Stable isotopes track biogeochemical processes under seasonal ice cover in a shallow,productive lake

Biogeochemical dynamics under seasonal ice cover were investigated in the shallow (<10 m) water column of highly productive Georgetown Lake, western Montana, USA. This high altitude (1,800 m) reservoir is well-mixed in summer, but becomes strongly stratified under ice cover (mid-November–mid-May). A rapid drop in dissolved oxygen (DO) concentration and rise in dissolved inorganic carbon (DIC) concentration was observed after the onset of ice, with a corresponding increase in δ¹⁸O-DO and decrease in δ¹³C-DIC, likely caused by respiration (R) of organic carbon. Photosynthesis/respiration ratios (P/R) estimated from simultaneous measurement of DO and δ¹⁸O-DO were near unity prior to ice formation but then systematically decreased with time and depth in the lake under ice cover. P/R in the water column was higher at a shallower monitoring site compared to a deeper site near the dam outlet, which may have been important for over-winter survival of salmonids. By March, the bottom 3 m of the water column at both sites was anoxic, with the bottom 1 m being euxinic. Elevated concentrations of dissolved sulfide, ammonium, phosphate, Fe²⁺, and Mn²⁺ in deep water suggest coupling of organic carbon degradation with reduction of a number of electron acceptors (e.g., Fe³⁺, \({\text{NO}}_{3}^{ - } ,\;{\text{SO}}_{4}^{2 - }\) ). The concentrations and δ³⁴S values of H₂S in the deep water and \({\text{SO}}_{4}^{2 - }\) in the shallow water were similar, indicating near-complete reduction of sulfate in the euxinic zone. Late in the winter, an influx of isotopically heavy DIC was noted in the deep water coincident with a buildup of dissolved CH₄ to concentrations >1 mM. These trends are attributed to acetoclastic methanogenesis in the benthic sediments. This pool of dissolved CH₄ was likely released from the lake to the atmosphere during spring ice-off and lake turnover.     

Vertical change in dissolved organic carbon and oxygen at the water table region of an aquifer recharged with stormwater: biological uptake or mixing?

Decreases in dissolved organic carbon (DOC) and dissolved oxygen (DO) with increasing depth below the groundwater table are often considered as evidence for aerobic respiration; however, they may reflect mixing of infiltrating water and groundwater. We found that groundwater DOC concentration was on average 0.3 mg C l^?1 higher and DO concentration 1.5 mg O₂ l^?1 lower at recharge sites replenished with stormwater than at reference sites fed by direct infiltration of rain water from the land surface. Groundwater DOC increased and DO decreased with increasing vadose zone thickness (VZT) at both recharge and reference sites. There was no significant interaction between the effects of stormwater infiltration and VZT. Vertical changes in DOC and DO below the groundwater table at recharge sites could account for by simple mixing of infiltrating stormwater and groundwater. Moreover, aquifer sediment respiration (SR) was not significantly higher at recharge sites than at reference sites. However, slow filtration column experiments showed that SR increased significantly with an increasing supply of easily biodegradable DOC. We conclude that the observed reduction in DOC below the groundwater table at recharge sites was essentially due to water mixing rather than biological uptake because of the low biodegradability of the DOC and the short transit time of stormwater in the upper layers of groundwater. Our results highlight the need to distinguish between the effect of hydrological and biological processes on DOC and DO patterns below the groundwater before conclusions are made on the efficiency of groundwater in degrading surface-derived DOC.     

Using oxygen stable isotopes to quantify ecosystem metabolism in northern lakes

In remote regions of the world, whole lake metabolic estimates are scarce, largely because long incubations, intensive sampling and deployment of monitoring equipment are impractical. The oxygen isotope (δ¹⁸O) mass balance approach represents a simple and efficient alternative to measure whole-lake gross primary production (GPP) and respiration (R) from a single point sample, yet this option has not been extensively explored in habitats such as remote northern lakes. Here, we explored the applicability of the method using a sensitivity analysis on simulated data, showing that in large, heterotrophic (i.e., R > GPP) lakes, model outputs are sensitive to input terms for isotopic fractionation and air–water gas exchange. Despite these sensitivities, field applications of the δ¹⁸O method generated promising results that were generally consistent with parallel, free-water diel DO metabolic modelling, but greater than in vitro incubation measurements. The isotopic approach captured both wide-ranging metabolic conditions in in situ experimental mesocosms, and the seasonal trends in GPP and R in a shallow, dystrophic lake. In a clearer, deeper heterotrophic lake, the isotope approach integrated a fraction of metalimnetic metabolism missed by diel DO metabolic estimates. Overall, metalimnetic contributions to surface δ¹⁸O–DO dynamics had the greatest impact on model outputs, but with accurate information on air–water gas exchange, mixing depth, and the vertical DO and light regime of a given system, these effects can be accounted for and the isotopic approach can yield well constrained, spatio-temporally integrated rates of GPP and R. The approach is clearly suitable for use in oligo- and mesotrophic lakes, especially in remote regions where sampling is logistically difficult.     

