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The Greenland Ice Sheet has a central role in the global climate system owing to its size, radiative effects and freshwater storage, and as a potential tipping point1. Weather stations show that the coastal regions are warming2, but the imprint of global warming in the central part of the ice sheet is unclear, owing to missing long-term observations. Current ice-core-based temperature reconstructions3–5 are ambiguous with respect to isolating global warming signatures from natural variability, because they are too noisy and do not include the most recent decades. By systematically redrilling ice cores, we created a high-quality reconstruction of central and north Greenland temperatures from ad 1000 until 2011. Here we show that the warming in the recent reconstructed decade exceeds the range of the pre-industrial temperature variability in the past millennium with virtual certainty (P ad 1800. The disproportionate warming is accompanied by enhanced Greenland meltwater run-off, implying that anthropogenic influence has also arrived in central and north Greenland, which might further accelerate the overall Greenland mass loss.

RationaleAnalysis of stable isotopes in tissue and excreta may provide information about animal diets and their nutritional state. As body condition may have a major influence on reproduction, linking stable isotope values to animal demographic rates may help unravel the drivers behind animal population dynamics.MethodsWe performed sequential analysis of δ15N values in guard hair from 21 muskoxen (Ovibos moschatus) from Zackenberg in high arctic Greenland. We were able to reconstruct the dietary history for the population over a 5-year period with contrasting environmental conditions. We examined the linkage between guard hair δ15N values in 12 three-month periods and muskox calf recruitment to detect critical periods for muskox reproduction. Finally, we conducted similar analyses of the correlation between environmental conditions (snow depth and air temperature) and calf recruitment.Resultsδ15N values exhibited a clear seasonal pattern with high levels in summer and low levels in winter. However, large inter-annual variation was found in winter values, suggesting varying levels of catabolism depending on snow conditions. In particular δ15N values during January–March were linked to muskox recruitment rates, with higher values coinciding with lower calf recruitment. δ15N values were a better predictor of muskox recruitment rates than environmental conditions.ConclusionsAlthough environmental conditions may ultimately determine the dietary δ15N signal in muskox guard hairs, muskox calf recruitment was more strongly correlated with δ15N values than ambient snow and temperature. The period January–March, corresponding to late gestation, appears particularly critical for muskox reproduction. RationaleAnalysis of stable isotopes in tissue and excreta may provide information about animal diets and their nutritional state. As body condition may have a major influence on reproduction, linking stable isotope values to animal demographic rates may help unravel the drivers behind animal population dynamics. MethodsWe performed sequential analysis of delta N-15 values in guard hair from 21 muskoxen (Ovibos moschatus) from Zackenberg in high arctic Greenland. We were able to reconstruct the dietary history for the population over a 5-year period with contrasting environmental conditions. We examined the linkage between guard hair delta N-15 values in 12 three-month periods and muskox calf recruitment to detect critical periods for muskox reproduction. Finally, we conducted similar analyses of the correlation between environmental conditions (snow depth and air temperature) and calf recruitment. Results delta N-15 values exhibited a clear seasonal pattern with high levels in summer and low levels in winter. However, large inter-annual variation was found in winter values, suggesting varying levels of catabolism depending on snow conditions. In particular delta N-15 values during January-March were linked to muskox recruitment rates, with higher values coinciding with lower calf recruitment. delta N-15 values were a better predictor of muskox recruitment rates than environmental conditions. ConclusionsAlthough environmental conditions may ultimately determine the dietary delta N-15 signal in muskox guard hairs, muskox calf recruitment was more strongly correlated with delta N-15 values than ambient snow and temperature. The period January-March, corresponding to late gestation, appears particularly critical for muskox reproduction.

Migratory species have geographically separate distributions during their annual cycle, and these areas can vary between populations and individuals. This can lead to differential stress levels being experienced across a species range. Gathering informa-tion on the areas used during the annual cycle of red- throated divers (RTDs; Gavia stel-lata) has become an increasingly pressing issue, as they are a species of concern when considering the effects of disturbance from offshore wind farms and the associated ship traffic. Here, we use light- based geolocator tags, deployed during the summer breeding season, to determine the non- breeding winter location of RTDs from breed-ing locations in Scotland, Finland, and Iceland. We also use δ15N and δ13C isotope signatures, from feather samples, to link population- level differences in areas used in the molt period to population- level differences in isotope signatures. We found from geolocator data that RTDs from the three different breeding locations did not overlap in their winter distributions. Differences in isotope signatures suggested this spatial separation was also evident in the molting period, when geolocation data were unavailable. We also found that of the three populations, RTDs breeding in Iceland moved the shortest distance from their breeding grounds to their wintering grounds.In contrast, RTDs breeding in Finland moved the furthest, with a westward migration from the Baltic into the southern North Sea. Overall, these results suggest that RTDs breeding in Finland are likely to encounter anthropogenic activity during the win-ter period, where they currently overlap with areas of future planned developments. Icelandic and Scottish birds are less likely to be affected, due to less ship activity and few or no offshore wind farms in their wintering distributions. We also demonstrate that separating the three populations isotopically is possible and suggest further work to allocate breeding individuals to wintering areas based solely on feather samples. Migratory species have geographically separate distributions during their annual cycle, and these areas can vary between populations and individuals. This can lead to differential stress levels being experienced across a species range. Gathering information on the areas used during the annual cycle of red-throated divers (RTDs; Gavia stellata) has become an increasingly pressing issue, as they are a species of concern when considering the effects of disturbance from offshore wind farms and the associated ship traffic. Here, we use light-based geolocator tags, deployed during the summer breeding season, to determine the non-breeding winter location of RTDs from breeding locations in Scotland, Finland, and Iceland. We also use delta N-15 and delta C-13 isotope signatures, from feather samples, to link population-level differences in areas used in the molt period to population-level differences in isotope signatures. We found from geolocator data that RTDs from the three different breeding locations did not overlap in their winter distributions. Differences in isotope signatures suggested this spatial separation was also evident in the molting period, when geolocation data were unavailable. We also found that of the three populations, RTDs breeding in Iceland moved the shortest distance from their breeding grounds to their wintering grounds. In contrast, RTDs breeding in Finland moved the furthest, with a westward migration from the Baltic into the southern North Sea. Overall, these results suggest that RTDs breeding in Finland are likely to encounter anthropogenic activity during the winter period, where they currently overlap with areas of future planned developments. Icelandic and Scottish birds are less likely to be affected, due to less ship activity and few or no offshore wind farms in their wintering distributions. We also demonstrate that separating the three populations isotopically is possible and suggest further work to allocate breeding individuals to wintering areas based solely on feather samples.

