Stable Isotope Values Research Articles

Lack of detectable chemosynthesis at a sponge dominated subarctic methane seep

We used high-resolution imagery within a Geographic Information System (GIS), free gas and porewater analyses and animal bulk stable isotope measurements to characterize the biotic and abiotic aspects of the newly discovered Vestbrona Carbonate Field (VCF) seep site on the Norwegian shelf (63°28′N, 6° 31′E, ∿270 m water depth). Free gas was mainly composed of microbial methane. Sediment porewater sulfide concentrations were in the millimolar range and thus high enough to sustain seep chemosymbiotrophic animals. Nonetheless, the VCF lacked chemosymbiotrophic animals despite an abundance of methane-derived carbonate crusts which are formed by the same anaerobic processes that sustain chemosymbiotrophic animals at seeps. Furthermore, none of the sampled taxa, across various trophic guilds exhibited a detectable contribution of chemosynthetically fixed carbon to their diets based on bulk stable isotope values, suggesting a predominantly photosynthetic source of carbon to the VCF seep food web. We link the absence of chemosymbiotrophic animals to highly localized methane flow pathways, which may act as a “shunt-bypass” of the anaerobic oxidation of methane (AOM) and by extension sulfide generation, thus leading to sediment sulfide concentrations that are highly heterogeneous over very short lateral distances, inhibiting the successful colonization of chemosymbiotrophic animals at the VCF seep. Instead, the seep hosted diverse biological communities, consisting of heterotrophic benthic fauna, including long lived taxa, such as soft corals (e.g., Paragorgia arborea) and stony corals (i.e., Desmophyllum pertusum, formerly known as Lophelia pertusa). Compared to the surrounding non-seep seafloor, we measured heightened megafaunal density at the seep, which we attribute to increased habitat heterogeneity and the presence of a variety of hard substrates (i.e., methane-derived authigenic carbonates, dropstones and coral rubble), particularly since the most abundant taxa all belonged to the phylum Porifera. Compared to the surrounding non-seep seafloor, marine litter was denser within the VCF seep, which we link to the more variable local topography due to authigenic carbonates, which can rip off parts of bottom trawling nets thereby making the seep act as catchment area for marine litter.

Rainfall intensity affects the recharge mechanisms of groundwater in a headwater basin of the North China plain

Understanding the origins of groundwater and its movement from mountain to plain during different high intensity rainfall events is critical for conserving water supplies, determining water-use policies and controlling pollution. These factors are also the keys for understanding the dominant processes in hydrological models. In this study, groundwater resources and recharge processes during heavy precipitation were explored by using stable isotope tracers in the hilly area of Taihang Mountain. It was found that the δ2H and δ18O values of precipitation exhibited obvious precipitation amount effect during different precipitation intensity events. The stable isotopic values in groundwater and river water showed significantly varied during the single extreme heavy precipitation and the continuous heavy precipitation events. In the rainy season, precipitation amounts greater than 40 mm/d could effectively recharge the shallow groundwater in the study area. By comparing the spatial isotopic distribution of groundwater, soil water, and river water with precipitation, we showed distinct groundwater recharge patterns in terms of their water resources, timing, and the degree of river water and groundwater interaction during the single extreme heavy precipitation and the continuous heavy precipitation. After the single extreme heavy precipitation, the δ2H and δ18O values of groundwater, soil water, and river water showed stable over time and had the same similar variations, suggested that the groundwater recharge was mainly dominated by precipitation with preferential flow or bypass flow. While after the continuous heavy precipitation, the variation of δ2H and δ18O in all water is consistent with the previous precipitation, which shown a mixing effect of previous enrich precipitation and depleted heavy precipitation, suggested that groundwater source was dominated by a continuous recharge of previous heavier precipitation with translatory flow. The groundwater main recharge mechanism is not constant, but changes with rainfall intensity. The rainfall intensity play an important role in groundwater recharge change affecting runoff process. Overall, this paper presents a new insight to understand the effect of rainfall intensity on hydrological process, which could be used to provide vital information in the semi-humid and semi-arid regions where water resources are critical in climate change adaptation strategies.

