2001 with daily fluxes reaching up to 4 g C m^?2. The cumulative growth respiration for the entire growth period gave total carbon losses of 170, 208, and 142 g C m^?2 for 1999, 2001, and 2002, respectively. Biochemical analysis of soluble carbohydrates, starch, cellulose, hemicellulose, proteins, lignin, and lipids for leaves and stems allowed calculation of the total construction costs of the different growth components, which yielded values of 154, 200, and 150 g C for 3 years, respectively, corresponding well to estimated growth respiration. Estimates of both leaf and stem growth showed very large interannual variation, although average growth respiration coefficients and average yield of growth processes were fairly constant over the 3 years and close to literature values. The time course of the growth respiration may be explained by the growth pattern of leaves and stems and by cambial activity. This approach has potential applications for interpreting the effects of climate variation, disturbances, and management practices on growth and ecosystem respiration.     

Hair of grazing cattle provides an integrated measure of the effects of site conditions and interannual weather variability on δ13C of temperate humid grassland

The carbon isotope composition (δ¹³C) of C3 ecosystems is sensitive to water availability, and provides important information for the assessment of terrestrial carbon (C) sink/source activity. Here, we report the effects of plant available soil water (PAW) on community ¹³C signatures of temperate humid grassland. The 5‐year study was conducted on pastures exhibiting a large range of PAW capacity that were located on two site types: peat and mineral soils. The data set included the centennial drought year 2003, and data from wet years (2000 and 2002). Seasonal variation of PAW was modeled using PAW capacity of each pasture, precipitation inputs and evapotranspiration estimates. Community ¹³C signatures were derived from the δ¹³C of vegetation and segments of tail switch hair of cattle grown while grazing pastures. Hair ¹³C signatures provided an assimilation‐weighted ¹³C signal that integrated both spatial (paddock‐scale) and temporal (grazing season) variation of ¹³C signatures on a pasture. The δ¹³C of hair and vegetation increased with decreasing modeled PAW in the same way on mineral and peat soils. But, at a given PAW, the δ¹³C of hair was 2.6‰ less negative than that of vegetation, reflecting the diet‐hair isotopic shift. Furthermore, the δ¹³C of hair and vegetation on peat soil pastures was 0.5‰ more negative than on pastures situated on mineral soil. This may have resulted from a ～10 ppm CO₂ enrichment of canopy air derived from ongoing peat mineralization. Community‐scale season‐mean ¹³C discrimination (Δ) exhibited a saturation‐type response towards season‐mean modeled PAW (r²=0.78), and ranged between 19.8‰ on soils with low PAW capacity during the drought year of 2003, and 21.4‰ on soils with high PAW capacity in a wet year. This indicated relatively small variation in season‐mean assimilation‐weighted p_i/p_a (0.68–0.75) between contrasting sites and years. However, this range is similar to that reported in other studies, which encompass the range from subtropical arid to humid temperate grassland. Furthermore, the tight relationship between season‐mean Δ and modeled mean PAW suggests that PAW may be used as proxy for Δ.     

Carbon isotopic composition of forest soil respiration in the decade following bark beetle and stem girdling disturbances in the Rocky Mountains

Bark beetle outbreaks are widespread in western North American forests, reducing primary productivity and transpiration, leading to forest mortality across large areas and altering ecosystem carbon cycling. Here the carbon isotope composition (δ¹³C) of soil respiration (δ_J) was monitored in the decade after disturbance for forests affected naturally by mountain pine beetle infestation and artificially by stem girdling. The seasonal mean δ_J changed along both chronosequences. We found (a) enrichment of δ_J relative to controls (<1 ‰) in near‐surface soils in the first 2 years after disturbance; (b) depletion (1‰ or no change) during years 3–7; and (c) a second period of enrichment (1–2‰) in years 8–10. Results were consistent with isotopic patterns associated with the gradual death and decomposition of rhizosphere organisms, fine roots, conifer needles and woody roots and debris over the course of a decade after mortality. Finally, δ_J was progressively more ¹³C‐depleted deeper in the soil than near the surface, while the bulk soil followed the well‐established pattern of ¹³C‐enrichment at depth. Overall, differences in δ_J between mortality classes (<1‰) and soil depths (<3‰) were smaller than variability within a class or depth over a season (up to 6‰).     

