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In ice-covered lakes, near-bottom oxygen concentration decreases for most of the wintertime, sometimes down to the point that bottom waters become hypoxic. Studies insofar have reached divergent conclusions on whether climate change limits or reinforces the extent and duration of hypoxia under ice, raising the need for a comprehensive understanding of the drivers of the dissolved oxygen (DO) dynamics under lake ice. Using high-temporal resolution time series of DO concentration and temperature across 14 mountain lakes, we showed that the duration of bottom hypoxia under ice varies from 0 to 236 d within lakes and among years. The variability of hypoxia duration was primarily explained by changes in the decay rate of DO above the lake bottom rather than by differences in DO concentration at the ice onset or in the ice-cover duration. We observed that the DO decay rate was primarily linked to physical controls (i.e., deep-water warming) rather than biogeochemical drivers (i.e., proxies for lake or catchment productivity). Using a simple numerical model, we provided a proof-of-concept that the near-bed stratification can be the mechanism tying the DO decay rate to the sediment heat release under the ice. We ultimately showed that the DO decay rate and hypoxia duration are driven by the summer light climate, with faster oxygen decline found under the ice of clearer cryostratified alpine lakes. We derived a framework theorizing how the hypoxia duration might change under the ice of alpine lakes in a warmer climate., Limnology and Oceanography, 68 (6), ISSN:0024-3590, ISSN:1939-5590

Electric fishing is an illegal hunting method, unfortunately widely used by poachers to paralyze fish and to catch many animals in a short time. In Italy, it is authorized only for scientific and conservative purposes. Between 2014 and 2018, the Ferrara section of the Experimental Zooprophylactic Institute of Lombardy and Emilia Romagna, Italy, received nine cases of potentially illegal electric fishing in Po river and its tributary rivers. Necropsies were performed following standard protocols and samples of different tissues were collected and examined using histochemical and immunohistochemical techniques. Gross lesions frequently observed were circulatory alteration phenomena (i.e. multi-organ hyperemia, hemorrhages and congestion, hemopericardium), also found histologically, in addition to multifocal degenerative and necrotic muscular processes that could be attributed to injuries from electric current, as already reported in literature. Immunohistochemical investigations confirmed degenerative and necrotic lesions with myoglobin depletion and a corresponding fibrinogen accumulation. Myoglobin globules were also detected in the renal parenchyma, as consequent of rhabdomyolysis. The results of this study allowed to correlate electric fishing to gross, histologic and immunohistochemical lesions, which together constitute a pathognomonic picture to be considered a reference standard in this type of illegal controversy.

The influence of human activities on the role of inland waters in the global carbon (C) cycle is poorly constrained. In this study, we investigated the impact of human land use on the sources and biodegradation of dissolved organic matter (DOM) and its potential impact on bacterial respiration in 10 independent catchments of the Lake Geneva basin. Sites were selected along a gradient of human disturbance (agriculture and urbanization) and were visited twice during the winter high-flow period. Bacterial respiration and DOM bioavailability were measured in the laboratory through standardized dark bioassays, and the influence of human land uses on DOM sources, composition and reactivity was assessed from fluorescence spectroscopy. Bacterial respiration was higher in agro-urban streams but was related to a short-term bioreactive pool (0–6 d of incubation) of autochthonous origin, whose relative contribution to the total DOM pool increased with the degree of human disturbance. On the other hand, the degradation of a long-term (6–28 d) bioreactive pool related to terrestrial DOM was independent from the catchment land use and did not contribute substantially to aquatic bacterial respiration. From a greenhouse gas emission perspective, our results suggest that human activities may have a limited impact on the net C exchanges between inland waters and the atmosphere, as most CO2 fixed by aquatic producers in agro-urban streams is cycled back to the atmosphere after biomineralization. Although seasonal and longitudinal changes in DOM sources must be considered, the implications of our results likely apply more widely as a greater proportion of autochthonous-DOM signature is a common feature in human-impacted catchments. Yet, on a global scale, the influence of human activities remains to be determined given the large diversity of effects of agriculture and urbanization on freshwater DOM depending on the local environmental context.

Additional file 1. Main spectrum profiles (MSPs) of the Streptococcus iniae isolate showing intensity (Y-axis) as a function of the mass-to-charge ratio (m/z, molecular weight for a single positive charge; X-axis). Peaks with intensities greater than 2000 are labelled.