The Mechanisms Controlling the CO2 Outgassing of a Karst Spring–River–Lake Continuum: Evidence from Baotuquan Spring Drainage Area, Jinan City, Northern China

The significance of CO2 emissions at the water–air interface from inland water bodies in the global carbon cycle has been recognized and is being studied more and more. Although it is important to accurately assess CO2 emission flux in a catchment, little research has been carried out to investigate the spatio-temporal variations in CO2 emissions in view of a water continuum. Here, we systematically compared the differences and control factors of CO2 degassing across the water–air interface of a spring–river–lake continuum in the discharge area of Baotuquan Spring in July 2017, which is a typical temperate karst spring area in Jinan city, northern China, using hydrogeochemical parameters, stable carbon isotope values, and CO2 degassing flux. Affected by the pCO2 concentration gradient between the water and ambient air, the spring water showed a high CO2 degassing flux (166.19 ± 91.91 mmol/(m2 d)). After the spring outlet, the CO2 degassing flux in the spring-fed river showed a slight increase (181.05 ± 155.61 mmol/(m2 d)) due to river flow rate disturbance. The river flow rate was significantly reduced by the “blockage” of the lake, which promoted the survival and reproduction of phytoplankton and provided favorable conditions for aquatic plant photosynthesis, increasing the plankton biomass in the lake to 3383.79 × 104/L. In addition, the significant decrease in the dissolved inorganic carbon (DIC) concentration and the increase in the δ13CDIC values in the lake also indicated that the photosynthesis of the lake’s aquatic plants resulted in a significant decrease in the pCO2 concentration, thus limiting the amount of CO2 off-gassing (90.56 ± 55.03 mmol/(m2 d)).

Assessing the effects of embedding resins on carbonate stable and clumped isotope analyses.

Embedding resins are widely used to fix carbonates for high-precision sample preparation and high-resolution sampling. However, these embedding materials are difficult to remove after sample preparation and are known to affect the accuracy of carbonate stable isotope analyses. Nevertheless, their impact on clumped isotope analysis, which is particularly sensitive to contamination artifacts, has so far not been tested. The observation that running resin-containing samples decreased the reproducibility of clumped isotope values for internal laboratory carbonate standards and increased the external standard deviation (SD 0.061-0.088‰) compared to the long-term observations (0.034‰), prompted us to set up an experiment to test the influence of resin addition on instrument performance. Here we analyzed the stable and clumped isotope composition of a pure calcium carbonate standard (ETH-4) mixed with three types of embedding resins in 2:1 and 1:1 proportions. Our aim was to assess how resin addition affects isotope analyses. We found that none of the stable isotopic values were significantly different. The δ13 C values were -10.22 ± 0.07‰ (mean ± SD) for pure ETH-4, while the δ13 C values of ETH-4 mixed with embedding resins in 2:1 and 1:1 proportions were -10.21 ± 0.06‰ and -10.18 ± 0.06‰, respectively (p> 0.05). The δ18 O values were -18.82 ± 0.11‰ for pure ETH-4 versus -18.81 ± 0.09‰ and -18.82 ± 0.08‰ for 2:1 and 1:1 ETH-4:resin mixtures, respectively (p> 0.05). Given the large uncertainty in our results, we did not find significant differences between different mixtures in the carbonate clumped isotope values (Δ47 ), with 0.458 ± 0.107‰, 0.464 ± 0.086‰, and 0.417 ± 0.089‰ in pure ETH-4 and ETH-4 with 2:1 and 1:1 resin mixtures, respectively (p> 0.05). However, a resin-related bias in the results might be masked by the large uncertainty. The measured ETH-4 values in our study are similar to the InterCarb values (δ13 C = -10.20‰, δ18 O = -18.81‰, Δ47 = 0.450‰, InterCarb-Carbon Dioxide Equilibrium Scale). However, the external SD of Δ47 in sessions measuring ETH-4 with resins is higher than in sessions without deliberate resin addition for the same measuring period. We find that the potential contamination from the resin addition leads to a larger variability for Δ47 values in sessions measuring ETH-4 including resins. We therefore recommend purification of embedded samples using a contamination trap with Porapak prior to analysis, if possible, or avoiding resins during sample preparation and workup, as well as monitoring the measurement quality during and after sessions with samples containing embedding resins.

