Carbon sources and pathways in lake pelagic food webs based on carbon stable isotope analysis of daphnids and their subfossil remains, a combined neo- and paleolimnological approach.

Les lacs sont des écosystèmes cruciaux pour la régulation du cycle global du carbone, pouvant fixer le carbone inorganique via la photosynthèse et minéraliser le carbone organique pour produire du CO2 et du CH4. L’intensification récente des pressions anthropiques a conduit à un bouleversement du cycle biogéochimique du carbone pour de nombreux lacs, se traduisant par une intensification des processus de minéralisation de la matière organique. Ces dynamiques peuvent conduire à un déséquilibre de la balance entre les processus d’émission, de stockage et de transport dans les réseaux trophiques du carbone. Les facteurs de contrôle et les mécanismes de transfert du carbone dans les réseaux trophiques restent cependant mal connus, notamment en ce qui concerne les réseaux trophiques pélagiques. Cette thèse vise donc à étudier les sources et voies de transferts du carbone aux consommateurs pélagiques à des échelles temporelles variables, et à évaluer l’influence des perturbations touchant les systèmes lacustres. La stratégie mise en œuvre combine des approches synchroniques et diachroniques et s’appuie sur l’analyse des isotopes stables du carbone des daphnies et de leurs restes subfossiles, les éphippies. Les résultats acquis au cours de cette thèse ont contribué à montrer une variabilité saisonnière des sources de carbone consommées par les daphnies avec notamment des transferts soutenus de carbone méthanogénique (C-CH4) en période hivernale. Nos résultats montrent également que la signature isotopique des éphippies reflète plus ou moins les transferts hivernaux de C-CH4 en fonction de leur période de production par rapport à la période du brassage automnal des lacs. Nous avons ainsi proposé une stratégie originale basée sur l’utilisation des éphippies combinée à celle d’autres restes subfossiles ayant une intégration temporelle plus large pour l’étude du cycle du carbone en paléolimnologie. Enfin, les approches néo- et paléolimnologiques développées dans cette thèse mettent en évidence l’influence de la stratification thermique, de l’occupation des bassins-versant et du niveau trophique des lacs sur les processus de production et de transfert du CO2 et du CH4 dans les réseaux trophiques pélagiques.Lakes are key ecosystems in the global carbon cycling, which can fix inorganic carbon via photosynthesis and mineralize organic carbon to produce CO2 and CH4. The recent intensification of anthropogenic pressures has led to a disruption of the biogeochemical carbon cycle for many lakes, resulting in an intensification of organic matter mineralization processes. These dynamics can then lead to an imbalance between the processes of emission, storage and transport of carbon in food webs. However, the control factors and mechanisms of carbon transfer in food webs remain poorly understood, especially for pelagic food webs. This thesis aims to study these mechanisms and pathways of carbon transfer to pelagic consumers at variable temporal scales, as well as its response to metabolic perturbations in these lake systems. The strategy implemented combines synchronic and diachronic approaches and relies on the analysis of stable carbon isotopes of daphnia and their subfossil remains, the ephippia. The results acquired during this thesis have contributed to show a seasonal variability of carbon sources consumed by daphnids, with sustained transfers of methanogenic carbon (C-CH4) during winter. Our results also show that the isotopic signature of the ephippia reflects more or less the winter transfers of C-CH4 according to their production period relative to the timing of lakes autumnal turnover. We have thus proposed an original strategy based on the use of ephippia combined with other subfossil remains with a wider temporal integration for the study of the carbon cycle in paleolimnology. Finally, the neo- and paleolimnological approaches developed in this thesis highlight the influence of thermal stratification, catchment occupation and lakes trophic level on the production and transfer processes of CO2 and CH4 in pelagic food webs

Influence of thermal regime, oxygen conditions and land use on source and pathways of carbon in lake pelagic food webs

International audienc

Impacts of anthropogenic eutrophication on the carbon transfers in lacustrine food webs: A paleolimnological assessment in four mountain lakes