RATIONALE: Dental calculus (mineralized dental plaque) is composed primarily of hydroxyapatite. We hypothesize that the carbonate component of dental calculus will reflect the isotopic composition of ingested simple carbohydrates. Therefore, dental calculus carbonates may be an indicator for sugar consumption, and an alternative to bone carbonate in isotopic paleodiet studies. METHODS We utilised FTIR-ATR analysis to characterise the composition and crystallisation of bone and dental calculus before isotope analysis of carbonate. Using a Sercon 20-22 mass spectrometer coupled with a Sercon GSL Sample Preparation System and an IsoPrime 100 dual inlet mass spectrometer plus Multiprep device to measure carbon, we tested the potential of dental calculus carbonate to identify C4 resources in diet through analysis of δ13C values in paired bone, calculus, and teeth mineral samples. - RESULTS: The modern population shows higher δ13C values in all three tissue carbonates compared to both archaeological populations. Clear differences in dental calculus δ13C values are observed between the modern and archaeological individuals suggesting potential for utilising dental calculus in isotope paleodiet studies. The offset between dental calculus and either bone or enamel carbonate δ13C values are large and consistent in direction, with no consistent offset between the δ13C values for the three tissues per individual. - CONCLUSIONS: Our results support dental calculus carbonate as a new biomaterial to identify C4 sugar through isotope analysis. Greater carbon fractionation in the mouth is likely due to the complex formation of dental calculus as a mineralized biofilm, which results in consistently high δ13C values compared to bone and enamel. 1 Introduction 1.1 Dental Calculus and potential for isotopic analysis 2 Mterial and methods 2.1 Materials 2.2 Methods 2.2.1 FTIR with attenuated total reflectance (ATR) 2.2.2 Bone collagen extraction 2.2.3 Bone preparation for carbonate analysis 2.2.4 Enamel preparation for carbonate analysis 2.2.5 Dental calculus preparation for carbonate analysis 2.3 Analytical measurement 2.3.1 Bone collagen 2.3.2 Bone, enamel, and dental calculus carbonates 2.4 Suess effect correction 2.5 Statistical analysis 3 Results and discussion 3.1 FTIR-ATR analysis of bone and dental calculus 3.1.1 Infrared splitting factor (IRSF) 3.1.2 Carbonate-to-phosphate ratio (C/P) 3.1.3 IRSF and C/P relationship 3.2 Isotopes 3.2.1 Correlations between tissues 3.2.2 Carbonate δ13C tissue offsets 3.2.3 Dietary interpretation from the three tissues 3.2.4 Causes of diet differences between bone and enamel 4 Conclusions

Interpreting phloem carbohydrate or xylem tissue carbon isotopic composition as measures of water-use efficiency or past tree productivity requires in-depth knowledge of the factors altering the isotopic composition within the pathway from ambient air to phloem contents and tree ring. One of least understood of these factors is mesophyll conductance (g(m)). We formulated a dynamic model describing the leaf photosynthetic pathway including seven alternative g(m) descriptions and a simple transport of sugars from foliage down the trunk. We parameterised the model for a boreal Scots pine stand and compared simulated g(m) responses with weather variations. We further compared the simulated delta C-13 of new photosynthates among the different g(m) descriptions and against measured phloem sugar delta C-13. Simulated g(m) estimates of the seven descriptions varied according to weather conditions, resulting in varying estimates of phloem delta C-13 during cold/moist and warm/dry periods. The model succeeded in predicting a drought response and a postdrought release in phloem sugar delta C-13 indicating suitability of the model for inverse prediction of leaf processes from phloem isotopic composition. We suggest short-interval phloem sampling during and after extreme weather conditions to distinguish between mesophyll conductance drivers for future model development.

Stable isotopologues of water are widely used to derive relative root water uptake (RWU) profiles and average RWU depth in lignified plants. Uniform isotope composition of plant xylem water (δxyl) along the stem length of woody plants is a central assumption of the isotope tracing approach which has never been properly evaluated. Here we evaluate whether strong variation in δxyl within woody plants exists using empirical field observations from French Guiana, northwestern China, and Germany. In addition, supported by a mechanistic plant hydraulic model, we test hypotheses on how variation in δxyl can develop through the effects of diurnal variation in RWU, sap flux density, diffusion, and various other soil and plant parameters on the δxyl of woody plants. The hydrogen and oxygen isotope composition of plant xylem water shows strong temporal (i.e., sub-daily) and spatial (i.e., along the stem) variation ranging up to 25.2 ‰ and 6.8 ‰ for δ2H and δ18O, respectively, greatly exceeding the measurement error range in all evaluated datasets. Model explorations predict that significant δxyl variation could arise from diurnal RWU fluctuations and vertical soil water heterogeneity. Moreover, significant differences in δxyl emerge between individuals that differ only in sap flux densities or are monitored at different times or heights. This work shows a complex pattern of δxyl transport in the soil–root–xylem system which can be related to the dynamics of RWU by plants. These dynamics complicate the assessment of RWU when using stable water isotopologues but also open new opportunities to study drought responses to environmental drivers. We propose including the monitoring of sap flow and soil matric potential for more robust estimates of average RWU depth and expansion of attainable insights in plant drought strategies and responses.

Understanding the processes that affect the triple oxygen isotope composition of atmospheric CO2during gas exchange can help constrain the interaction and fluxes between the atmosphere and the biosphere. We conducted leaf cuvette experiments under controlled conditions using three plant species. The experiments were conducted at two different light intensities and using CO2with different δ17O. We directly quantify the effect of photosynthesis on δ17O of atmospheric CO2for the first time. Our results demonstrate the established theory for δ18O is applicable to δ17O.CO2/at leaf level, and we confirm that the following two key factors determine the effect of photosynthetic gas exchange on the δ17O of atmospheric CO2. The relative difference between δ17O of the CO2entering the leaf and the CO2in equilibrium with leaf water and the back-diffusion flux of CO2from the leaf to the atmosphere, which can be quantified by the cm=ca ratio, where ca is the CO2mole fraction in the surrounding air and cm is the one at the site of oxygen isotope exchange between CO2and H2O. At low cm=ca ratios the discrimination is governed mainly by diffusion into the leaf, and at high cm=ca ratios it is governed by back-diffusion of CO2that has equilibrated with the leaf water. Plants with a higher cm=ca ratio modify the 117O of atmospheric CO2more strongly than plants with a lower cm=ca ratio. Based on the leaf cuvette experiments, the global value for discrimination against δ17O of atmospheric CO2during photosynthetic gas exchange is estimated to be-0:57±0:14% using cm=ca values of 0.3 and 0.7 for C4and C3plants, respectively. The main uncertainties in this global estimate arise from variation in cm=ca ratios among plants and growth conditions.