The prospects of poop: a review of past achievements and future possibilities in faecal isotope analysis.

What can the stable isotope values of human and animal faeces tell us? This often under-appreciated waste product is gaining recognition across a variety of disciplines. Faecal isotopes provide a means of monitoring diet, resource partitioning, landscape use, tracking nutrient inputs and cycling, and reconstructing past climate and environment. Here, we review what faeces are composed of, their temporal resolution, and how these factors may be impacted by digestive physiology and efficiency. As faeces are often used to explore diet, we clarify how isotopic offsets between diet and faeces can be calculated, as well as some differences among commonly used calculations that can lead to confusion. Generally, faecal carbon isotope (δ13 C) values are lower than those of the diet, while faecal nitrogen isotope values (δ15 N) values are higher than in the diet. However, there is considerable variability both within and among species. We explore the role of study design and how limitations stemming from a variety of factors can affect both the reliability and interpretability of faecal isotope data sets. Finally, we summarise the various ways in which faecal isotopes have been applied to date and provide some suggestions for future research. Despite remaining challenges, faecal isotope data are poised to continue to contribute meaningfully to a variety of fields.

Charring effects on stable carbon and nitrogen isotope values on C4 plants: Inferences for archaeological investigations

Experimental studies demonstrated that charring affects stable isotope values of plant remains. Therefore, it is necessary to consider the impact of charring to reliably interpret δ13C and δ15N values in archaeobotanical remains before using this approach to reconstruct past water management, paleoclimatic changes, and infer paleodietary patterns. Research so far has focused mostly on C3 plants while the charring effect on C4 plants is less understood. This study explored the effects of charring on δ13C, δ15N, %C, %N, and C:N in grains of two C4 species, Sorghum bicolor (L.) Moench (NADP-ME) and Cenchrus americanus (L.) Morrone (heterotypic synonym Pennisetum glaucum (L.) R.Br.) (NAD-ME), grown under the same controlled environmental conditions (watering, light, atmospheric humidity). Sorghum and pearl millet grains were charred from 1 to 3 h at 200–300 °C. Comparing first the uncharred grains, the results show that sorghum has lower δ15N and higher δ13C values than pearl millet. This evidence is also recorded in the charred grains. The charring experiments indicate that the temperature to which the grains are exposed has a higher impact than time on the preservation, mass loss, %C, %N, C:N, and δ13C and δ15N values. Every 50 °C of increase resulted in significant increases of δ15N (+0.37‰) and of δ13C (+0.06‰) values. Increasing the duration of charring to 3 h resulted in significant changes of δ15N (+0.17‰) and no significant changes for δ13C (−0.04‰) values. The average charring effects estimated in our experiment is 0.27‰ (95% CI between −0.02 and 0.56) for δ15N and −0.18‰ (95% CI between −0.30 and −0.06‰) for δ13C. Considering the average values, our data show that pearl millet is more affected by charring than sorghum; however, according to the standard deviations, sorghum shows a greater variability charring effect than pearl millet. This study provides new information to correctly assessing the isotopic values obtained from ancient C4 crops, providing a charring offset specific for C4 plants. Furthermore, it suggests that NAD-ME and NADP-ME species present isotopic differences under the same growing conditions and this must be taken into account in analytical works on ancient C4 crops.