Chloroplast development is initiated by light-signals triggering the expression of nuclear encoded chloroplast genes in a first phase, followed by massive structural changes in the transition from proplastids to mature chloroplasts in the second phase. While the molecular players involved in the first phase are currently emerging, regulatory components of the second phase, demanding high plastid translational capacity and RNA processing, are still enigmatic. This is mostly due to the very limited amount of plant material at the early phases of development that makes biochemical studies such as identifying protein interaction networks very difficult. To overcome this problem, we developed a TurboID-based proximity labelling workflow that requires only very limited sample amounts to obtain mechanistic insights into protein interaction networks present in the early stages of plastid development. We used the CGL20a protein, a novel factor involved in chloroplast development, as bait forin vivoproximity labelling in developing seedlings 7 days after germination. We found that CGL20a resides in a nexus of RNA binding proteins mainly associated to ribosomal RNA (rRNA) including different ribosome-associated proteins.One-sentence summaryThe use of plastid-specific in vivo proximity labelling in Arabidopsis seedlings allows to identify novel components in chloroplast development in higher plants.

Whiting events are massive calcite precipitation events turning hardwater lake waters to a milky turquoise color. Herein, we use a multispectral remote sensing approach to describe the spatial and temporal occurrences of whitings in Lake Geneva from 2013 to 2021. Landsat-8, Sentinel-2, and Sentinel-3 sensors are combined to derive the AreaBGR index and identify whitings using appropriate filters. 95% of the detected whitings are located in the northeastern part of the lake and occur in a highly reproducible environmental setting. An extended time series of whitings in the last 60 years is reconstructed from a random forest algorithm and analyzed through a Bayesian decomposition for annual and seasonal trends. The annual number of whiting days between 1958 and 2021 does not follow any particular monotonic trend. The inter-annual changes of whiting occurrences significantly correlate to the Western Mediterranean Oscillation Index. Spring whitings have increased since 2000 and significantly follow the Atlantic Multidecadal Oscillation index. Future climate change in the Mediterranean Sea and the Atlantic Ocean could induce more variable and earlier whiting events in Lake Geneva.

Speckle tracking technology has been applied to assess ventricular deformation throughout the cardiac cycle. An electronic four dimensional probe that allows rapid acquisition of electronic spatio-temporal image correlation volumes (eSTIC) has been recently introduced.The aim of our study was to investigate whether e-STIC acquisition improves deformation analyses reproducibility.We recruited fetuses between 20 and 40 weeks of gestation. We obtained a 2D video clip and an e-STIC volume of a four-chamber view. We focused on left ventricular global strain (LV-GS) and left ventricular ejection fraction (LV-FE). Intraobserver, interobserver and intermethod agreement were assessed by means of intraclass correlation coefficient (ICC) and illustrated by Bland-Altman plots. Systematic differences between measurements were assessed using a paired t-test.The mean difference between LV-GS values obtained with e-STIC and 2D analysis was -0.10 (95% CI -2.28, 2.08). No systematic differences were found between the two techniques for LV-GS values (e-STIC seems to be a better technique than 2D analysis for intra-rater reliability of LV-GS. 4D acquisition might improve intrinsic limitations of speckle tracking echocardiography.

In hardwater lakes, calcite precipitation is an important yet poorly understood process in the lacustrine carbon cycle, in which catchment-derived alkalinity (Alk) is both transformed and translocated. While the physico-chemical conditions supporting the supersaturation of water with respect to calcite are theoretically well described, the magnitude and conditions underlying calcite precipitation at fine temporal and spatial scales are poorly constrained. In this study, we used high frequency, depth-resolved (0-30 m) data collected over 18 months (June 2019 - November 2020) in the deeper basin of Lake Geneva to describe the dynamics of calcite precipitation fluxes at a fine temporal resolution (day to season) and to scale them to carbon fixation by primary production. Calcite precipitation occurred during the warm stratified periods when surface water CO 2 concentrations were below atmospheric equilibrium. Seasonally, the extent of Alk loss due to calcite precipitation (i.e., [30-42] g C m -2 ) depended upon the level of Alk in surface waters. Moreover, interannual variability in seasonal calcite precipitation depended on the duration of stratification, which determined the volume of the water layer susceptible to calcite precipitation. At finer timescales, calcite precipitation was characterized by marked daily variability with dynamics strongly related to that of planktonic autotrophic metabolism. Increasing daily calcite precipitation rates (i.e., maximum values 9 mmol C m -3 d -1 ) coincided with increasing net ecosystem production (NEP) during periods of enhanced water column stability. In these conditions, calcite precipitation could remove as much inorganic carbon from the productive layers as NEP. This study provides mechanistic insights into the conditions driving pelagic calcite precipitation, and quantifies its essential contribution to the coupling of organic and inorganic carbon cycling in lakes.