Growing evidence suggests that global climate change promotes the dominance of mixotrophic algae especially in oligotrophic aquatic ecosystems. While theory predicts that mixotrophy increases trophic transfer efficiency in aquatic food webs, deleterious effects of some mixotrophs on consumers have also been reported. Here, using a widespread mixotrophic algal genus Dinobryon, we aimed to quantify how colonial taxa contribute to secondary production in lakes. We, therefore, studied the dietary effects of Dinobryon divergens on Cladocera (Daphnia longispina) and Copepoda (Eudiaptomus gracilis), representing two main taxonomic and functional groups of zooplankton. In feeding experiments, we showed that Dinobryon was largely grazing resistant and even inhibited the uptake of the high-quality reference food in Daphnia. Eudiaptomus could to some extent compensate with selective feeding, but a negative long-term food quality effect was also evident. Besides, Eudiaptomus was more sensitive to the pure diet of Dinobryon than Daphnia. Low lipid content and high C:P elemental ratio further supported the low nutritional value of the mixotroph. In a stable isotope approach analysing a natural plankton community, we found further evidence that carbon of Dinobryon was not conveyed efficiently to zooplankton. Our results show that the increasing dominance of colonial mixotrophs can result in reduced dietary energy transfer to consumers at higher trophic levels. In a wider perspective, global climate change favours the dominance of some detrimental mixotrophic algae which may constrain pelagic trophic transfer efficiency in oligotrophic systems, similarly to cyanobacteria in eutrophic lakes.

Nature Communications, 13, ISSN:2041-1723

Time-controlled quantitative climate data are crucial for reconstructing past environmental contexts of human history. In west Anatolia, the Simav Graben used to be occupied by a lake. However, today it is drained and associated with Holocene sediments that record lake level changes. An initial drill-core at the lake's centre reveals an arid phase before ca. 14 ka (i.e. prior to the Late Glacial warming) followed by a wet phase (marshes and lake) throughout the Holocene. Along the lake's margin, two additional sediment sequences contain a detailed Holocene record of changes in humidity throughout the Simav Graben's watershed. Classification of the shoreline facies from these sequences records drought events marked by colluvium prograding towards the lake. Using 13 radiocarbon dates, sedimentary facies suggest rapid lake contraction phases grossly around 8.0, 3.9, 2.9, 2.4 and 2.0 ka. After the drier periods, recorded by phases of terrestrial progradation, lake level increase is marked by retrogradational lake muds and peats onlapping the colluvium, consistent with increases in total lake volume. Based on modern lake morphology, climate and runoff data, and the elevations recorded by the shoreline facies identified within the marginal Holocene sedimentary sequence, we constructed a paleohydrologic balance model and a precipitation curve. According to the model, annual precipitation may have been fallen below 460 mm during the 8.2 ka event, and 468-478 mm during the 4.2 ka and 3.2 ka events demonstrating an extreme decrease in precipitation compared to the earlier wet phases. In turn, Holocene Archaeology in West Anatolia outlines (i) a widespread abandonment and/or notable fire events at the end of the Early Bronze Age III (ca. 4.2 ka), and (ii) a high level of social instability at the beginning of the Early Iron Age (ca. 3.2 ka). The correlation between the timing of lake level falls at Simav (water volume depletion caused by drying trends) at 8.0 ka, 3.9 ka and 2.8 ka and cultural fluctuations suggest that drought events may have played a major role in these socio-political changes in western Anatolia. Furthermore, the high lake-level stages (associated with precipitation >500 mm) coincide with the Late Chalcolithic to Early Bronze Age I transition as well as the Hellenistic and Roman periods, both times of economic stability and growth in the region., Commission for Scientific Research at Eskisehir Osmangazi University, Turkey [2016-15050], The authors are grateful to Dr. Fatima Makodem (CNRS) for her help in the fieldwork, and Ays egul Dikme (ESOGU) for her contribution to the laboratory studies. Clay Campbell (University of Kansas) kindly edited the manuscript. This study was financially supported by the Commission for Scientific Research at Eskis ehir Osmangazi University, Turkey (Grant number: 2016-15050).

There is still a paucity of hydrological data explaining the relationship between (rapid, millennial‐scale) climate forcing and Mediterranean rainfall since the Last Glacial. We show that distinct lake‐level fluctuations at Lake Trasimeno (Italy) are associated with changing aridity in the central Mediterranean during the last ~47 800 years. The lake‐level fluctuations are reconstructed based on carbonate mineral content and carbonate mineral species, as well as the stable oxygen and carbon isotope (δ18O and δ13C) geochemistry of endogenic carbonates. Low lake levels are linked to high carbonate, Mg‐calcite and aragonite contents, and high δ18O and δ13C values. Inferred hydrological changes are linked to glacial–interglacial and, tentatively within the limitations of our chronology, to millennial‐scale climate variability as well as the intensity of the Atlantic Meridional Overturning Circulation (AMOC). Prior to the Last Glacial Maximum (LGM), during intervals equivalent to Marine Isotope Stage 3 (MIS 3), a stronger AMOC associated with Greenland interstadial periods (Dansgaard/Oeschger (D/O) warm periods) and stronger Asian monsoon probably coincide with increased precipitation in central Italy as inferred from high lake levels at Lake Trasimeno. Periods of weak AMOC intensity such as during Greenland stadials (D/O cold periods), during Heinrich events, and weak Asian monsoons are correlated with lake level lowstands, which imply relatively dry conditions in central Italy. Lake Trasimeno’s water level during the LGM and the Lateglacial (MIS 2) is relatively stable, with recorded changes showing distinct similarities to orbital configurations. Although muted, high latitude climate forcing is still evident in the data during peak glacial conditions. The transition from D/O‐like hydrological variability at Lake Trasimeno during MIS 3 to orbitally controlled fluctuations during the Lateglacial to Holocene transition coincides with an increasing amplitude in local winter and summer insolation, probably indicating increasing seasonality and a larger temperature gradient between low‐ and high‐latitude settings., Deutscher Akademischer Austauschdienst http://dx.doi.org/10.13039/501100001655