Protocols for sample preparation and compound-specific stable-isotope analyses (δ2H, δ13C) of fatty acids in biological and environmental samples

Compound-specific stable-isotope analysis (CSIA) of fatty acids is a powerful tool to better understand the trophic transfer of fatty acids and their biochemical fate in and across ecosystems, including tracing animal migration and understanding physiological processes. The non-exchangeable nature of CH bonds in acyl chains, hydrogen (δ2H) and carbon (δ13C) stable-isotope values of fatty acids (FA) provide independent information about the origins of fatty acids. Several technical obstacles must be overcome to ensure accurate and reproducible measurements of FA-CSIA can be made. This protocol describes the sample preparation process for successful stable-isotope analyses of fatty acids obtained from environmental and biological samples. Numerous techniques for the preanalytical processing of fatty acid samples are available, and these often have minimal impact on δ values. Here, we provide an in-depth guide detailing our well-established laboratory protocols, ranging from the initial sample preparation, lipid extraction, and transmethylation to the instrumental arrangement, data collection, and analysis.•Protocol from obtaining a sample to standardized fatty acid specific δ2H and δ13C values.•Separate GC analysis procedures for C and H are recommended for optimal performance.

Radical-Driven Methane Formation in Humans Evidenced by Exogenous Isotope-Labeled DMSO and Methionine.

Methane (CH4), which is produced endogenously in animals and plants, was recently suggested to play a role in cellular physiology, potentially influencing the signaling pathways and regulatory mechanisms involved in nitrosative and oxidative stress responses. In addition, it was proposed that the supplementation of CH4 to organisms may be beneficial for the treatment of several diseases, including ischemia, reperfusion injury, and inflammation. However, it is still unclear whether and how CH4 is produced in mammalian cells without the help of microorganisms, and how CH4 might be involved in physiological processes in humans. In this study, we produced the first evidence of the principle that CH4 is formed non-microbially in the human body by applying isotopically labeled methylated sulfur compounds, such as dimethyl sulfoxide (DMSO) and methionine, as carbon precursors to confirm cellular CH4 formation. A volunteer applied isotopically labeled (2H and 13C) DMSO on the skin, orally, and to blood samples. The monitoring of stable isotope values of CH4 convincingly showed the conversion of the methyl groups, as isotopically labeled CH4 was formed during all experiments. Based on these results, we considered several hypotheses about endogenously formed CH4 in humans, including physiological aspects and stress responses involving reactive oxygen species (ROS). While further and broader validation studies are needed, the results may unambiguously serve as a proof of concept for the endogenous formation of CH4 in humans via a radical-driven process. Furthermore, these results might encourage follow-up studies to decipher the potential physiological role of CH4 and its bioactivity in humans in more detail. Of particular importance is the potential to monitor CH4 as an oxidative stress biomarker if the observed large variability of CH4 in breath air is an indicator of physiological stress responses and immune reactions. Finally, the potential role of DMSO as a radical scavenger to counteract oxidative stress caused by ROS might be considered in the health sciences. DMSO has already been investigated for many years, but its potential positive role in medical use remains highly uncertain.

Global investigation of lake habitat coupling by fishes.

Habitat coupling, where consumers acquire resources from different habitats, plays an important role in ecosystem functioning. In this study, we provide a global investigation of lake habitat coupling by freshwater fishes between littoral (nearshore) and pelagic (open water) zones and elucidate the extent to which magnitude of coupling varies according to environmental context and consumer traits. We consider the influence of lake factors (surface area, depth, shoreline complexity, and annual temperature), relative trophic position of consumers, fish community species richness, and fish morphological traits on habitat coupling by fishes. Using a worldwide dataset consisting of fish stable isotope values (δ13C and δ15N), we developed an index of habitat coupling, and used Bayesian hierarchical and non-hierarchical beta regressions to estimate the effects of environmental lake context and morphological traits on habitat coupling by fishes. Our results show high rates of habitat coupling among fishes globally with marked taxonomic differences in the magnitude and variation. Habitat coupling was higher in lower elevation lakes and in regions characterized by relatively colder climates, whereas other environmental context factors had little or no effects on habitat coupling. Furthermore, habitat coupling was associated with several locomotion and feeding traits, but independent from species maximum body length. Overall, we highlight the prevalence of multiple resources supporting fish populations and suggest future research identify implications to ecosystem functioning that may result from alterations to habitat coupling by fishes.