International audienceOver the last decades, the intensification of anthropogenic activities and associated disturbances on lake watersheds have led to major changes in lakes trophic functioning through accelerated eutrophication. In many lakes, these changes are characterized by an unprecedented increase in organic carbon fluxes, potentially leading to a shift in biogeochemical cycles and in the balance between carbon sequestration and greenhouse gas emissions. Understanding the response of the carbon cycle to natural and anthropogenic environmental changes is becoming a crucial challenge in the context of increasing global pressures. In this study, we reconstructed the changes in the trophic functioning of the benthic and pelagic food web, in response to accelerated eutrophication in four lakes over the last millenium. Changes in carbon pathways in food webs were assessed using stable carbon isotope analysis of chitinous subfossil remains of Daphnia, Bosmina, and Chironomini archived in sedimentary records. Changes in the trophic state were inferred from sedimentary geochemical analysis, including carbon accumulation rates, Chl a accumulation rates and carbon-to-nitrogen ratios. Agro-pastoral activities were tracked by analysis of coprophilous ascospores. Results provided by this multi-proxy approach highlight recent disruptions in carbon sources and transfer pathways in lakes food webs. In particular, changes in the carbon isotopic signature of pelagic consumers suggest a recent increase in the use of 13 C-depleted carbon sources such as methanogenic or respiration-derived carbon linked to recent changes in trophic state under the intensification of anthropogenic pressures

Spatial distribution of sediment archaeal and bacterial communities relates to the source of organic matter and hypoxia – a biogeographical study on Lake Remoray (France)

International audienceBottom waters hypoxia spreads in many lakes worldwide causing severe consequences on whole lakes trophic network. Here, we aimed at understanding the origin of organic matter stored in the sediment compartment and the related diversity of sediment microbial communities in a lake with deoxygenated deep water layers. We used a geostatistical approach to map and compare both the variation of organic matter and microbial communities in sediment. Spatialisation of C/N ratio and δ13C signature of sediment organic matter suggested that Lake Remoray was characterized by an algal overproduction which could be related to an excess of nutrient due to the close lake-watershed connectivity. Three spatial patterns were observed for sediment microbial communities after the hypoxic event, each characterized by specific genetic structure, microbial diversity and composition. The relative abundance variation of dominant microbial groups across Lake Remoray such as Cyanobacteria, Gammaproteobacteria, Deltaproteobacteria and Chloroflexi provided us important information on the lake areas where hypoxia occurs. The presence of methanogenic species in the deeper part of the lake suggests important methane production during hypoxia period. Taken together, our results provide an extensive picture of microbial communities' distribution related to quantity and quality of organic matter in a seasonally hypoxic lake

Spatial and seasonal variability of the carbon isotopic signature of Daphnia and their ephippia in four French lakes: Implications for the study of carbon transfers in lake food webs