Water transit time is now a standard measure in catchment hydrological and ecohydrological research. The last comprehensive review of transit time modeling approaches was published 15+ years ago. But since then the field has largely expanded with new data, theory and applications. Here, we review these new developments with focus on water-age-balance approaches and data-based approaches. We discuss and compare methods including StorAge-Selection functions, well/partially mixed compartments, water age tracking through spatially distributed models, direct transit time estimates from controlled experiments, young water fractions, and ensemble hydrograph separation. We unify some of the heterogeneity in the literature that has crept in with these many new approaches, in an attempt to clarify the key differences and similarities among them. Finally, we point to open questions in transit time research, including what we still need from theory, models, field work, and community practice., Water Resources Research, 58 (11), ISSN:0043-1397, ISSN:1944-7973

Environmental change, including joint effects of increasing dissolved organic carbon (DOC) and total phosphorus (TP) in boreal northern lakes could potentially affects food web energy sources and the biochemical composition of organisms. These environmental stressors are enhanced by anthropogenic land-use and can decrease the quality of polyunsaturated fatty acids (PUFAs) in seston and zooplankton, and therefore, possibly cascading up to fish. In contrast, the content of mercury in fish increases with lake browning potentially amplified by intensive forestry practises. However, there is little evidence on how these environmental stressors simultaneously impact beneficial omega-3 fatty acid (n3-FA) and total mercury (THg) content of fish muscle for human consumption. A space-for-time substitution study was conducted to assess whether environmental stressors affect Eurasian perch (Perca fluviatilis) allochthony and muscle nutritional quality [PUFA, THg, and their derivative, the hazard quotient (HQ)]. Perch samples were collected from 31 Finnish lakes along pronounced lake size (0.03–107.5 km2), DOC (5.0–24.3 mg L−1), TP (5–118 μg/ L) and land-use gradients (forest: 50.7–96.4%, agriculture: 0–32.6%). These environmental gradients were combined using principal component analysis (PCA). Allochthony for individual perch was modelled using source and consumer δ2H values. Perch allochthony increased with decreasing lake pH and increasing forest coverage (PC1), but no correlation between lake DOC and perch allochthony was found. Perch muscle THg and omega-6 fatty acid (n6-FA) content increased with PC1 parallel with allochthony. Perch muscle DHA (22:6n3) content decreased, and ALA (18:3n3) increased towards shallower murkier lakes (PC2). Perch allochthony was positively correlated with muscle THg and n6-FA content, but did not correlate with n3-FA content. Hence, the quality of perch muscle for human consumption decreases (increase in HQ) with increasing forest coverage and decreasing pH, potentially mediated by increasing fish allochthony. peerReviewed

Generalist fish species can feed on a wide resource spectrum and across trophic levels depending on resource availability and trophic interactions. Crucian carp (Carassius carassius) represents a good candidate species to investigate variation in the trophic ecology of generalist fish as it can be found in highly variable fish communities and its resource use is well documented. In this study, we explored the trophic ecology of crucian carp at the individual and population levels using stable isotope and gut content analysis. We tested if trophic resource use varied according to lake productivity, predation risk, intra- and interspecific competition, or individual fish size. We found that crucian carp resource preference was highly variable among and within lakes. In predator-free lakes, small crucian carp occurred in high densities, showed increased interindividual specialisation, and relied mainly on pelagic zooplankton. In presence of predators, large crucian carp occurred in low densities and included greater proportions of benthic macroinvertebrates in their diet. This shift in resource use was further favoured in productive, shallow lakes where littoral prey were probably abundant. Resource partitioning was an important factor determining crucian carp niche use, as fish had higher trophic position in absence of other cyprinids. Crucian carp showed highly dynamic resource use and food preferences in response to variable environmental conditions. Overlooking complex diet preferences of generalist fish may lead to an oversimplification of freshwater community dynamics.

Large marine predators exhibit high concentrations of mercury (Hg) as neurotoxic methylmercury, and the potential impacts of global change on Hg contamination in these species remain highly debated. Current contaminant model predictions do not account for intraspecific variability in Hg exposure and may fail to reflect the diversity of future Hg levels among conspecific populations or individuals, especially for top predators displaying a wide range of ecological traits. Here, we used Hg isotopic compositions to show that Hg exposure sources varied significantly between and within three populations of white sharks (Carcharodon carcharias) with contrasting ecology: the north-eastern Pacific, eastern Australasian, and south-western Australasian populations. Through Δ200Hg signatures in shark tissues, we found that atmospheric Hg deposition pathways to the marine environment differed between coastal and offshore habitats. Discrepancies in δ202Hg and Δ199Hg signatures among white sharks provided evidence for intraspecific exposure to distinct sources of marine methylmercury, attributed to population and ontogenetic shifts in foraging habitat and prey composition. We finally observed a strong divergence in Hg accumulation rates between populations, leading to three times higher Hg concentrations in large Australasian sharks compared to north-eastern Pacific sharks, and likely due to different trophic strategies adopted by adult sharks across populations. This study illustrates the variety of Hg exposure sources and bioaccumulation patterns that can be found within a single species and suggests that intraspecific variability needs to be considered when assessing future trajectories of Hg levels in marine predators.

Subarctic ecohydrological processes are changing rapidly, but detailed and integrated ecohydrological investigations are not as widespread as necessary. We introduce an integrated research catchment site (Pallas) for atmosphere, ecosystems, and ecohydrology studies in subarctic conditions in Finland that can be used for a new set of comparative catchment investigations. The Pallas site provides unique observational data and high-intensity field measurement datasets over long periods. The infrastructure for atmosphere- to landscape-scale research in ecosystem processes in a subarctic landscape has recently been complemented with detailed ecohydrological measurements. We identify three dominant processes in subarctic ecohydrology: (a) strong seasonality drives ecohydrological regimes, (b) limited dynamic storage causes rapid stream response to water inputs (snowmelt and intensive storms), and (c) hydrological state of the system regulates catchment-scale dissolved carbon dynamics and greenhouse (GHG) fluxes. Surface water and groundwater interactions play an important role in regulating catchment-scale carbon balances and ecosystem respiration within subarctic peatlands, particularly their spatial variability in the landscape. Based on our observations from Pallas, we highlight key research gaps in subarctic ecohydrology and propose several ways forward. We also demonstrate that the Pallas catchment meets the need for sustaining and pushing the boundaries of critical long-term integrated ecohydrological research in high-latitude environments.