Whiting events are transient phenomena commonly occurring in hardwater lakes and manifesting as a turquoise coloration of surface waters during massive calcium carbonate precipitation. While biological and physico-chemical drivers of carbonate precipitation are known, their relative contributions in controlling whiting events' timing and spatial extent remain poorly understood. Coupling spatially resolved data obtained for two sampling surveys using multiple analytical techniques and geochemical modeling, this study investigated the mechanisms underlying a whiting event during the early summer of 2019 in Lake Geneva. Satellite observations showed that the phenomenon started during a snowmelt period in the catchment at the Rhône River delta before spreading along the lake's northern shore and covering vast areas of its deeper basin. Authigenic calcite precipitated at the river mouth during mixing of warmer calcite super-saturated lake surface waters with colder snowmelt-diluted, sediment-rich river water containing detrital carbonates as potential nucleation sites. The development of the whiting event depended upon the thermal stratification of the water column and the existence of a physically stable metalimnion, within which a river interflow transported finer particles across the lake. During transport, the whiting plume enriched in authigenic carbonates by settling of coarser detrital particles and additional precipitation likely both on the fine-grained carbonate fraction and through biologically induced mechanisms in the superficial layers of the lake. This study provides novel mechanistic insights on the conditions controlling whiting events in lakes, highlighting a tight coupling of their dynamics with processes acting at the catchment scale.

We present a case of prenatal diagnosis of critical congenital aortic valve stenosis with progressive systolic left ventricular failure. An ultrasound-guided balloon aortic valvuloplasty was performed at 28 weeks of gestational age because of left ventricular dysfunction associated with signs of fetal heart failure. There were no significant post-procedural complications and the pregnancy was carried to term with elective cesarean section at 38 weeks of gestational age. At birth, an echocardiogram showed severe aortic valve stenosis with global hypokinesia of the left ventricle. Therefore a percutaneous balloon aortic valvuloplasty was repeated through transseptal approach with prompt improvement of the antegrade aortic flow and of the left ventricular systolic function. The baby is currently 2 months old and he is doing fine.

Whiting events, i.e., sudden and massive calcite precipitation turning lake waters to a milky turquoise colour, are unpredictable in space and time. This lack of understanding arises from the transitory nature of whiting events and their variable spatial extent, which are not well captured by traditional monitoring. We use a multi-spectral remote sensing approach to reconstruct the occurrence, timing, and spatial extent of whiting events in Lake Geneva from 2013 to 2021. Landsat-8 and Sentinel-2 multispectral sensors were combined and intercalibrated to derive the AreaBGR index and identify whiting events over the studied period. Then, intercalibrated remote sensing was used to estimate spatial extents and temporal occurrences of whiting events in Lake Geneva. Principal Component Analysis (PCA) identified environmental drivers of typical features of whiting events (such as meteorological, Rhône River and lake physical conditions). KNN (K-nearest neighbours) was used to reconstruct the full time-series of whiting events over the last 50 years. Ongoing results show that the combination of multi-spectral sensors is suitable for monitoring whiting events in Lake Geneva, based on the description of their spatial and temporal occurrences.

Even low productive, high-altitude lakes experience deep water hypoxia under ice-cover. While the changing ice phenology is expected to ripple on the magnitude of under-ice hypoxia, the lack of a mechanistic framework linking the physical impact of ice loss to biogeochemical properties has led to seemingly contradictory conclusions. Biogeochemical and physical processes constrain the Dissolved Oxygen (DO) dynamic at the sediment-water interface under lake ice. On the one hand, the biogeochemical hypothesis envisions a primary control of DO decay under the ice by sediment oxygen uptake, which arises from benthic microbial respiration and the release of reduced compounds. On the other hand, the physical hypothesis assumes a greater DO decay when sediment heat release reinforces the inverse stratification; the stronger is the sediment heat release, the more the bottom layer, from which oxygen is consumed, gets isolated from potential diffusive resupply from the upper layers. The outcome of a shorter ice-cover on the under-ice DO dynamics depends on the dominance of either biogeochemical or physical processes.Based on in-situ observations of DO and temperature, we assessed the relative share of biogeochemical and physical processes on decay under the ice of 14 high-altitude lakes in the French Alps. We found highly variable DO decay rates across the different lakes and years, with exponential coefficients ranging from 1.10-3 to 6.10-2 d-1. The under-ice DO decay rates increased, within years and lakes, with sediment heat release, while biogeochemical factors played only a marginal role. We tested through a reaction-diffusion model on an archetypal, testbed lake the individual effects of biogeochemical versus physical processes on DO decay. We confirmed that the sediment heat flux at ice-on is a major driver of DO decay under the ice, explaining one mechanism by which shallower or more transparent lakes experience greater DO decay under the ice.