International audienceCarbon isotope analysis (delta C-13) of Daphnia resting-eggs (ephippia) in sedimentary records can be used to reconstruct past carbon transfers in pelagic food webs in lakes. However, there may be seasonal variability of cladoceran delta C-13 and ephippia production that could affect their use as palaeoecological indicators of pelagic carbon transfers. This is particularly likely in stratified lakes where availability of different pelagic carbon sources is seasonal. In addition, there are currently no studies on spatial variability of ephippia delta C-13 in deep areas of lakes and its implications for sampling strategies in paleolimnological studies. Four French lakes were sampled for over a year to evaluate seasonal variation of the carbon sources consumed by Daphnia using analyses of the Daphnia carbon isotope signature (delta C-13(Daphnia)) and suspended organic matter signature (delta C-13(seston)). Daphnia ephippia were also collected from surface sediments at maximum depth in the four lakes. Ephippia signatures (delta C-13(ephippia)) were compared to the delta C-13(Daphnia) to evaluate ephippia production periods and the implications for the use of delta C-13(ephippia) in trophic functioning studies. In addition, spatial variability of delta C-13(Daphnia) among the four lakes and the variability of delta C-13(ephippia) in the deep area of one lake were assessed to determine the relevance of a single sampling point in the context of paleolimnological studies. A similar pattern was observed in all four lakes: delta C-13(Daphnia) was close to delta C-13(seston) during the summer stratification period, but delta C-13(Daphnia) became much lower than delta C-13(seston) after the autumnal turnover. Ephippia seem to be produced either during the summer stratification period or after autumnal turnover depending on the lake. No spatial variability was observed among the sampling points in delta C-13(Daphnia) in the four lakes, and delta C-13(ephippia) seems to be spatially randomly distributed in the deep zone of the studied lake. In the four lakes, summer delta C-13(Daphnia) values were never below -40 parts per thousand, reflecting mainly the consumption of phytoplankton, which in turn uses varying amounts of CO2 from respiration (depending on the lake). After autumnal turnover, the delta C-13(Daphnia) values reached values far below -40 parts per thousand, reflecting the transfer of a non-negligible part of C-CH4 for three of the lakes. Seasonal stratification and turnover mechanisms seem to influence the availability of carbon sources in the pelagic compartment of the four lakes. This study shows that the timing of ephippia production affects the information provided by delta C-13(ephippia) due to seasonal differences in the mechanisms (stratification, autumnal turnover) that determine which carbon sources are available in the pelagic compartment. Therefore, using delta C-13(ephippia) to study past pelagic transfers of carbon in stratified lakes may entail uncertainty if used alone and requires multi-proxy studies. Finally, delta C-13(ephippia) appear to be homogeneously distributed in the deepest area of a medium-sized, single basin.A single core retrieved from the deepest part of the lake should therefore provide a representative sample of the ephippia produced in a lake

Spatial and seasonal variability of the carbon isotopic signature of Daphnia and their ephippia in four French lakes: Implications for the study of carbon transfers in lake food webs

International audienceIntensification of anthropogenic activities in many lake catchments during the twentieth century led to increased autochthonous organic matter sedimentation and degradation of hypolimnetic oxygen conditions due to the intensification of heterotrophic processes. These processes can be amplified by the effect of climate warming on thermal stratification in lakes. This study aimed to assess how metabolic disruptions affect carbon sources and pathways in lake pelagic food webs, focusing on methanogenic carbon. The studied lakes showed strong seasonal variations of carbon source availability and transfers to pelagic food webs, characterized by increased methanogenic carbon transfers to Daphnia populations in winter. The magnitude of these winter transfers seems to largely depend on the amount of methane stored in the hypolimnion during the stratification period, and thus on the amount of methane released with autumnal turnover. Methane production, storage and transfer mechanisms partly depend on thermal stratification intensity, but also on external factors such as land use. This study provides new insights into the impacts of global changes on the sources and pathways of carbon in pelagic food webs through their influence on lake metabolism and thermal regimes. These functional changes may lead to greater production and release of greenhouse gases into the atmosphere

Spatial and seasonal variability of the carbon isotopic signature of Daphnia and their ephippia in four French lakes: Implications for the study of carbon transfers in lake food webs

International audienceIntensification of anthropogenic activities in many lake catchments during the twentieth century led to increased autochthonous organic matter sedimentation and degradation of hypolimnetic oxygen conditions due to the intensification of heterotrophic processes. These processes can be amplified by the effect of climate warming on thermal stratification in lakes. This study aimed to assess how metabolic disruptions affect carbon sources and pathways in lake pelagic food webs, focusing on methanogenic carbon. The studied lakes showed strong seasonal variations of carbon source availability and transfers to pelagic food webs, characterized by increased methanogenic carbon transfers to Daphnia populations in winter. The magnitude of these winter transfers seems to largely depend on the amount of methane stored in the hypolimnion during the stratification period, and thus on the amount of methane released with autumnal turnover. Methane production, storage and transfer mechanisms partly depend on thermal stratification intensity, but also on external factors such as land use. This study provides new insights into the impacts of global changes on the sources and pathways of carbon in pelagic food webs through their influence on lake metabolism and thermal regimes. These functional changes may lead to greater production and release of greenhouse gases into the atmosphere

Origin and fate of dissolved inorganic carbon in a karst groundwater fed peatland using δ13CDIC