The rule of thumb in marine trophodynamics indicates that the bigger an organism, the higher its trophic level (TROPH). This generalization leads to the assumption that fishes increase their TROPH with growth. However, a recent review showed that for many species, TROPH does not increase with body size, suggesting that size-independent feeding is not rare in marine fishes. Here, we assessed some morphological traits of marine fishes that could potentially be used as indicators for the ability to vary TROPH with body size. Stable isotope values of nitrogen were used as a proxy TROPH. The specific objectives were as follows: (a) to evaluate the relationship between the oral gape and the trophodynamic trend and (b) to quantify the relevance of 11 external morphological traits in determining the size-based trophodynamics. We used random forest models to identify the morphological traits that would help predict which species would have the potential to increase TROPH with growth and which would have not. The selected traits included the pelvic fin relative position, the dorsal fin shape, total length, and relative mouth size. Our results also showed a marked relationship between the rate of increase in the oral gape and TROPH. The analyses presented here provide the first comprehensive and quantitative review aiming at linking the trophodynamics of marine fishes with external morphological traits. Fil: Rios, Maria Florencia. Universidad Nacional de la Patagonia. Facultad de Ciencias Naturales. Sede Puerto Madryn; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Centro para el Estudio de Sistemas Marinos; Argentina Fil: Venerus, Leonardo Ariel. Universidad Nacional de la Patagonia "San Juan Bosco"; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Centro para el Estudio de Sistemas Marinos; Argentina Fil: Karachle, Paraskevi K.. Hellenic Centre for Marine Research. Institute of Marine Biological Resources and Inland Waters; Grecia Fil: Reid, William D. K.. University of Newcastle; Reino Unido Fil: Erzini, Karim. Universidad de Algarve; Portugal Fil: Stergiou, Konstantinos I.. Aristotle University of Thessaloniki. Department of Zoology. School of Biology; Grecia Fil: Galvan, David Edgardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Centro para el Estudio de Sistemas Marinos; Argentina

Eutrophication causes tremendous losses to seagrass around the globe. The effects of nutrient loading vary along environmental gradients, and wave forces especially are expected to affect meadow stability, nutrient status, and responses to nutrient supply. Here, we surveyed the pristine subtropical intertidal seagrass system of Banc d’Arguin, Mauritania, to characterise Zostera noltii in terms of morphology (biomass allocation, leaf length and area, rhizome internode length), nutrient content (carbon: C, nitrogen: N, phosphorus: P, iron: Fe, aluminium: Al), and patterns in δ13C and δ15N across a wide gradient of hydrodynamic conditions. We subsequently assessed temporal variability in seagrass stability and nutrient fluxes, as well as responses to experimental fertilisation (pulses of + N, + P, + N + P) on three meadows representing different degrees of wave-force (exposed, intermediate and sheltered). The large-scale survey revealed a marked increase in N and P limitation with increasing wave energy. The overall low leaf %N (1.74 ± 0.04; mean ± se) and N:P ratio (8.67 ± 0.14) suggests that the area is N-limited. Seasonal variation in seagrass cover and biomass showed the exposed site to be the most stable and the sheltered site the least. Variation in δ15N signatures indicates seasonal shifts in N sources at the exposed site only. Fertilisation with + N and + N + P induced seagrass mortality at the exposed site, while at the sheltered site it was + P that degraded seagrass. Collectively, our results indicate that with increasing wave forces, the degree of stability of seagrass beds increases, but nutrient limitation and vulnerability to eutrophication increase as well.

Carbonate clumped isotope thermometry is based on the thermodynamically dependent relative abundance of 13C 18O bonds (quantified as Δ47) within the carbonate crystal lattice. The clumping of 13C 18O in carbonates is based on a self-reaction of isotope exchange that occurs rapidly at near neutral pH. Similar Δ47-temperature relationships between biogenic and inorganically precipitated carbonate in initial studies led to the promise of a proxy free of biologically driven disequilibrium effects, commonly referred to as “vital effects”. This has been largely the case for most organisms investigated. Biologically mediated disequilibrium precipitation has however been reported in corals and cephalopods and brachiopod molluscs. Echinoderms, despite their complex inter-cellular bio-mineralization strategy, large inter-skeletal fractionation of δ18O, δ13C and rapid calcite precipitation have however not been previously investigated with regards to their clumped isotope composition. We present clumped isotopic composition (Δ47) of 25 inter-skeletal elements of 5 echinoid species with varying growth temperatures. We found no statistically significant inter-skeletal variation in Δ47 in all echinoid species measured, a surprising find given the important inter-skeletal variability reported for δ13C and δ18O. Our echinoid Δ47-temperature calibration however shows a statistically significant positive offset from Δ47-temperature calibration for inorganic calcite of 0.014‰. The pattern of isotopic fractionation in δ18O and Δ47 of echinoderms is not consistent with CO2 hydration or hydroxylation, diffusion or the high-Mg calcite composition of echinoid calcite. Positive offsets in the Δ47 of echinoid calcite may however relate to deviations in the pH of the calcifying fluid from the pH at which equilibrium calcite is precipitated.

International audience; Elephant seals, Mirounga spp., are highly dimorphic, having different energetic requirements according to age and sex, and foraging in various ecological and oceanographic contexts. Resource partitioning has been shown for the sub‐Antarctic populations of southern elephant seals, M. leonina, where colonies are surrounded by narrow shelves that deepen abruptly. In contrast, seals from Península Valdés (Argentina), in the northernmost extent of the breeding range, face an extended, shallow, temperate, and productive continental shelf. We integrated tracking data from 98 animals (juveniles and adults, males and females) gathered over more than two decades, and found that although all available habitats were used, individuals segregated by age and sex. Juvenile males favored shelf habitats, whereas subadult and adult males also used the shelf break. Juvenile females preferred the shelf and the more distant Argentine Basin used by postbreeding and postmolt adult females. Males showed the highest proportion of area‐restricted search locations, suggesting more spatially concentrated feeding activity, and likely reflecting a preference for foraging habitat and prey. Our results are consistent with those from other populations, implying that elephant seals show remarkable similarities in habitat use by age and sex classes, despite broad differences in the offshore habitats between sub‐Antarctic and temperate ecosystems.

WOS:000684005500003; International audience; Coastal habitats are interlinked by ecological connectivity, defined as the exchange of organic matter or organisms between habitats. The degree of this connectivity will depend in particular on the geometric properties of the landscape. The increasing anthropogenic alterations in estuaries therefore raises the need to understand how their morphological characteristics influence fluxes between habitats. We used stable isotopes to investigate the variability of ecological connectivity between three estuaries and their adjacent coastal areas by tracking the origin of the organic matter (estuarine vs coastal) underlying the diet of the migrant species Eucinostomus argenteus. The chosen estuaries were geomorphologically distinct, exhibiting, in particular, differences in their degree of connection to the sea, corresponding to the morphological features (shape, mouth width) controlling key physico-chemical variables in this habitat (e.g. salinity). The sampling of the basal food sources contributing to the food web was performed in the three estuaries and in their adjacent coasts. The variability in stable isotope ratios between estuaries was examined for both fish and sources of organic matter. In the three estuaries, Bayesian models were applied for each season (dry and rainy) to quantify the relative contribution of sources from estuarine and coastal environments supporting the diet of the silver mojarra. The share of coastal organic matter increased with the degree of sea connection, indicating that the properties of the seascape can regulate the intensity of interactions between ecosystems. Variations in ecological connectivity are likely to affect the functioning of ecosystems as they influence trophic pathways and energy flows between adjacent habitats. Morphological modifications could thus significantly disturb ecosystems by altering the structure of food web, thereby affecting certain ecosystem services such as the availability of living marine resources.