Dissolved inorganic carbon (DIC) is rarely regarded as a limiting factor of gross primary production (GPP) because atmospheric CO2 invasion is expected to compensate for the DIC demands at high GPP levels. However, from May to August, the CO2 concentrations in the surface waters of Lake Geneva, a large and deep hardwater lake, can be as low as 100 ppm, i.e. threshold values for carbon limitation of gross primary production (GPP). Because GPP values remain high during summer in Lake Geneva, we questioned whether CO2 invasion was sufficient to supply GPP in inorganic carbon, or whether additional inorganic carbon was supplied by alkalinity, through CO2 released by the background calcite precipitation. Using the paired O2–CO2 departure from the atmospheric equilibrium coupled to alkalinity data in littoral and pelagic environments, we investigated the sources of the inorganic carbon fixed by primary production, along three hypotheses. The first hypothesis (H1) is that atmospheric CO2 invasion is sufficient to compensate for the CO2 demand. The second hypothesis (H2) builds on H1 while including the chemical enhancement of CO2 influxes. The third hypothesis (H3) consists in adding to H2 CO2 released by calcite precipitation. We find first that daily molar changes in DO are unrelated to daily molar changes in CO2, but to molar changes in alkalinity for most of the warm season. For days for which we could compute GPP, we found that for GPP < 30 µmol C L–1 d–1, H1 and H2 are sufficient to maintain the CO2 demand, in both the littoral and pelagic habitats. Above 30 µmol C L–1 d–1, even chemically enhanced CO2 invasion cannot compensate for GPP, and CO2 demand can only be meet adding CO2 supplied by alkalinity through calcite precipitation. In Lake Geneva, alkalinity could thus supply inorganic carbon to high GPP up to ~ 40% during the stratified season for littoral and pelagic primary producers.

Nodular Gill Disease (NGD) in highly intensive farming systems plays a major role in production losses and its outbreaks can result in cumulative mortality exceeding 60%, especially during the winter months and early spring. NGD was described in rainbow trout farms of North America and Europe and appears to be caused by different species of amoebae, for example both testate (Roghostoma minus) and naked amoebae belonging to five genera. In the last few years, NGD is widespread in rainbow trout farms of northern and central Italy. In March 2017, during the epidemiological investigations into NGD in salmonid farms of northern Italy, the first cases of branchitis referable to amoebic infection in juvenile brown trout (Salmo trutta L.) were observed. Twenty‐five live and moribund brown trout from each tank were collected at monthly intervals from March to May 2017 for a total of 150 fish. The samples were subjected to necropsy, parasitological analysis, histology and bacteriological and virological examinations for diagnostic purposes. Histological examination of gills from diseased and moribund brown trout showed characteristic pathological signs of NGD infection. The most significant change was the development of heavy gill lesions with variable degrees of lamellar fusion, especially in the distal part of filaments (clubbing), with obliteration of interlamellar spaces. A limited number of amoebae were detected along the surface of the gills. The lesions observed in fresh mounts and in histological sections obtained from the gills of the infected brown trout confirmed to be less serious than those detected in affected rainbow trout of the same farm. Therefore, this investigation, which reports for the first time NGD in brown trout, has shown that S. trutta would be less susceptible to this pathology than rainbow trout.