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.     

Within-lake variability in carbon and nitrogen stable isotope signatures

1. We assessed spatial and temporal variation in carbon and nitrogen isotopic signatures in different compartments of a single lake ecosystem. Stable isotope analyses were made on samples of particulate organic matter (POM), zooplankton, periphyton, macrophytes, macroinvertebrates and fish collected from several locations throughout the ice‐free period. 2. No spatial variation in δ¹³C or δ¹⁵N values was found for pelagic samples of POM and zooplankton. However, pelagic δ¹⁵N signatures increased steadily through the summer resulting in an almost 6‰ average increase in POM and zooplankton. A concurrent decrease in epilimnetic nitrate concentrations suggested that the increase in δ¹⁵N of POM and zooplankton could have resulted from a progressive ¹⁵N‐enrichment of the available inorganic nitrogen pool as the size of this pool was reduced. 3. Significant spatial variation in isotopic ratios was observed within littoral and profundal communities. Some spatial differences were likely related to lake‐specific characteristics, such as a major inlet and a small harbour area and some were interconnected with temporal events. 4. Marked differences between spring and autumn δ¹⁵N and δ¹³C values of fish at one site probably reflected a spring spawning immigration from a larger downstream lake and also indicated limited dispersal of these immigrants. 5. Our results indicate that restricted sampling of ecosystem components from lakes may provide misleading single values for the isotope end members needed for quantitative uses of stable isotopes in mixing models and for estimating trophic position. Hence we strongly advise that studies of individual lakes, or multiple lake comparisons, that utilise stable isotope analyses should pay more attention to potential within lake spatial and temporal variability of isotope ratios.     

Determination of zooplankton dietary shift following a zebra mussel invasion, as indicated by stable isotope analysis

1. Freshwaters with established zebra mussel populations typically exhibit reduced chlorophyll a concentrations, but the subsequent impacts upon zooplankton are varied. We hypothesised that in an invaded system with less phytoplankton but available allochthonous subsidy, zooplankton may utilise greater proportions of allochthonous matter and that this could be traced by analysis of stable carbon and nitrogen isotopes. 2. We used archived zooplankton samples which had been consistently preserved and which spanned the invasion period of an Irish lake, Lough Erne. Increasing reliance upon allochthonous resources would be reflected in an increase in zooplankton δ¹³C away from phytoplankton which is relatively ¹³C‐depleted in humic‐stained L. Erne. 3. Analysis of a series of monthly samples (1992–96, 1999–2003) revealed significant ¹³C‐enrichment of mixed zooplankton, Eudiaptomus gracilis and Mysis relicta post‐zebra mussel invasion; δ¹³C values approached ?27‰ typical of terrestrial organic matter during spring and autumn. Changes in zooplankton elemental composition also suggested a switch to a lower quality diet. However, analysis of zooplankton δ¹³C from an annual, single‐point (June) time series spanning 28 years (1977–2004) suggested that when phytoplankton was sufficiently abundant, zooplankton used this resource and their δ¹³C remained relatively constant around ?32‰. Post‐invasion enrichment of mysid δ¹⁵N may reflect a shift towards carnivory, but planktonic prey abundance was reduced and a subsequent loss of body condition could result in the same isotopic changes. 4. Our results indicate that in L. Erne, when phytoplankton was reduced by zebra mussel filtering, zooplankton assimilated more from allochthonous matter, and potentially sustained a higher population than would otherwise be possible. Thus, zebra mussel impact on foodweb structure and function is likely to be different in lakes subject to varying subsidy levels.     

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.     

Isotopic biosignatures in carbonate‐rich,cyanobacteria‐dominated microbial mats of the Cariboo Plateau,B.C.