The planktonic food web in the Gulf of Naples based on the analysis of carbon and nitrogen stable isotope ratios

AbstractPlankton play a key role in marine food webs by producing and transferring organic matter and energy to higher trophic levels. To define the trophic structure and interactions within the planktonic communities in the Gulf of Naples, we determined carbon and nitrogen stable isotope ratios in particulate organic matter (POM, <20 μm), phytoplankton (20–200 μm), and bulk (unsorted) and sorted mesozooplankton (200–2000 μm) on a weekly basis in 2019. The stable isotope values of POM and, to a lesser extent, phytoplankton reflected the short‐term (weekly) alternation between offshore and coastal waters within seasonal variability. Although the isotopic signatures of coastal and offshore sources were still detectable in bulk mesozooplankton, δ13C and δ15N of single groups remained almost unchanged throughout the year. The trophic niches of mesozooplankton groups defined using the Stable Isotope Bayesian Ellipses in R software showed a high degree of overlap among them, which was corroborated by the tendency of most groups toward omnivory, beyond their known feeding habits, as indicated by MixSIAR models. Collectively, our results highlight that the complex network of interactions within the planktonic community in the Gulf of Naples can buffer natural variability due to the specific hydrographic features of the system. This characteristic makes the trophic structure of this community an ideal model for monitoring the response of planktonic food webs to climate‐ and anthropic‐driven changes.

Оценка состава водных масс восточной части Баренцева моря с использованием современных данных о параметрах δ18O и δ2Н

This paper presents the results of measurements of δ18O and δ2H stable isotope values in the waters of the Barents Sea, which were carried out in March–April 2021. Based on the data presented, the ratio of base water content – atlantic (fa), river (fr) and melt water (fi), as well as the volume of water withdrawn for ice formation in the studied water area was assessed. The comparative estimation of base water content, calculated on values “δ18O–salinity” and “δ2H–salinity” is presented and obtained values do not exceed limits of mean value ±sd. However, average values of Atlantic and ice water content calculated using δ2H parameter are higher than those using δ18O, and river water content is lower. The use of δ18O and δ2H parameters in oceanographic studies in the calculation of balance characteristics is preferable to temperature and salinity because of their greater conservativity.

Stable Isotope Signatures in Tehran’s Precipitation: Insights from Artificial Neural Networks, Stepwise Regression, Wavelet Coherence, and Ensemble Machine Learning Approaches

This study investigates the impact of precipitation on Middle Eastern countries like Iran using precise methods such as stable isotope techniques. Stable isotope data for precipitation in Tehran were obtained from the Global Network of Isotopes in Precipitation (GNIP) station and sampled for two periods: 1961–1987 and 2000–2004. Precipitation samples were collected, stored, and shipped to a laboratory for stable isotope analyses using the GNIP procedure. Several models, including artificial neural networks (ANNs), stepwise regression, and ensemble machine learning approaches, were applied to simulate stable isotope signatures in precipitation. Among the studied machine learning models, XGboost showed the most accurate simulation with higher R2 (0.84 and 0.86) and lower RMSE (1.97 and 12.54), NSE (0.83 and 0.85), AIC (517.44 and 965.57), and BIC values (531.42 and 979.55) for 18O and 2H compared to other models, respectively. The uncertainty in the simulations of the XGboost model was assessed using the bootstrap technique, indicating that this model accurately predicted stable isotope values. Various wavelet coherence analyses were applied to study the associations between stable isotope signatures and their controlling parameters. The BWC analysis results show coherence relationships, mainly ranging from 16 to 32 months for both δ18O–temperature and δ2H–temperature pairs with the highest average wavelet coherence (AWC). Temperature is the dominant predictor influencing stable isotope signatures of precipitation, while precipitation has lower impacts. This study provides valuable insights into the relationship between stable isotopes and climatological parameters of precipitation in Tehran.