If inland waters have been acknowledged as significant CO2 emitters and reactors of the global carbon cycle, light has been essentially shed on wetlands, permafrost, and humic lakes, under the overwhelming paradigm that lake supersaturation with CO2 arises from metabolic processes. Within this picture, large and deep hardwater lakes such as Lake Geneva have been largely overlooked, considered neutral to the atmosphere. However, those conceptions rely on data that are poorly resolved in both time and space, leading to a deficient understanding of the fine-scale surface CO2 dynamics in large, deep hardwater lakes and major uncertainties on their estimated annual lake-wide CO2 emissions. Using Lake Geneva as a model for large, deep hardwater lake, the main objectives of this doctoral thesis are (i) to reach a high-resolution understanding of the surface CO2 dynamics and fluxes at the lake, (ii) to untie the physical and biogeochemical processes controlling the CO2 fluxes at the lake– atmosphere interface, and (iii) to derive methodological guidelines on the frequency at which the different components of the CO2 fluxes should be monitored to reach representative estimates of annual CO2 fluxes. The CO2 fluxes at the surface of lakes operate through a net diffusive transport, obeying the first Fickian law often expressed as 𝐹 = 𝑘(𝐶w − 𝐶sat), where F is the CO2 gas flux, k is the gas transfer velocity, 𝐶w is the CO2 concentration at the water surface, and 𝐶sat is the CO2 concentration at saturation with the atmosphere. The guideline of this work is the Fickian equation which is decomposed in its individual terms to quantify the role of physical and biogeochemical processes on their dynamics. In that objective, the temporal variation of the lake surface CO2 and gas exchange velocity was measured at an hourly resolution while their spatial component was addressed by comparing the pelagic and littoral environments. This work benefited from the ongoing initiative of off-shore and in-shore stations for high-frequency monitoring: the LéXPLORE platform (110 m depth) and the Buchillon mast (4 m depth), representative of the two environments. The first study is dedicated to the drivers of the gas transfer velocity (k). Direct and continuous measures of k are technically challenging, so that k values introduced within annual estimates of CO2 fluxes for lakes are modelled. Insofar, k models in lakes accounted for the effect of wind shear (all lake sizes) and convection (small lakes). Unlike oceanographic studies, the effect of surface waves is typically not included in lake k models although those can occur in large lakes when the wind fetch is long enough. Herein, we demonstrate that accounting for surface waves generated during windy events (> 5 m s–1) significantly improves the accuracy of k estimates in large lakes (i.e. fetch > 15 km). The computation of a new improved k model over a 1-year time period shows that episodic extreme events with surface waves can generate more than 20% of annual cumulative k and more than 25% of annual net CO2 fluxes in Lake Geneva. Moreover, the integration of the spatial variability of k is proposed using spatial meteorological model. Because all the terms of the flux equation are challenging to measure or parameterise at fine-time and space scale resolution over annual periods, few studies can simultaneously link the variabilities of CO2 flux, water CO2 and k. The aim of this second study is to assess the minimal sampling frequency of inputs data that is necessary to reach representative of estimates annual CO2 fluxes at the surface of a large lake. Herein we show that representative estimates of CO2 fluxes require high-frequency computations of k (hourly), all year round, to capture intense but short-lived turbulence events. Daily and weekly measurements of water CO2 are necessary during shoulder periods, while the CO2 sampling frequency can be loosened during periods of stability such as summer. Besides, we show that littoral CO2 fluxes, which are one order of magnitude greater than pelagic fluxes, contribute significantly to the total lake emissions even where they represent of very small share of the total lake surface. Finally, we propose solutions to improve these CO2 gas exchange quantifications using currently available numerical tools such as spatial weather model, hydrodynamical model, and data reconstruction. The last study is dedicated to the interaction between alkalinity and biological processes, in relation to surface CO2 dynamics. In alkaline freshwater systems such as Lake Geneva, the apparent absence of carbon limitation to gross primary production (GPP) at low CO2 concentrations suggests that bicarbonates can support GPP. However, the contribution of bicarbonates to GPP has never been quantified in lakes along the seasons. We can demonstrate for the first time that the available CO2 at the lake surface is not sufficient to maintain GPP for two-thirds of the year in Lake Geneva. To support the high rate of O2 production, aquatic primary producers withdraw bicarbonate from the alkalinity pool as a carbon supply for GPP. The neglected role of alkalinity in the freshwater carbon cycle is highlighted throughout an annual cycle. In addition, we show that bicarbonate-fixation by primary producers, far from being anecdotical, can be the dominant model for hardwater lakes. Finally, all the results of these three studies coupled with the existing literature allow us to propose a conceptual carbon cycle for large and deep hardwater lakes. It highlights the complex interaction of physical and biogeochemical processes responsible for CO2 emissions over an annual cycle and demonstrates that the lake can be considered as an active carbon transformer. To conclude, the limits and perspectives of this research are discussed with an emphasis on future estimates of CO2 fluxes from lakes integrated in time and space using new numerical tools such as the coupling of physical and biogeochemical models, and Deep Learning. -- Si les eaux intérieures ont été reconnues comme d'importants émetteurs de CO2 et réacteurs du cycle global du carbone, la lumière a été essentiellement faite sur les zones humides, le pergélisol et les lacs humiques, sous le paradigme selon lequel la sursaturation des lacs en CO2 découle de processus métaboliques. Dans ce tableau, les grands lacs profonds, avec des duretés de l’eau élevées, tels que le lac Léman, ont été largement négligés, considérés comme neutres vis à vis de l'atmosphère. Cependant, ces