Planktonic Microbial Communities Associated with Fracture-Derived Groundwater in a Deep Gold Mine of South Africa

The vertical distribution and function of terrestrial planktonic microbial communities at depths greater than 600 m remain poorly established. Culture-independent methods using 16S rRNA genes and geochemical approaches were employed to investigate the heterogeneity and potential function of microbial communities residing within fractures at 0.7 to 1.4 kilometers below land surface of Beatrix Au Mine, South Africa. The salinity (26 to 47 mM Cl^?), temperature (33 to 40°C) and age (1 to 5 Ma) of these fracture water increased with depth. The δD and δ¹⁸O values of fracture water ranged from ?44 to ?39‰ and from ?7 to ?4‰ VSMOW, respectively, and exhibited a mixing trend with fracture water collected from the same mine in a previous study where isotopic signatures were indicative of hydrothermal origin. Fracture water from Beatrix Mine was distinct from the groundwater in the overlying Karoo sedimentary strata in terms of its Cl^?, He and CH₄ concentrations, and its δD and δ¹⁸O signatures and from Vaal River (source of service water) in terms of its δD and δ¹⁸O signatures. The differences constrain the maximum amount of mixing with service water or shallow groundwater to be less than 4%. The 16S rDNA analyses revealed diverse and numerous novel 16S rRNA genes affiliated with Proteobacteria, Firmicutes, Nitrospira, Chlorobi, Thermus, Candidate Division OP3 and Euryarchaeota. The proportion of each phylum in clone libraries varied markedly among samples and suggests km-scale, spatial heterogeneity in community structures. Potential metabolisms inferred from the presence of 16S rRNA genes are generally consistent with estimates of the available free energy.     

Oxygen supply and the adaptations of animals in groundwater

1. The first part of this review focuses on the oxygen status of natural groundwater systems (mainly porous aquifers) and hyporheic zones of streams. The second part examines the sensitivity of groundwater organisms, especially crustaceans, to low oxygen concentrations (< 3.0 mg L^?1 O₂). 2. Dissolved oxygen (DO) in groundwater is spatially heterogeneous at macro- (km), meso- (m) and micro- (cm) scales. This heterogeneity, an essential feature of the groundwater environment, reflects changes in sediment composition and structure, groundwater flow velocity, organic matter content, and the abundance and activity of micro-organisms. Dissolved oxygen also exhibits strong temporal changes in the hyporheic zone of streams as well as in the recharge area of aquifers, but these fluctuations should be strongly attenuated with increasing distance from the stream and the recharge zone. 3. Dissolved oxygen gradients along flow paths in groundwater systems and hyporheic zones vary over several orders of magnitude (e.g. declines of 9 × 10^?5 to 1.5 ×10^?2 mg L^?1 O₂ m^?1 in confined aquifers and 2 × 10^?2 to 1 mg L^?1 O₂ m^?1 in parafluvial water). Several factors explain this strong variation. Where the water table is close to the surface, oxygen is likely to be consumed rapidly in the first few metres below the water table because of incomplete degradation of soil-generated labile dissolved organic carbon (DOC) in the vadose zone. Where the water table is far from the surface, strong oxygen depletion in the vicinity of the water table does not occur, DO being then gradually consumed as groundwater flows down the hydraulic gradient. In unconfined groundwater systems, oxygen consumption along flow paths may be compensated by down-gradient replenishment of DO, resulting either from the ingress of atmospheric oxygen or water recharge through the vadose zone. In confined groundwater systems, where replenishment of oxygen is impossible, the removal time of DO varies from a few years to more than 10 000 years, depending mainly on the organic carbon content of the sediment. Comparison of the hyporheic zones between systems also revealed strong differences in the removal time and length of underground pathways for DO. This strong variability among systems seems related to differences in contact time of water with sediment. 4. Although groundwater macro-crustaceans are much more resistant to hypoxia than epigean species, they cannot survive severe hypoxia (DO < 0.01 mg L^?1 O₂) for very long (lethal time for 50% of the population ranged from 46.7 to 61.7 h). In severe hypoxia, none of the hypogean crustaceans examined utilized a high-ATP yielding metabolic pathway. High survival times are mainly a result of the combination of three mechanisms: a high storage of fermentable fuels (glycogen and phosphagen), a low metabolic rate in normoxia, and a further reduction in metabolic rate by reducing locomotion and ventilation. It is suggested here that the low metabolic rate of many hypogean species may be an adaptation to low oxygen and not necessarily result from an impoverished food supply. 5. An interesting physiological feature of hypogean crustaceans is their ability to recover from anaerobic stress and, more specifically, rapidly to resynthesize glycogen stores during post-hypoxic recovery. A high storage and rapid restoration of fermentable fuels (without feeding) allows groundwater crustaceans to exploit a moving mosaic of suboxic (< 0.3 mg L?1 O₂), dysoxic (0.3–3.0 mg L^?1 O₂) and oxic (> 3 mg L^?1 O₂) patches. 6. It is concluded that although hypogean animals are probably unsuited for life in extensively or permanently suboxic groundwater, they can be found in small or temporarily suboxic patches. Indeed, their adaptations to hypoxia are clearly suited for life in groundwater characterized by spatially heterogeneous or highly dynamic DO concentrations. Their capacity to survive severe hypoxia for a few days and to recover rapidly would explain partly why ecological field studies often reveal the occurrence of interstitial taxa in groundwater with a wide range of DO.     