International audienceContinental hydrosystems and in particular peatlands play an important role in the carbon cycle of the Critical Zone (CZ). Peatlands are important sinks for organic carbon and have therefore been extensively studied. However, peatlands are not only important for the fate of organic carbon, but they also affect the cycle of Dissolved Inorganic Carbon (DIC) of the peatland and the surrounding watershed. The fate of DIC is particularly complex in peatlands in limestone-dominated regions, because bicarbonate concentrations in surface and groundwater are high and the interaction between peatlands and surrounding hydrosystems are facilitated by the presence of highly permeable karst aquifers. In the present paper we study the origin and the fractionation of DIC in a peatland located on top of a karst aquifer. The study is based on hydrochemical and isotopic (δ13CDIC) data from samples recovered during 2 campaigns (low flow, high flow) at various depths within the Forbonnet peatland (Jura Mountains, eastern France), at the peatland outlet and at adjacent karst springs representing the underlying aquifer. In order to evaluate secondary fractionation processes, the measured δ13CDIC compositions were compared to modeled values considering the origin of DIC and potentially associated fractionation and speciation processes. The main results are: (1) DIC is lost at the bog surface by CO2 outgassing. (2) The δ13CDIC compositions of deep catotelm pore waters from the bog were much heavier than the modeled values. We relate this discrepancy to methanogenesis and show that this process is favored by reduced conditions at pH ~ 6 and a HCO3 − content of ~1 mmol/L, most probably due to punctual groundwater inflows at the base of the bog. Finally, contrasted δ13CDIC compositions between the bog and the fen of the peatland reveal an additional ecohydrological control on DIC speciation

From microbiological to ecosystemic scale evaluation of carbon-based (CO2, CH4) greenhouse gas sources, production, and transfers in temperate peatlands: a pluridisciplinary week at the playground for Critical Zonists in Frasne, Jura Mountains

International audienceDespite covering only 3% of the global land surface, peatlands are an active part of the Critical Zone (CZ) exchanging large water and greenhouse gas (GHG) fluxes with the surrounding aquifers, surface waters, and the atmosphere. While ecosystem services of peatlands (carbon and water storage, buffering of local climate) are essential to address 21st century challenges regarding climate, biodiversity, and water resources, they are directly threatened by human activities at global (climate change) and local (drainage for agriculture, forestry and peat harvesting) scales. Understanding the hydrological, biogeochemical, and ecological mechanisms of peatlands functioning at different spatiotemporal scales is therefore fundamental to mitigate these impacts. In order to characterize the mechanisms and factors controlling GHG sources, production and transfers in peatlands, we organized an interdisciplinary field campaign at the Frasne peatland. The site (7 ha, 46.826 N, 6.1754E, 840 m a.s.l) is a long-term observatory since 2008 and one of the four French peatland observatories (SNO Tourbi`eres) of the French CZ research infrastructure (OZCAR). The peatland is also an observatory of the Zone Atelier of Arc Jurassien dedicated to exploring the interrelationships between human and nature.This campaign is supported by the TERRA FORMA project, aiming at designing and testing in-situ smart, connected, low-cost, low-impact and socially appropriated environmental sensors to capture the trajectory of the CZ in the Anthropocene. This field campaign will combine microbiological characterization (membrane lipid analysis to trace the involved microbial metabolisms) with hydrogeochemical analyses of peat pore water (major elements, DOC (quantity and quality), CO2, CH4, δ18OH2O-δ2HH2O, δ13CDIC, δ13CDOC, δ13CCH4, δ2HCH4, δ13CCO2) along upstream-downstream and surface-depth gradients. In parallel, GHG fluxes will be measured from the plot to the ecosystem scale, by combining dissolved gas profiles, chamber measurements, eddy-covariance and unmanned aerial vehicle characterization. This multiscale campaign will have the potential to address various challenges faced by Critical Zonists and environmental managers: (1) assessing 3D carbon fluxes (lateral and vertical) at the peatland scale; (2) characterizing biological processes and in particular how they favor or limit GHG production; (3) and transfers and developing affordable and user-friendly tools to face the above-mentioned topics