conceptions reposent sur des données mal résolues dans le temps et dans l'espace, ce qui conduit à une compréhension déficiente de la dynamique du CO2 de surface à petite échelle dans les grands lacs d'eau dure profonds et à des incertitudes majeures sur leurs émissions annuelles de CO2 estimées à l'échelle du lac. En utilisant le lac Léman comme site modèle, les principaux objectifs de cette thèse de doctorat sont (i) d'obtenir une compréhension à haute résolution de la dynamique et des flux de CO2 de surface au lac, (ii) de dénouer les liens physiques et processus biogéochimiques contrôlant les flux de CO2 à l'interface lac-atmosphère, et (iii) de dériver des directives méthodologiques sur la fréquence à laquelle les différentes composantes des flux de CO2 doivent être surveillées pour obtenir des estimations représentatives des flux annuels de CO2. Les flux de CO2 à la surface des lacs opèrent par un transport diffusif net, obéissant à la première loi de Fick, souvent exprimée comme 𝐹 = 𝑘(𝐶w − 𝐶sat), où F est le flux de gaz CO2, k est la vitesse de transfert du gaz, 𝐶w est la concentration de CO2 à la surface de l'eau, et 𝐶sat est la concentration de CO2 à saturation avec l'atmosphère. La ligne directrice de ce travail est la décomposition des termes de l'équation Fickienne pour quantifier le rôle des processus physiques et biogéochimiques sur leur dynamique. Dans cet objectif, la variation temporelle du CO2 de surface du lac et la vitesse d’échange de gaz a été mesurée à une résolution horaire tandis que leur composante spatiale a été abordée en comparant les environnements pélagiques et littoraux. Ces travaux ont bénéficié de l'initiative continue des stations off-shore et in-shore de surveillance haute fréquence : la plateforme LéXPLORE (profondeur 110 m) et le mât de Buchillon (profondeur 4 m), représentatifs des deux milieux. La première étude est dédiée aux processus impliqués dans la vitesse de transfert du gaz (k). Les mesures directes et continues de k sont techniquement difficiles, de sorte que les valeurs de k introduites dans les estimations annuelles des flux de CO2 pour les lacs sont modélisées plutôt que quantifiées sur le terrain. Jusqu'à présent, les modèles k dans les lacs ont tenu compte de l'effet du cisaillement du vent (toutes tailles de lacs) et de la convection (petits lacs). Contrairement aux études océanographiques, l'effet des vagues de surface, bien qu’occasionnellement présent dans les grands lacs lorsque le fetch du vent (distance de bord à bord d’un lac ou distance d’un bord à un point donnée sur le lac) est suffisamment long, n'est généralement pas inclus dans les modèles de k pour les lacs. Ici, nous démontrons que la prise en compte des vagues de surface générées lors d'événements venteux (> 5 m s– 1) améliore considérablement la précision des estimations de k dans les grands lacs (fetch > 15 km). L’application sur une période de 1 an du nouveau modèle k amélioré montre que des événements extrêmes épisodiques avec des vagues de surface peuvent générer plus de 20% du k cumulé annuel et plus de 25% des flux nets annuels de CO2 dans le lac Léman. De plus, l'intégration de la variabilité spatiale du k est proposée à l'aide d'un modèle météorologique spatial. Étant donné que tous les termes de l'équation de flux sont difficiles à mesurer ou à paramétrer à une résolution fine à l'échelle temporelle et spatiale sur des périodes annuelles, peu d'études peuvent relier simultanément les variabilités du flux de CO2, du CO2 de l'eau et de k. L'objectif de cette deuxième étude est d'évaluer la fréquence minimale d'échantillonnage des données d'entrée, nécessaire aux estimations représentatives des flux annuels de CO2 à la surface d'un grand lac. Ici, nous montrons que des estimations représentatives des flux de CO2 nécessitent des k modèles à haute fréquence (horaire), tout au long de l'année, pour capturer des événements de turbulence intense mais de courte durée. Des mesures quotidiennes et hebdomadaires du CO2 de l'eau sont nécessaires pendant les périodes de transitions (printemps et automne), tandis que la fréquence d'échantillonnage du CO2 peut être relâchée pendant les périodes de stabilité comme l'été. En outre, nous montrons que les flux de CO2 littoraux, qui sont supérieurs d'un ordre de grandeur aux flux pélagiques, contribuent de manière significative aux émissions totales du lac même lorsque le littoral ne représente qu'une très petite part de la surface totale du lac. Enfin, nous proposons des solutions pour améliorer ces quantifications des échanges de gaz de CO2 en utilisant des outils numériques actuels tels que les modèles météorologiques spatiaux, les modèles hydrodynamiques et la reconstruction de données. La dernière étude est consacrée à l'interaction entre l'alcalinité et les processus biologiques, en relation avec les dynamiques de CO2 de surface. Dans les systèmes d'eau douce alcalins tels que le lac Léman, l'apparente absence de limitation du carbone à la production primaire brute (GPP) à de faibles concentrations de CO2 suggère que les bicarbonates peuvent soutenir la GPP. Cependant, la contribution des bicarbonates à la GPP n'a jamais été quantifiée dans les lacs au fil des saisons. Nous pouvons démontrer pour la première fois que le CO2 disponible à la surface du lac n'est pas suffisant pour maintenir la GPP pendant les deux tiers de l'année dans le lac Léman. Pour soutenir le taux élevé de production d'O2, les producteurs primaires aquatiques pompent les bicarbonates de l'alcalinité pour soutenir la GPP. Le rôle négligé de l'alcalinité dans le cycle du carbone de l'eau douce est mis en évidence tout au long d'un cycle annuel. De plus, nous montrons que la fixation des bicarbonates par les producteurs primaires, loin d'être anecdotique, peut être le modèle dominant pour les lacs d'eau dure. Finalement, l'ensemble des résultats de ces trois études, couplé à la littérature existante, permet de proposer un cycle conceptuel du carbone pour les grands lacs alcalins profonds. Il met en évidence l'interaction complexe des processus physiques et biogéochimiques responsables des émissions de CO2 sur un cycle annuel et démontre que le lac peut être considéré comme un transformateur de carbone actif. Pour conclure, les limites et les perspectives de cette recherche sont discutées en mettant l'accent sur les estimations futures des flux de CO2 des lacs intégrés dans le temps et dans l'espace à l'aide de nouveaux outils numériques tels que le couplage de modèles physiques et biogéochimiques, et le Deep Learning.