Fertilizer restrictions are not sufficient to mitigate nutrient pollution and harmful algal blooms in the Indian River Lagoon, Florida

In Florida's Indian River Lagoon (IRL), anthropogenic eutrophication has resulted in harmful algal blooms and catastrophic seagrass losses. Hoping to improve water quality, policy makers enacted fertilizer bans, assuming that this would reduce the nitrogen (N) load. To assess the effectiveness of these bans, seawater and macroalgal samples were collected at 20 sites “pre” and ~ five-years “post” bans and analyzed to determine concentrations of dissolved nutrients and stable nitrogen isotope values (δ15N). Higher concentrations of ammonium and nitrate were observed post-ban and macroalgal δ15N values increased. A comparison of nutrient concentrations and δ15N between brown tide (Aureoumbra lagunensis) blooms indicated that the post-ban bloom was more strongly N-enriched with higher δ15N values than the pre-ban bloom, which had depleted values in the range of fertilizers. These data indicate a primary role of human waste influence in the IRL, suggesting that current management actions have been insufficient at mitigating eutrophication.

Movement and population dispersal of dolphinfish (Coryphaena hippurus) across the Eastern Tropical Pacific inferred from carbon and nitrogen stable isotope analyses.

The dolphinfish (Coryphaena hippurus) is a globally distributed marine predator that supports one of the most important coastal fisheries along the Eastern Tropical Pacific (ETP), but its spatial movements in this area are poorly understood. Stable isotope values (δ13 C and δ15 N) of white muscle from dolphinfish (n = 220) captured at different locations across the ETP (i.e., Mexico, Costa Rica, Ecuador, Peru and oceanic areas) were normalized to copepod baseline stable isotope values to estimate dolphinfish trophic position, movements and population dispersal. Movement or residence patterns were inferred from the difference in δ15 N values (Δ15 Ndolphinfish-copepod ‰) between copepods and dolphinfish muscle. Baseline corrected isotope values (δ13 Cdolphinfish-copepod and δ15 Ndolphinfish-copepod ) of dolphinfish muscle were used to estimate isotopic niche metrics and infer population dispersal across isoscapes. Values of δ13 C and δ15 N differed between juvenile and adult dolphinfish and across the ETP. Trophic position estimates ranged from 3.1 to 6.0 with a mean of 4.6. Adults and juveniles had similar trophic position estimates, whereas isotopic niche areas (SEA ‰2 ) of adults were greater relative to juveniles in every location. Adult dolphinfish showed "moderate movement by some individuals" in all locations based on Δ15 Ndolphinfish-copepod values, except for Costa Rica where adults were classified with "high degree of movement by some individuals" whereas juveniles showed "limited movement" in all areas except Mexico. Population dispersal based on Δ15 Ndolphinfish-copepod values showed "moderate" and "high" dispersal for adults and "no dispersal" for most juveniles, except for Mexico. This study provides insight into potential dolphinfish spatial mobility across an area of interest for multiple nations, which can help to improve stock assessments and management of the species.

Molecular and isotopic signatures of indigenous and non-indigenous hydrocarbons in Campanian sediments from the Tano Basin, Ghana

The Campanian source strata of the Tano Basin, Ghana, have garnered exploration interest based on the recent discoveries of hydrocarbon shows in some offshore wells. This study reports on the bulk geochemical composition (Total organic carbon content and Rock-Eval pyrolysis parameters), molecular markers, and stable carbon isotope (δ13C) composition of eleven Campanian sediments from TP–1 and Ankobra–1 well, a shallow and deep-water well in the Tano Basin respectively. Based on the analysis of total organic carbon content and Rock-Eval pyrolysis parameters (S1, S2, and Tmax), it was found that the TP-1 Campanian sediments contain indigenous hydrocarbons. Contrariwise, the Campanian samples from the Ankobra-1 well are mainly composed of non-indigenous hydrocarbons, possibly originating from offshore drilling fluid contaminants. Distribution of saturated and aromatic hydrocarbons, including normal alkanes, acyclic isoprenoids, steranes, naphthalenes, phenanthrenes, dibenzothiophene, dibenzofuran, fluorenes, and their alkyl derivatives, as well as stable carbon isotope (δ13C) values, were analyzed to gain insight into the biological source input, depositional environment, and thermal maturity of the organic matter (OM) in the sediments. The molecular and isotope results indicated that the OM in the indigenous hydrocarbons is from mixed organic sources, with a predominance of terrestrial plants, whereas lower marine organisms dominate the non-indigenous hydrocarbons. Moreover, the molecular ratios obtained from our analysis indicated that the OM present in the indigenous hydrocarbons was deposited under suboxic conditions in a marginal marine environment. Contrarily, the non-indigenous hydrocarbons exhibit a strong association with marine carbonate facies. Additionally, the maturity assessment based on biomarker ratios, indicated that the OM within the Campanian sediments from TP-1 well is in the early phase of oil generation whereas the OM in the non-indigenous hydrocarbons is in the peak-late mature phase. In conclusion, our geochemical findings have revealed the hydrocarbon potential and heterogeneity of hydrocarbons within the Tano Basin's Campanian source strata induced by non-indigenous hydrocarbons. The geochemical data obtained from our study provide evidence of the potential influence of non-indigenous hydrocarbons on the misrepresentation of source input, depositional environment, and thermal maturity of organic matter (OM) in source rock analysis.