Identifying drivers of leaf water and cellulose stable isotope enrichment in <Emphasis Type="Italic">Eucalyptus</Emphasis> in northern Australia

Several previous studies have investigated the use of the stable hydrogen and oxygen isotope compositions in plant materials as indicators of palaeoclimate. However, accurate interpretation relies on a detailed understanding of both physiological and environmental drivers of the variations in isotopic enrichments that occur in leaf water and associated organic compounds. To progress this aim we measured δ¹⁸O and δ²H values in eucalypt leaf and stem water and δ¹⁸O values in leaf cellulose, along with the isotopic compositions of water vapour, across a north-eastern Australian aridity gradient. Here we compare observed leaf water enrichment, along with previously published enrichment data from a similar north Australian transect, to Craig–Gordon-modelled predictions of leaf water isotopic enrichment. Our investigation of model parameters shows that observed ¹⁸O enrichment across the aridity gradients is dominated by the relationship between atmospheric and internal leaf water vapour pressure while ²H enrichment is driven mainly by variation in the water vapour—source water isotopic disequilibrium. During exceptionally dry and hot conditions (RH < 21%, T > 37 °C) we observed strong deviations from Craig–Gordon predicted isotope enrichments caused by partial stomatal closure. The atmospheric–leaf vapour pressure relationship is also a strong predictor of the observed leaf cellulose δ¹⁸O values across one aridity gradient. Our finding supports a wider applicability of leaf cellulose δ¹⁸O composition as a climate proxy for atmospheric humidity conditions during the leaf growing season than previously documented.     

Variations of the nitrate isotopic composition in the St. Lawrence River caused by seasonal changes in atmospheric nitrogen inputs

We present 42 dual-isotope nitrate analyses of fresh water samples collected in the St. Lawrence River between June 2006 and July 2008. Measured δ¹⁵N–NO₃ ^? and δ¹⁸O–NO₃ ^? values correlate negatively, while δ¹⁸O–NO₃ ^? displays no negative correlation with nitrate concentration. This suggests that nitrate uptake and/or elimination by denitrification is not the main driver of observed variations in nitrate concentration and isotopic signature in the St. Lawrence River. In addition, δ¹⁸O–NO₃ ^? is negatively correlated with the seasonally variable δ¹⁸O of ambient water, indicating that the variation in the isotopic signature of nitrate is barely modulated by in-stream nitrate regeneration (nitrification). It rather is constrained by along-river changes in the external sources of nitrate. Given the distinct nitrogen (N) and oxygen (O) isotopic signature of atmospheric nitrate, we argue that observed seasonal variations of δ¹⁵N–NO₃ ^? and δ¹⁸O–NO₃ ^? in the St. Lawrence River are due to variable contributions of snowmelt-derived water. Based on a N and O isotope mass balance, we show that total nitrate loading in the St. Lawrence River is dominated by a N input from the Great Lakes (47 ± 28 %) and from nitrate regeneration of both internal and external N (48 ± 22 %). While temporal nitrate N and O isotope dynamics in the St. Lawrence River are mainly influenced by the atmospheric N input fluctuations, with an increase in atmospheric loading during spring, atmospheric N plays overall a rather insignificant role with regards to the N budget (5 ± 4 %).     