Photosynthetic activity in carbonate‐rich benthic microbial mats located in saline, alkaline lakes on the Cariboo Plateau, B.C. resulted in pCO₂ below equilibrium and δ¹³C_DIC values up to +6.0‰ above predicted carbon dioxide (CO₂) equilibrium values, representing a biosignature of photosynthesis. Mat‐associated δ¹³C_carb values ranged from ~4 to 8‰ within any individual lake, with observations of both enrichments (up to 3.8‰) and depletions (up to 11.6‰) relative to the concurrent dissolved inorganic carbon (DIC). Seasonal and annual variations in δ¹³C values reflected the balance between photosynthetic ¹³C‐enrichment and heterotrophic inputs of ¹³C‐depleted DIC. Mat microelectrode profiles identified oxic zones where δ¹³C_carb was within 0.2‰ of surface DIC overlying anoxic zones associated with sulphate reduction where δ¹³C_carb was depleted by up to 5‰ relative to surface DIC reflecting inputs of ¹³C‐depleted DIC. δ¹³C values of sulphate reducing bacteria biomarker phospholipid fatty acids (PLFA) were depleted relative to the bulk organic matter by ~4‰, consistent with heterotrophic synthesis, while the majority of PLFA had larger offsets consistent with autotrophy. Mean δ¹³C_org values ranged from ?18.7 ± 0.1 to ?25.3 ± 1.0‰ with mean Δ¹³C_inorg‐org values ranging from 21.1 to 24.2‰, consistent with non‐CO₂‐limited photosynthesis, suggesting that Precambrian δ¹³C_org values of ~?26‰ do not necessitate higher atmospheric CO₂ concentrations. Rather, it is likely that the high DIC and carbonate content of these systems provide a non‐limiting carbon source allowing for expression of large photosynthetic offsets, in contrast to the smaller offsets observed in saline, organic‐rich and hot spring microbial mats.     

Diurnal variation in the stable isotope composition of water and dry matter in fruiting Lupinus angustifolius under field conditions

In this paper, we present an integrated account of the diurnal variation in the stable isotopes of water (δD and δ¹⁸O) and dry matter (δ¹⁵N, δ¹³C, and δ¹⁸O) in the long‐distance transport fluids (xylem sap and phloem sap), leaves, pod walls, and seeds of Lupinus angustifolius under field conditions in Western Australia. The δD and δ¹⁸O of leaf water showed a pronounced diurnal variation, ranging from early morning minima near 0‰ for both δD and δ¹⁸O to early afternoon maxima of 62 and 23‰, respectively. Xylem sap water showed no diurnal variation in isotopic composition and had mean values of ?13·2 and ?2·3‰ for δD and δ¹⁸O. Phloem sap water collected from pod tips was intermediate in isotopic composition between xylem sap and leaf water and exhibited only a moderate diurnal fluctuation. Isotopic compositions of pod wall and seed water were intermediate between those of phloem and xylem sap water. A model of average leaf water enrichment in the steady state (Craig & Gordon, pp. 9–130 in Proceedings of a Conference on Stable Isotopes in Oceanographic Studies and Palaeotemperatures, Lischi and Figli, Pisa, Italy, 1965; Dongmann et al., Radiation and Environmental Biophysics 11, 41–52, 1974; Farquhar & Lloyd, pp. 47–70 in Stable Isotopes and Plant Carbon–Water Relations, Academic Press, San Diego, CA, USA, 1993) agreed closely with observed leaf water enrichment in the morning and early afternoon, but poorly during the night. A modified model taking into account non‐steady‐state effects (Farquhar and Cernusak, unpublished) gave better predictions of observed leaf water enrichments over a full diurnal cycle. The δ¹⁵N, δ¹³C, and δ¹⁸O of dry matter varied appreciably among components. Dry matter δ¹⁵N was highest in xylem sap and lowest in leaves, whereas dry matter δ¹³C was lowest in leaves and highest in phloem sap and seeds, and dry matter δ¹⁸O was lowest in leaves and highest in pod walls. Phloem sap, leaf, and fruit dry matter δ¹⁸O varied diurnally, as did phloem sap dry matter δ¹³C. These results demonstrate the importance of considering the non‐steady‐state when modelling biological fractionation of stable isotopes in the natural environment.