The vertical distribution of absorbing roots is one of the most influential plant traits determining plant strategy to access below ground resources. Yet little is known of natural variability in root distribution since collecting field data is challenging and labor-intensive. Studying stable water isotope compositions in plants could offer a cost-effective and practical solution to estimate the absorbing root surfaces distribution. However, such an approach requires developing realistic inverse modeling techniques that enable robust estimation of rooting distributions and associated uncertainty from xylem water isotopic composition observations. This study introduces an inverse modeling method that supports the assessment of the root allocation parameter (β) that defines the exponential vertical decay of a plants’ absorbing root surfaces distribution with soil depth. The method requires measurements obtained from xylem and soil water isotope composition, soil water potentials, and sap flow velocities when plants’ xylem water is sampled at a certain height above the rooting point. In a simulation study, we show that the approach can provide unbiased estimates of β and its associated uncertainty due to measuring errors and unmeasured environmental factors that can impact the xylem water isotopic data. We also recommend improving the accuracy and power of β estimation, highlighting the need for considering accurate soil water potential and sap flow monitoring. Finally, we apply the inverse modeling method to xylem water isotope data of lianas and trees collected in French Guiana. Our work shows that the inverse modeling procedure provides a robust analytical and statistical framework to estimate β. The method accounts for potential bias due to extraction errors and unmeasured environmental factors, which improves the viability of using stable water isotope compositions to estimate the distribution of absorbing root surfaces complementary to the assessment of relative root water uptake profiles.

Increased numbers of great cormorants (Phalacrocorax carbo) in the Baltic Sea may have local impacts on fisheries and salmonid hatcheries. We studied spatial and temporal variability in cormorant diet, and potential consumption of hatchery salmonids, by analysing knee subcutaneous adipose tissue fatty acids (FA) of specimens (N = 77) collected along Swedish and Finnish coasts in different seasons during 2013–2017. The FA profiles of the subspecies sinensis and carbo were similar, with large individual variation. The proportion of C18 polyunsaturated FAs (PUFAs) was the largest in the north, whereas the proportion of C20–22 monounsaturated FAs (MUFAs) increased towards the south, reflecting diminishing freshwater and increasing marine food web characteristics towards the south. As an exception, the C20–22 MUFA percentage was high in sinensis collected in June 2017 from the northern Baltic Sea. The source of C20–22 MUFAs was probably hatchery salmonids, raised on ocean fish hatchery feed and released 10 days before, near the cormorant capture site. The FA profiles of northern and southern cormorants differed from each other both in early and late summer samples, suggesting spatially different diets. The largest individual variation was found in 22:1n-11, characteristic of ocean zooplanktivorous fish, and likely originating from Atlantic wild or Baltic Sea hatchery-reared fish. This study shows that adipose tissue FA profiles can be used as proxies for seabird diet monitoring and indicators of predation on hatchery-reared fish. Obtaining quantitative estimates on the proportions of dietary fish species requires future feeding experiments, allowing calibration between the FA compositions and diet.

We report the first radiocarbon datings and carbon (δ13C) and oxygen (δ18O) stable isotopes data to reconstruct the paleoecology of medium to large herbivorous mammals from late Quaternary of southwestern Amazon (Acre and Rondonia states, Brazil). AMS 14C dates for Neochoerus sp. (29,072 - 27,713 Cal yr BP), Notiomastodon platensis (25,454 - 24,884 Cal yr BP) and Eremotherium laurillardi (11,320 - 11,131 Cal yr BP) support the Lujanian ages. All fossils have low δ13C and δ18O isotopic values that suggest C3-dominated environments from closed canopy forests to wooded savannas, agreeing with paleovegetation reconstitution. Most species were browsers (piC3=100%; Niche breadth, BA=0), where the key species with the largest body mass, N. platensis (∼6,300 kg) and E. laurillardi (∼3,500 kg), possibly had a more generalized browser diet in closed-canopies to woodlands. Their diet distinguished from the C3/C4 generalist Trigodonops lopesi (∼1,900 kg), which foraged in wooded savannas (piC3=70%; BA=0.72), similarly with its relative Toxodon platensis (∼1,800 kg) that had a browse-dominated mixed feeder diet (piC3 ≥ 84%, BA ≤ 0.38) in Peruvian and Bolivian Amazon localities. Palaeolama major (∼280 kg) was possibly a strictly folivorous within forest canopies, whereas Tapirus sp. (∼250 kg) and Mazama sp. (∼40 kg) were browsers in closed-canopies to woodlands. Holmesina rondoniensis (∼120 kg) was a browser but not restricted, where could also feed on herbaceous from understories in woodlands, and Neochoerus sp. (∼200 kg) feeding predominantly herbaceous plants in wooded savannas (piC3=∼69%; BA=0.75). We estimate that the interspecific competition could have been avoid by different feeding strategies, although more investigations are still needed to better understand their ecological interactions in the habitats of the southwestern Amazon during the late Quaternary.

International audience; The growth of yellowfin tuna has been the subject of considerable research efforts since the early 1960s. Most studies support a complex two-stanza growth pattern with a sharp acceleration departing from the von Bertalanffy growth curve used for most fish populations. This growth pattern has been assumed to result from a combination of physiological, ecological and behavioral factors but the role and contribution of each of them have not been addressed yet. We developed a bioenergetic model for yellowfin tuna in the context of Dynamic Energy Budget theory to mechanistically represent the processes governing yellowfin tuna growth. Most parameters of the model were inferred from Pacific bluefin tuna using body-size scaling relationships while some essential parameters were estimated from biological data sets collected in the Indian Ocean. The model proved particularly suitable for reproducing the data collected during the Pacific yellowfin tuna farming experience conducted by the Inter-American Tropical Tuna Commission at the Achotines Laboratory in Panama. In addition, model predictions appeared in agreement with knowledge of the biology and ecology of wild yellowfin tuna. We used our model to explore through simulations two major assumptions that might explain the existence of growth stanzas observed in wild yellowfin tuna: (i) a lower food supply during juvenile stage in relation with high intra- and inter-species competition and (ii) ontogenetic changes in food diet. Our results show that both assumptions are plausible although none of them is self-sufficient to explain the intensity of growth acceleration observed in wild Indian Ocean yellowfin tuna, suggesting that the two factors may act in concert. Our study shows that the yellowfin growth pattern is likely due to behavioral changes triggered by the acquisition of physiological abilities and anatomical traits through ontogeny that result in a major change in intensity of schooling and in a shift in the biotic habitat and trophic ecology of this commercially important tuna species.