The water balance and water sources of a Eucalyptus plantation over shallow saline groundwater

Eucalypt plantations have been trialled in recent years as a control measure for shallow groundwater associated with secondary salinity. Uncertainty still remains as to the potential growth and water use of these plantations; these relate mainly to the problems associated with drought stress and accumulated solutes in the root zone resulting from saline groundwater uptake. This study investigates the water balance and identifies water sources of a 21 year old unirrigated Eucalyptus grandis W. Hill ex Maiden (flooded or rose gum) and E. camaldulensis Dehnh. (river red gum) plantation over shallow saline groundwater in the Shepparton Irrigation Region of northern Victoria, Australia. Water sources used by the plantation were identified using a monthly water balance approach, together with investigations of stable isotopes of water (deuterium and oxygen-18), soil water and chloride. We found these trees to be heavily reliant on rainwater, and derive approximately 15% of their transpiration requirements from saline (10,000 mg l^?1) groundwater at the capillary fringe. Rainfall at the site is relatively low (465 mm year^?1 on average) and groundwater uptake provides a stable water source that leads to a slight extension of the growing period of these trees. There is little potential for recharge, with subsurface water moving into the groundwater depression created by tree water uptake.     

Assimilation and nitrification in pelagic waters: insights using dual nitrate stable isotopes (δ<Superscript>15</Superscript>N, δ<Superscript>18</Superscript>O) in a shallow lake

Nitrate dual stable isotopes (δ¹⁵N and δ¹⁸O of NO₃ ^?) have proven to be a powerful technique to elucidate nitrogen (N) cycling pathways in aquatic systems. We applied this technique for the first time in the pelagic zone of a small temperate meso-eutrophic lake to identify the dominant N cycling pathways, and their spatial and temporal variability. We measured the lake NO₃ ^? δ¹⁵N and δ¹⁸O signatures over an annual cycle and compared them to that of the watershed. Both δ¹⁵N and δ¹⁸O of NO₃ ^? in the lake increased during summer relative to the inputs. Relationships between lake NO₃ ^? isotopic composition and concentrations were different across thermal strata with an apparent isotope effect in the epilimnion of ¹⁵ε_epi = 4.6‰ and ¹⁸ε_epi = 10.9‰. We found a strong deviation of the lake NO₃ ^? δ¹⁸O and δ¹⁵N from the expected 1:1 line for assimilation (slope = 1.73) suggesting that nitrification was co-occurring. We estimated that nitrification could support between 5 and 30% of nitrate-based production during the growing season, but was negligible in early spring and fall, and probably more dominant under ice. We showed that the technique is promising to study N processes at the ecosystem scale in shallow lakes, particularly during winter. Our results suggest that recycled NO₃ ^? could support primary productivity and influence phytoplankton composition in the surface waters of small lakes.     

Vertical and Horizontal Distribution of Bacterial Communities in Alluvial Groundwater of the Nakdong River Bank

As a collaborative effort to characterize a pilot test site for managed aquifer recharge (MAR), vertical and horizontal distributions of microbial communities in the river bank subsurface were investigated to assess the ecological effects after the operation of the MAR using the river water adjacent to the site. Along with a geochemical analysis, barcoded pyrosequencing was performed using the genomic DNAs extracted from the subsurface groundwater/sediment samples retrieved from three multilevel wells among the installed cluster of 14 boreholes. A total of 9 samples from 3 depths (10, 15–25, and 33 m below the ground surface) of each borehole showed higher bacterial abundance and diversity in the shallow (10 m) depths than in the deep (33 m) groundwater. In addition, there was a slight separation of the microbial communities between the depths based on the nonmetric multidimensional scaling analysis of the Yue and Clayton distance and the distance-weighted UniFrac analysis. The phylum Proteobacteria was dominant in all the samples at the sequence abundance of 64.0–97.8% with the total operational taxonomic units of 3375 at the species level, while among the total 288 genera, the genus Pseudomonas and an unclassified genus from Betaproteobacteria were the most abundant across the samples. The community separation between the shallow and the deep groundwater seemed to be correlated with depth differences, supported by differences in the dissolved oxygen (DO) concentration and oxidation-reduction potential (ORP). In the study site, unusually high values of electrical conductivity (EC) were found in the deep groundwater, but those values were unlikely to contribute to the community separation between the shallow and deep groundwater, unlike the DO and ORP values, which were found to influence the community differences.     