Predicting the spatial distribution of stable isotopes in precipitation using a machine learning approach: a comparative assessment of random forest variants

Stable isotopes of hydrogen and oxygen are important natural tracers with a wide variety of environmental applications (e.g., the exploration of the water cycle, ecology and food authenticity). The spatially explicit predictions of their variations are obtained through various interpolation techniques. In the present work, a classical random forest (RF) and two of its variants were applied. RF and a random forest version employing buffer distance (RFsp) were applied to each month separately, while a random forest model was trained using all data employing month and year as categorical variables (RFtg). Their performance in predicting the spatial variability of precipitation stable oxygen isotope values for 2008–2017 across Europe was compared. In addition, a comparison was made with a publicly available alternative machine learning model which employs extreme gradient boosting. Input data was retrieved from the Global Network of Isotopes in Precipitation (GNIP; no. of stations: 144) and other national datasets (no. of stations: 127). Comparisons were made on the basis of absolute differences, median, mean absolute error and Lin’s concordance correlation coefficient. All variants were capable of reproducing the overall trends and seasonal patterns over time of precipitation stable isotope variability measured at each chosen validation site across Europe. The most important predictors were latitude in the case of the RF, and meteorological variables (vapor pressure, saturation vapor pressure, and temperature) in the case of the RFsp and RFtg models. Diurnal temperature range had the weakest predictive power in every case. In conclusion, it may be stated that with the merged dataset, combining GNIP and other national datasets, RFsp yielded the smallest mean absolute error 1.345‰) and highest Lin’s concordance correlation coefficient (0.987), while with extreme gradient boosting (based on only the GNIP data) the mean absolute error was 1.354‰, and Lin’s concordance correlation coefficient was 0.984, although it produced the lowers overall median value (1.113‰), while RFsp produced 1.124‰. The most striking systematic bias was observed in the summer season in the northern validation stations; this, however, diminished from 2014 onward, the point after which stations beyond 55° N are available in the training set.

Holocene relationships between climate, waterfowl, and lacustrine nutrient cycling at Kettle Lake, North Dakota, USA

Avian populations can substantially influence lacustrine nutrient loading and biogeochemical cycling through guano deposition. Here, we examine the influence of climate-forced avian migration throughout the Holocene on Kettle Lake, North Dakota, using stable nitrogen and carbon isotope values (δ15N, δ13C) of lake sediment organic matter. Carbon content and δ13C values are negatively correlated with δ15N and appear to be driven by changes in charcoal abundance and watershed vegetation, respectively. We find enriched δ15N values when the guano mineral struvite is present in the lake sediment core in the early to Mid-Holocene. A strong δ15N-percent Nitrogen content relationship during periods with struvite, relative to periods without struvite, indicates that guano deposition from mass bird visitations altered past nitrogen cycle processes, likely through enhanced denitrification. These results attest to the ability of waterfowl to alter lacustrine N-cycling in a mid-continental North American lake, and indicate that paleo-N data in this particular lake are recording a unique history that does not necessarily represent regional paleoenvironmental conditions. However, a significant, positive relationship between δ15N and Ambrosia and Amaranthaceae pollen abundance suggests avian visitation and its impacts on the N-cycle occurred during periods of anomalously wet summers superimposed on the background conditions of early to Mid-Holocene drought.