ispartof: ESTUARINE COASTAL AND SHELF SCIENCE vol:267 status: published

WOS:000691601000007; International audience; This study addresses the temporal variability of couplings between pelagic and benthic habitats for fish assemblages at five periods in a shallow epicontinental sea, the Eastern English Channel (EEC). Organic matter fluxes fueling fish assemblages and the relative contribution of their different sources were assessed using stable isotope analysis and associated isotopic functional metrics. Couplings between benthic and pelagic realms appeared to be a permanent feature in the EEC, potentially favored by shallow depth and driven by the combination of two trophic processes. First, trophic interactions exhibited plasticity and revealed resource partitioning. Second, changes in the composition of fish assemblages did not impact benthic-pelagic couplings, as most dominant species were generalists during at least one time period, allowing complete use of available resources. Examining both unweighted and biomass-weighted indices was complementary and permitted a better understanding of trophic interactions and energy fluxes.

Torrential rain and extreme flooding caused by Atlantic hurricanes mobilize a large pool of organic matter (OM) from coastal forested watersheds in the southeastern United States. However, the mobilization of toxic metals such as mercury (Hg) that are associated with this vast pool of OM are rarely measured. This study aims to assess the variations of total Hg (THg) and methylmercury (MeHg) levels and the isotopic compositions of Hg in a blackwater river (Waccamaw River, SC, U.S.A.) during two recent extreme flooding events induced by Hurricane Joaquin (October 2015) and Hurricane Matthew (October 2016). We show that extreme flooding considerably increased filtered THg and MeHg concentrations associated with aromatic dissolved organic matter. During a 2-month sampling window each year (October-November), we estimate that about 27% (2015) and 78% (2016) of the average amount of Hg deposited atmospherically during these 2 months was exported via the river. The isotopic composition of Hg in the river waters was changed only slightly by the substantial inputs of runoff from surrounding landscapes, in which mass-dependent fractionation (as delta Hg-202) decreased from -1.47 to -1.67 parts per thousand and mass-independent fractionation (as increment Hg-199) decreased from -0.15 to -0.37 parts per thousand. The slight variations in Hg isotopic composition during such extreme flooding events imply that sources of Hg in the river are nearly unchanged even under the very high wet deposition of Hg derived from the intensive rainfall. The majority of Hg exported by the river (74-85%) is estimated to have been derived from dry deposition to the watersheds. An increase in frequency and intensity of Atlantic hurricanes is expected in the next few decades due to further warming of ocean surface waters. We predict that increased hurricanes will mobilize more dry-deposited Hg and in situ produced MeHg from these coastal watersheds where MeHg can be extensively bioaccumulated and biomagnified in the downstream aquatic food webs.

International audience; Species of the family Sternoptychidae (hatchetfishes) occur worldwide and play critical roles by sequestering carbon, recycling nutrients, and acting as a key trophic link between epipelagic primary consumers and higher trophic levels in marine ecosystems. Nevertheless, basic knowledge on their ecology is still lacking and their functional ecology remains understudied with respect to composition, organization, functions and environment interactions. Here we integrated comprehensive information collected in the western Tropical Atlantic on the diversity, abundance, distribution and trophic ecology of hatchetfishes, including physicochemical features of their habitats and extensive carbon and nitrogen stable isotope data on its main prey groups. On this basis we defined five functional groups of hatchetfishes with different diet preference, isotopic composition, and vertical abundance peaks and reveal a possible high resource partitioning. Additionally, these species might have a different feeding tie chronology. Hence, hatchetfishes segregate in different ecological groups responding differently to environmental constraints including oxygen concentration and presenting diverse functional roles. As deep-sea species that migrate to epipelagic waters, hatchetfishes may play a key role in the transfer of subsurface photoassimilated carbon to deeper waters, a pathway through which the effects of climate change at the surface are transferred to the deep ocean. Moreover, as consumers of gelatinous organisms, these species convert "gelatinous energy" into "fish energy" readily usable by higher trophic levels, including endangered and commercially important species. This is a crucial trophic relationship that has been historically underestimated due to methodology limitations (e.g., quickly digested gelatinous organisms were probably underestimated in previous studies, based solely on stomach contents). Considering in ecosystem models this trophic relationship, as well as the functional organization of hatchetfishes, is important to properly answer key ecological questions including resource use, carbon transportation, and influence of mesopelagic community in climate change process.

Understanding the processes that affect the triple oxygen isotope composition of atmospheric CO2during gas exchange can help constrain the interaction and fluxes between the atmosphere and the biosphere. We conducted leaf cuvette experiments under controlled conditions using three plant species. The experiments were conducted at two different light intensities and using CO2with different δ17O. We directly quantify the effect of photosynthesis on δ17O of atmospheric CO2for the first time. Our results demonstrate the established theory for δ18O is applicable to δ17O.CO2/at leaf level, and we confirm that the following two key factors determine the effect of photosynthetic gas exchange on the δ17O of atmospheric CO2. The relative difference between δ17O of the CO2entering the leaf and the CO2in equilibrium with leaf water and the back-diffusion flux of CO2from the leaf to the atmosphere, which can be quantified by the cm=ca ratio, where ca is the CO2mole fraction in the surrounding air and cm is the one at the site of oxygen isotope exchange between CO2and H2O. At low cm=ca ratios the discrimination is governed mainly by diffusion into the leaf, and at high cm=ca ratios it is governed by back-diffusion of CO2that has equilibrated with the leaf water. Plants with a higher cm=ca ratio modify the 117O of atmospheric CO2more strongly than plants with a lower cm=ca ratio. Based on the leaf cuvette experiments, the global value for discrimination against δ17O of atmospheric CO2during photosynthetic gas exchange is estimated to be-0:57±0:14% using cm=ca values of 0.3 and 0.7 for C4and C3plants, respectively. The main uncertainties in this global estimate arise from variation in cm=ca ratios among plants and growth conditions.