Carbonate facies‐specific stable isotope data record climate,hydrology, and microbial communities in Great Salt Lake,UT

Organic and inorganic stable isotopes of lacustrine carbonate sediments are commonly used in reconstructions of ancient terrestrial ecosystems and environments. Microbial activity and local hydrological inputs can alter porewater chemistry (e.g., pH, alkalinity) and isotopic composition (e.g., δ¹⁸O_water, δ¹³C_DIC), which in turn has the potential to impact the stable isotopic compositions recorded and preserved in lithified carbonate. The fingerprint these syngenetic processes have on lacustrine carbonate facies is yet unknown, however, and thus, reconstructions based on stable isotopes may misinterpret diagenetic records as broader climate signals. Here, we characterize geochemical and stable isotopic variability of carbonate minerals, organic matter, and water within one modern lake that has known microbial influences (e.g., microbial mats and microbialite carbonate) and combine these data with the context provided by 16S rRNA amplicon sequencing community profiles. Specifically, we measure oxygen, carbon, and clumped isotopic compositions of carbonate sediments (δ¹⁸O_carb, δ¹³C_carb, ?₄₇), as well as carbon isotopic compositions of bulk organic matter (δ¹³C_org) and dissolved inorganic carbon (DIC; δ¹³C_DIC) of lake and porewater in Great Salt Lake, Utah from five sites and three seasons. We find that facies equivalent to ooid grainstones provide time‐averaged records of lake chemistry that reflect minimal alteration by microbial activity, whereas microbialite, intraclasts, and carbonate mud show greater alteration by local microbial influence and hydrology. Further, we find at least one occurrence of ?₄₇ isotopic disequilibrium likely driven by local microbial metabolism during authigenic carbonate precipitation. The remainder of the carbonate materials (primarily ooids, grain coatings, mud, and intraclasts) yield clumped isotope temperatures (T(?₄₇)), δ¹⁸O_carb, and calculated δ¹⁸O_water in isotopic equilibrium with ambient water and temperature at the time and site of carbonate precipitation. Our findings suggest that it is possible and necessary to leverage diverse carbonate facies across one sedimentary horizon to reconstruct regional hydroclimate and evaporation–precipitation balance, as well as identify microbially mediated carbonate formation.     

Oxygen isotope fractionations across individual leaf carbohydrates in grass and tree species

Almost no δ¹⁸O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ¹⁸O relationship between leaf water and cellulose. We measured δ¹⁸O values of bulk leaf water (δ¹⁸O_LW) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ¹⁸O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally ¹⁸O‐enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (ε_bio) of more than 27‰ relative to δ¹⁸O_LW, which might be explained by isotopic leaf water and sucrose synthesis gradients. δ¹⁸O_LW and δ¹⁸O values of carbohydrates and cellulose in grasses were strongly related, indicating that the leaf water signal in carbohydrates was transferred to cellulose (ε_bio = 25.1‰). Interestingly, damping factor p_exp_x, which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ¹⁸O_LW but not if modelled directly from δ¹⁸O of individual carbohydrates. We conclude that δ¹⁸O_LW is not always a good substitute for δ¹⁸O of synthesis water due to isotopic leaf water gradients. Thus, compound‐specific δ¹⁸O analyses of individual carbohydrates are helpful to better constrain (post‐)photosynthetic isotope fractionation processes in plants.     