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The gas transfer velocity (k) is a major source of uncertainty when assessing the magnitude of lake gas exchange with the atmosphere. For the diversity of existing empirical and process-based k models, the transfer velocity increases with the level of turbulence near the air–water interface. However, predictions for k can vary by a factor of 2 among different models. Near-surface turbulence results from the action of wind shear, surface waves, and buoyancy-driven convection. Wind shear has long been identified as a key driver, but recent lake studies have shifted the focus towards the role of convection, particularly in small lakes. In large lakes, wind fetch can, however, be long enough to generate surface waves and contribute to enhance gas transfer, as widely recognised in oceanographic studies. Here, field values for gas transfer velocity were computed in a large hard-water lake, Lake Geneva, from CO2 fluxes measured with an automated (forced diffusion) flux chamber and CO2 partial pressure measured with high-frequency sensors. k estimates were compared to a set of reference limnological and oceanic k models. Our analysis reveals that accounting for surface waves generated during windy events significantly improves the accuracy of k estimates in this large lake. The improved k model is then used to compute k over a 1-year time period. Results show that episodic extreme events with surface waves (6 % occurrence, significant wave height > 0.4 m) can generate more than 20 % of annual cumulative k and more than 25 % of annual net CO2 fluxes in Lake Geneva. We conclude that for lakes whose fetch can exceed 15 km, k models need to integrate the effect of surface waves.