Cat remains from Poland dated to 4,200 to 2,300 y BCE are currently the earliest evidence for the migration of the Near Eastern cat (NE cat), the ancestor of domestic cats, into Central Europe. This early immigration preceded the known establishment of housecat populations in the region by around 3,000 y. One hypothesis assumed that NE cats followed the migration of early farmers as synanthropes. In this study, we analyze the stable isotopes in six samples of Late Neolithic NE cat bones and further 34 of the associated fauna, including the European wildcat. We approximate the diet and trophic ecology of Late Neolithic felids in a broad context of contemporary wild and domestic animals and humans. In addition, we compared the ecology of Late Neolithic NE cats with the earliest domestic cats known from the territory of Poland, dating to the Roman Period. Our results reveal that human agricultural activity during the Late Neolithic had already impacted the isotopic signature of rodents in the ecosystem. These synanthropic pests constituted a significant proportion of the NE cat's diet. Our interpretation is that Late Neolithic NE cats were opportunistic synanthropes, most probably free-living individuals (i.e., not directly relying on a human food supply). We explore niche partitioning between studied NE cats and the contemporary native European wildcats. We find only minor differences between the isotopic ecology of both these taxa. We conclude that, after the appearance of the NE cat, both felid taxa shared the ecological niches. ispartof: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA vol:117 issue:30 pages:17710-17719 ispartof: location:United States status: published

In Southeast Asia, biodiversity-rich forests are being extensively logged and converted to oil palm monocultures. Although the impacts of these changes on biodiversity are largely well documented, we know addition to samples we collected in 201 little about how these large-scale impacts affect freshwater trophic ecology. We used stable isotope analyses (SIA) to determine the impacts of land-use changes on the relative contribution of allochthonous and auto chthonous basal resources in 19 stream food webs. We also applied compound-specific SIA and bulk SIA to determine the trophic position of fish apex predators and meso-predators (invertivores and omnivores). There was no difference in the contribution of autochthonous resources in either consumer group (70–82%) among streams with different land-use type. There was no change in trophic position for meso-predators, but trophic position decreased significantly for apex predators in oil palm plantation streams compared to forest streams. This change in maximum food chain length was due to turnover in identity of the apex predator among land-use types. Disruption of aquatic trophic ecology, through reduction in food chain length and shift in basal resources, may cause significant changes in biodiversity as well as ecosystem functions and services. Understanding this change can help develop more focused priorities for mediating the negative impacts of human activities on freshwater ecosystems

Temporal trends of total mercury (THg) were examined in female polar bear (Ursus maritimus) hair (n = 199) from the Barents Sea in 1995-2016. In addition, hair values of stable isotopes (n = 190-197) of carbon (δ13C), sulfur (δ34S), and nitrogen (δ15N) and information on breeding status, body condition, and age were obtained. Stable isotope values of carbon and sulfur reflect dietary source (e.g., marine vs terrestrial) and the nitrogen trophic level. Values for δ13C and δ34S declined by -1.62 and -1.18‰ over the time of the study period, respectively, while values for δ15N showed no trend. Total Hg concentrations were positively related to both δ13C and δ34S. Yearly median THg concentrations ranged from 1.61 to 2.75 μg/g and increased nonlinearly by 0.86 μg/g in total over the study. Correcting THg concentrations for stable isotope values of carbon and sulfur and additionally breeding status and age slightly accelerated the increase in THg concentrations; however, confidence intervals of the raw THg trend and the corrected THg trend had substantial overlap. The rise in THg concentrations in the polar bear food web was possibly related to climate-related re-emissions of previously stored Hg from thawing sea-ice, glaciers, and permafrost.

Beech trees growing in biodiverse patches in Belgium have higher radial growth and are less physiologically sensitive to drought than those in monocultures. Forest diversification therefore alleviates the negative effects of drought on beech. Common beech, a widespread and economically important tree species in Europe, is known to be drought sensitive. For ensuring its survival under increasing future drought conditions, we need to advance our understanding on the relationships between drought and its growth performance. Diversifying forests has been proposed as a management strategy to mitigate the effects of drought, because a more complete use of the available water is expected. We made use of a tree species diversity gradient in Belgium to study if beech trees growing in diverse forest patches are more resistant and resilient to drought than beech trees in monocultures. Combining dendrochronological and stable carbon (δ13C) and oxygen isotope (δ18O) data allowed for studying the effect of tree species diversity on the response of beech growth to drought regarding stem radial growth and physiological performance. Up to 62% enhanced stem radial growth strongly increased growth stability, and higher resistance to drought was observed for beech trees in diverse forest patches. Beech performs best in three-species mixtures, particularly those with oak and maple. In drought years, beech growth is more reduced in monocultures than mixtures. During these drought years, δ13C values increased, and the increase was weaker in beech trees of diverse stands compared to monospecific stands, indicating enhanced stomatal conductance and growth continuation in mixtures. δ18O patterns did not indicate a clear effect of diversity or the response of beech trees to drought. Overall, our results indicate that until now, still the positive effects of diversity on beech growth outperform the negative effects induced by drought.

RATIONALE For clumped isotope analysis (Δ47 ), hydrocarbon and organic molecules present an important contaminant that cannot always be removed by CO2 purification through a Porapak-Q trap. Low-temperature oxygen plasma ashing (OPA) is a quick and easy approach for treatment; however, the impact of this treatment on the original carbonate clumped isotope values has never been fully studied. METHODS We tested the isotopic impact of OPA using three natural samples with a large range of initial Δ47 values. Crushed and sieved (125 μm mesh) samples were placed into a Henniker Plasma HPT-100 plasma system and treated at a flow rate of 46 mL/min and a power of 100 W at a vacuum of 0.2 mbar for 10, 20, 30 and 60 min before clumped isotope analysis using two MAT 253 isotope ratio mass spectrometers modified to measure masses 44-49. RESULTS OPA treatment for 30 min or more on calcite powder samples has the potential to alter the clumped isotopic composition of the samples beyond analytical error. A systematic positive offset is observed in all samples. The magnitude of this alteration translates to a temperature offset from known values ranging from 4°C to 13°C. We postulate that the observed positive offset in Δ47 occurs because the bonds within lighter isotopologues are preferentially broken by plasma treatment, leading to an artificial increase in the 'clumping' value of the sample. CONCLUSIONS We recommend that any laboratory performing OPA treatments should reduce the runs to 10-20 min or carry out successive runs of 10 min followed by sample stirring, as this procedure showed no alteration in the initial Δ47 values. Our results validate the use of OPA for clumped isotope applications and will allow future research to use clumped isotopes for challenging samples such as oil-stained carbonates, bituminous shales or host rocks with very high organic carbon content.