Modest Gaseous Nitrogen Losses Point to Conservative Nitrogen Cycling in a Lowland Tropical Forest Watershed

Primary tropical rainforests are generally considered to be relatively nitrogen (N) rich, with characteristically large hydrologic and gaseous losses of inorganic N. However, emerging evidence suggests that some tropical ecosystems can exhibit tight N cycling, with low biologically available losses. In this study, we combined isotopic data with a well-characterized watershed N mass balance to close the N budget and characterize gaseous N losses at the ecosystem scale in a lowland tropical rainforest on the Osa Peninsula in southwestern Costa Rica. We measured δ¹⁵N and δ¹⁸O of nitrate (NO₃ ^?) in precipitation, surface, shallow and deep soil lysimeters and stream water biweekly for 1 year. Enrichment of both isotopes indicates that denitrification occurs predominantly as NO₃ ^? moves from surface soil down to 15 cm depth or laterally to stream water, with little further processing in deeper soil. Two different isotopic modeling approaches suggested that the gaseous fraction comprises 14 or 32% of total N loss (2.7 or 7.5 kg N ha^?1 y^?1), though estimates are sensitive to selection of isotopic fractionation values. Gas loss estimates using the mass balance approach (3.2 kg N ha^?1 y^?1) fall within this range and include N₂O losses of 0.9 kg N ha^?1 y^?1. Overall, gaseous and soluble hydrologic N losses comprise a modest proportion (~ 25%) of the total N inputs to this ecosystem. By contrast, relatively large, episodic hydrologic losses of non-biologically available particulate N balance the majority of N inputs and may contribute to maintaining conservative N cycling in this lowland tropical forest. Similar patterns of N cycling may occur in other tropical forests with similar state factor combinations—high rainfall, steep topography, relatively fertile soils—such as the western arc of the Amazon Basin and much of IndoMalaysia, but this hypothesis remains untested.     

Isotopic heterogeneity of water in transpiring leaves: identification of the component that controls the δ18O of atmospheric O2 and CO2

Two direct but independent approaches were developed to identify the average δ¹⁸O value of the water fraction in the chloroplasts of transpiring leaves. In the first approach, we used the δ¹⁸O value of CO₂ in isotopic equilibrium with leaf water to reconstruct the δ¹⁸O value of water in the chloroplasts. This method was based on the idea that the enzyme carbonic anhydrase facilitates isotopic equilibrium between CO₂ and H₂O predominantly in the chloroplasts, at a rate that is several orders of magnitude faster than the non-catalysed exchange in other leaf water fractions. In the second approach, we measured the δ¹⁸O value of O₂ from photosynthetic water oxidation in the chloroplasts of intact leaves. Since O₂ is produced from chloroplast water irreversibly and without discrimination, the δ¹⁸O value of the O₂ should be identical to that of chloroplast water. In intact, transpiring leaves of sunflower (Helianthus annuus cv. giant mammoth) under the experimental conditions used, the average δ¹⁸O value of chloroplasts water was displaced by 3—10 % (depending on relative humidity and atmospheric composition) below the value predicted by the conventional Craig & Gordon model. Furthermore, this δ¹⁸O value was always lower than the δ¹⁸O value that was measured for bulk leaf water. Our results have implications for a variety of environmental studies since it is the δ¹⁸O value of water in the chloroplasts that is the relevant quantity in considering terrestrial plants influence on the δ¹⁸O values of atmospheric CO₂ and O₂, as well as in influencing the δ¹⁸O of plant organic matter.     

Stromatolite records of environmental change in perennially ice-covered Lake Joyce,McMurdo Dry Valleys,Antarctica

Calcite-rich columnar stromatolites grew in perennially ice-covered Lake Joyce in the McMurdo Dry Valleys, Antarctica, during a period of environmental change associated with rising lake level. Stromatolite calcite contains carbon and oxygen isotope records of changes to microbial activity in response to variable light environments and water chemistry through time. The stromatolites grew synchronously with correlative calcite zones. The innermost (oldest) calcite zone has a wide range of δ¹³C_calcite values consistent with variable photosynthetic effects on local DIC ¹³C/¹²C. Subsequent calcite zones preserve a progressive enrichment in δ¹³C_calcite values of approximately + 2.6‰ through time, with δ¹³C_calcite values becoming less variable. This enrichment likely records the removal of ¹²C by photosynthesis from the DIC reservoir over decades, with photosynthetic effects decreasing as light levels became lower and more consistent through time. Mean δ¹⁸O_calcite values of the innermost calcified zone were at least 1‰ lower than those of the other calcified zones (t test p-level < 0.001). The significant difference in δ¹⁸O_calcite values between the innermost and other calcified zones could be a product of mixing source waters with different isotopic values associated with the initiation of lake stratification associated with rising lake level. Overall, Lake Joyce stromatolites record significant lateral variability in relative photosynthetic rate and long-lived lake water stratification with microbial modification of the DIC pool. Such processes provide criteria for interpreting microbial activity within polar paleolake deposits and may shed light on variability in lake environments associated with changing climate in the McMurdo Dry Valleys.     