Deep roots mitigate drought impacts on tropical trees despite limited quantitative contribution to transpiration

Deep rooting is considered a central drought-mitigation trait with vast impact on ecosystem water cycling. Despite its importance, little is known about the overall quantitative water use via deep roots and dynamic shifts of water uptake depths with changing ambient conditions. Knowledge is especially sparse for tropical trees. Therefore, we conducted a drought, deep soil water labeling and re-wetting experiment at Biosphere 2 Tropical Rainforest. We used in situ methods to determine water stable isotope values in soil and tree water in high temporal resolution. Complemented by soil and stem water content and sap flow measurements we determined percentages and quantities of deep-water in total root water uptake dynamics of different tree species. All canopy trees had access to deep-water (max. uptake depth 3.3 m), with contributions to transpiration ranging between 21 % and 90 % during drought, when surface soil water availability was limited. Our results suggest that deep soil is an essential water source for tropical trees that delays potentially detrimental drops in plant water potentials and stem water content when surface soil water is limited and could hence mitigate the impacts of increasing drought occurrence and intensity as a consequence of climate change. Quantitatively, however, the amount of deep-water uptake was low due to the trees' reduction of sap flow during drought. Total water uptake largely followed surface soil water availability and trees switched back their uptake depth dynamically, from deep to shallow soils, following rainfall. Total transpiration fluxes were hence largely driven by precipitation input.

Isotopic and biochemical composition of Western Mediterranean macroalgae

ABSTRACT Among marine primary producers, macroalgae support complex and productive coastal food webs, but coastal primary production relies on terrigenous inputs and remineralized organic matter which both vary seasonally. An approach combining stable isotope and biochemical analyses enables a better characterization of macroalgae specificities and highlights environmental influences on their chemical signature. This study compared the isotopic signature and biochemical composition of 22 Mediterranean macroalgae belonging to Rhodophyta (red algae), Phaeophyceae (brown algae) and Chlorophyta (green algae) between March and November 2010 to capture the differences in species chemical signatures potentially driven by metabolic traits or environmental drivers. Carbon stable isotope values were evidenced as a good proxy of specific carbon metabolism: low values observed in red algae could be related to the reported absence of carbon concentrating mechanisms (CCMs) in this group while higher values were driven by strong CCM activity in green algae. Biochemical patterns also differed between groups: soluble carbohydrates were a major component for red algae, while lipids and proteins dominated in brown algae, and insoluble carbohydrate concentrations were high in green algae. Variation within species across two collection times could be related to environmental changes and algal metabolism. δ15N values confirm the efficiency of this parameter as a proxy of the impact of human influence in the Bay of Marseille.

Do stable carbon and nitrogen isotope values of Nitella flexilis differ between softwater and hardwater lakes?

The composition of stable carbon and nitrogen isotopes (δ13C and δ15N) is widely used for tracking the origin of organic matter (OM) present in lacustrine sediments. These isotopes also define the evolution of OM in food web loops. Charophyte species Nitella flexilis C. Agardh, 1824 can be found in different aquatic environments where it contributes significantly to sediment formation and influences biota function. Therefore, it is crucial to study more about δ13C and δ15N in different lake types. Here, we present the results of the first comprehensive study of N. flexilis δ13C and δ15N, which add to the knowledge of the C and N isotope records of charophytes. We obtained the δ13C and δ15N records of N. flexilis OM from hardwater and softwater lakes and checked for differences between these records. We also analyzed the differences in physical and chemical parameters. Finally, we compared the δ13C and δ15N records with physical and chemical parameters to identify the variables that have the highest influence on N. flexilis δ13C and δ15N values. Our study showed that both δ13C and δ15N did not differ significantly in the two types of lakes, although the lakes had significant differences in several physical and chemical parameters (pH, Ca2+, dissolved inorganic carbon, total phosphorus, conductivity). However, we observed that δ13C values were influenced by light conditions (photosynthetic active radiation, depth, dissolved OM), while δ15N values were influenced by the total nitrogen concentration in water.