Stable isotopes of lakes and precipitation along an altitudinal gradient in the Eastern Alps

Altitude encompasses broad environmental gradients that influence the isotopic composition of lake water. We selected 55 lakes in the Eastern Alps along an altitudinal gradient [214–2,532 m above sea level (a.s.l.)] to model the isotopic signal of surface water dependent on intrinsic (lake geomorphometry) and extrinsic (air temperature, precipitation) factors. Ordinary and generalised least squared regression were used for statistical analysis. The isotope signal of lake water was lower in spring than in summer and decreased with altitude (?0.21 δ¹⁸O ‰/100 m; ?1.5 δ²H ‰/100 m). This pattern largely depended on temperature and a pseudo-latitude effect. The isotopic signal in monthly precipitation (12 stations; altitudinal gradient 90–2,730 m a.s.l.) generally showed the expected pattern of less enriched values with altitude; however, unusual values were related to weather anomalies. The local meteoric water line was similar to the global meteoric water line as shown by overlapping confidence intervals. By discriminating different elevational bands, we could show that high elevation lakes (>1,500 m a.s.l.) experience different patterns of evaporation with respect to low elevation lakes (<1,500 m a.s.l.). Our study showed that lakes have a unique isotopic fingerprint along an altitudinal gradient, potentially useful for tracing ecological processes and for paleoclimatic studies.     

Past climate changes and ecophysiological responses recorded in the isotope ratios of saguaro cactus spines

The stable isotope composition of spines produced serially from the apex of columnar cacti has the potential to be used as a record of changes in climate and physiology. To investigate this potential, we measured the δ¹⁸O, δ¹³C and F¹⁴C values of spines from a long-lived columnar cactus, saguaro (Carnegiea gigantea). To determine plant age, we collected spines at 11 different heights along one rib from the stem apex (3.77 m height) to the base of a naturally occurring saguaro. Fractions of modern carbon (F¹⁴C) ranged from 0.9679 to 1.5537, which is consistent with ages between 1950 and 2004. We observed a very strong positive correlation (r = 0.997) between the F¹⁴C age of spines and the age of spines determined from direct and repeated height measurements taken on this individual over the past 37 years. A series of 96 spines collected from this individual had δ¹⁸O values ranging from 38‰ to 50‰ [Vienna standard mean ocean water (VSMOW)] and δ¹³C values from ?11.5‰ to ?8.5‰ [Vienna Peedee belemnite (VPDB)]. The δ¹⁸O and δ¹³C values of spines were positively correlated (r = 0.45, P < 0.0001) and showed near-annual oscillations over the ～15-year record. This pattern suggests that seasonal periods of reduced evaporative demand or greater precipitation input may correspond to increased daytime CO₂ uptake. The lowest δ¹⁸O and δ¹³C values of spines observed occurred during the 1983 and 1993 El Niño years, suggesting that the stable isotope composition recorded in spine tissue may serve as a proxy for these climate events. We compared empirical models and data from potted experimental cacti to validate these observations and test our hypotheses. The isotopic records presented here are the first ever reported from a chronosequence of cactus spines and demonstrate that tissues of columnar cacti, and potentially other long-lived succulents, may contain a record of past physiological and climatic variation.     

Reconstructing the δ18O of atmospheric water vapour via the CAM epiphyte Tillandsia usneoides: seasonal controls on δ18O in the field and large‐scale reconstruction of δ18Oa

Using both oxygen isotope ratios of leaf water (δ¹⁸O_L) and cellulose (δ¹⁸O_C) of Tillandsia usneoides in situ, this paper examined how short‐ and long‐term responses to environmental variation and model parameterization affected the reconstruction of the atmospheric water vapour (δ¹⁸O_a). During sample‐intensive field campaigns, predictions of δ¹⁸O_L matched observations well using a non‐steady‐state model, but the model required data‐rich parameterization. Predictions from the more easily parameterized maximum enrichment model (δ¹⁸O_L–M) matched observed δ¹⁸O_L and observed δ¹⁸O_a when leaf water turnover was less than 3.5 d. Using the δ¹⁸O_L–M model and weekly samples of δ¹⁸O_L across two growing seasons in Florida, USA, reconstructed δ¹⁸O_a was ?12.6 ± 0.3‰. This is compared with δ¹⁸O_a of ?12.4 ± 0.2‰ resolved from the growing‐season‐weighted δ¹⁸O_C. Both of these values were similar to δ¹⁸O_a in equilibrium with precipitation, ?12.9‰. δ¹⁸O_a was also reconstructed through a large‐scale transect with δ¹⁸O_L and the growing‐season‐integrated δ¹⁸O_C across the southeastern United States. There was considerable large‐scale variation, but there was regional, weather‐induced coherence in δ¹⁸O_a when using δ¹⁸O_L. The reconstruction of δ¹⁸O_a with δ¹⁸O_C generally supported the assumption of δ¹⁸O_a being in equilibrium with precipitation δ¹⁸O (δ¹⁸O_ppt), but the pool of δ¹⁸O_ppt with which δ¹⁸O_a was in equilibrium – growing season versus annual δ¹⁸O_ppt – changed with latitude.     

Using stable hydrogen and oxygen isotopes to study water movement in soil-plant-atmosphere continuum at Poyang Lake wetland,China

Water movement in the soil-plant-atmosphere continuum (SPAC) has a significant effect on the biogeochemical process in wetlands. This study investigated the water movement in the SPAC in Poyang Lake wetland, which is a protected area with an important ecological function within the Yangtze River basin, under different water-level conditions by analyzing the responses of river, groundwater, soil and plants to precipitation using stable hydrogen and oxygen isotopes. The results show that the stable hydrogen and oxygen isotopic compositions (δ¹⁸O and δD) of soil water decrease with increasing depth due to the near surface evaporation. During the dry season the water-level in Poyang Lake is low, when it rains the influencing depth of precipitation and evaporation on soil water isotopic signatures was 20 cm below the ground surface. The rain water infiltrates into the soil, recharges groundwater and flows to the river. When the water-level in Poyang Lake is low, the Xiu River is recharged by the groundwater, which recharges the soil water by capillary rise. During the flood season, the water-level is high and the water in Poyang Lake reaches or covers the meadows, recharges the groundwater and soil water. In the meantime, the water in Poyang Lake can be recharged by rain water when it rains. During the dry season when it doesn’t rain, plants mainly use groundwater, but soil water is preferred and plants don’t use rainwater directly when it rains. When the lake water-level is extremely low, the plants in Poyang Lake wetland may suffer from water stress, which is harmful for plant growth.     

Fatty acid and stable isotope characteristics of sea ice and pelagic particulate organic matter in the Bering Sea: tools for estimating sea ice algal contribution to Arctic food web production

We determined fatty acid (FA) profiles and carbon stable isotopic composition of individual FAs (δ¹³C_FA values) from sea ice particulate organic matter (i-POM) and pelagic POM (p-POM) in the Bering Sea during maximum ice extent, ice melt, and ice-free conditions in 2010. Based on FA biomarkers, differences in relative composition of diatoms, dinoflagellates, and bacteria were inferred for i-POM versus p-POM and for seasonal succession stages in p-POM. Proportions of diatom markers were higher in i-POM (16:4n-1, 6.6–8.7 %; 20:5n-3, 19.6–25.9 %) than in p-POM (16:4n-1, 1.2–4.0 %; 20:5n-3, 5.5–14.0 %). The dinoflagellate marker 22:6n-3/20:5n-3 was highest in p-POM. Bacterial FA concentration was higher in the bottom 1 cm of sea ice (14–245 μg L^?1) than in the water column (0.6–1.7 μg L^?1). Many i-POM δ¹³C_FA values were higher (up to ~10 ‰) than those of p-POM, and i-POM δ¹³C_FA values increased with day length. The higher i-POM δ¹³C_FA values are most likely related to the reduced dissolved inorganic carbon (DIC) availability within the semi-closed sea ice brine channel system. Based on a modified Rayleigh equation, the fraction of sea ice DIC fixed in i-POM ranged from 12 to 73 %, implying that carbon was not limiting for primary productivity in the sympagic habitat. These differences in FA composition and δ¹³C_FA values between i-POM and p-POM will aid efforts to track the proportional contribution of sea ice algal carbon to higher trophic levels in the Bering Sea and likely other Arctic seas.