Carrtel

Extreme weather events may be just as important as gradual trends for the long-term trajectories of ecosystems. For alpine lakes, which are exposed to both exacerbated atmospheric warming and intense episodic weather events, future conditions might not be appropriately forecast by only climate change trends, i.e., warming, if extreme events have the potential to deflect their thermal and metabolic states from their seasonal ranges. We used high-frequency monitoring data over three open-water seasons with a one-dimensional hydrodynamic model of the high-altitude Lake Muzelle (France) to show that rainstorms or windstorms, notwithstanding their intensity, did not trigger long-lasting consequences to the lake characteristics when light penetration into the lake was not modified. In contrast, storms associated with high turbidity input from the watershed ("turbid storms") strongly modified the lacustrine hydrodynamics and metabolism for the rest of the open-water season throug...

Using 3D modeling and remote sensing capabilities for a better understanding of spatiotemporal heterogeneities of phytoplankton abundance in large lakes. Journal of Great Lakes Research, 44(4), 756-764. https://doi.Bastiaan W Ibelings +41 22 379 3013 [email protected] Dominique Trevisan +33 4 50 26 78 30 +33 4 50 26 07 60 [email protected] Rob Uittenbogaard +31 88 335 8181 [email protected] Orlane Anneville +33 4 50 26 78 04 +33 4 50 26 07 60 [email protected] Abstract 26

This study deals with the impact of spatio-temporal heterogeneities on the assessment of lake ecological status according to the European water framework directive (WFD). A method, based on three-dimensional coupled hydrodynamic and ecological modeling, is presented to assess the variability of lake ecological status, and to locate the most representative sampling station of Lake Geneva (France/Switzerland). Five variables used in the lake ecological status evaluation were simulated by using the free software Delft3D. The numerical simulation results showed that the simulated ecological status based on chlorophyll a and total phosphorus concentrations measured at the regulatory monitoring station depend on the choice of the sampling date. Results also indicated a strong spatial heterogeneity in ecological status that varies from “poor” to “good” along an East-West gradient. Finally, the numerical simulation results showed that the most representative point of a mean theoretical ecol...

Extreme weather events may be just as important as gradual trends for the long-term trajectories of ecosystems. For alpine lakes, which are exposed to both exacerbated atmospheric warming and intense episodic weather events, future conditions might not be appropriately forecast by only climate change trends, i.e., warming, if extreme events have the potential to deflect their thermal and metabolic states from their seasonal ranges. We used high-frequency monitoring data over three open-water seasons with a one-dimensional hydrodynamic model of the high-altitude Lake Muzelle (France) to show that rainstorms or windstorms, notwithstanding their intensity, did not trigger long-lasting consequences to the lake characteristics when light penetration into the lake was not modified. In contrast, storms associated with high turbidity input from the watershed ("turbid storms") strongly modified the lacustrine hydrodynamics and metabolism for the rest of the open-water season through reduced light penetration. The long-lasting effects of turbid storms were related to the inputs and in-lake persistence of very-light glacial suspensoids from the watershed. The occurrence of the observed turbid storms was not related to the wind or rain intensities during the events. Instead, the turbid storms occurred after dry and atypically warm spells, i.e., meteorological conditions expected to be more frequent in this alpine region in the upcoming decades. Consequently, storm events, notwithstanding their intensity, are expected to strongly imprint the future ecological status of alpine lakes under climate warming.

Extreme weather events may be just as important as gradual trends for the long-term trajectories of ecosystems. For alpine lakes, which are exposed to both exacerbated atmospheric warming and intense episodic weather events, future conditions might not be appropriately forecast by only climate change trends, i.e., warming, if extreme events have the potential to deflect their thermal and metabolic states from their seasonal ranges. We used high-frequency monitoring data over three open-water seasons with a one-dimensional hydrodynamic model of the high-altitude Lake Muzelle (France) to show that rainstorms or windstorms, notwithstanding their intensity, did not trigger long-lasting consequences to the lake characteristics when light penetration into the lake was not modified. In contrast, storms associated with high turbidity input from the watershed ("turbid storms") strongly modified the lacustrine hydrodynamics and metabolism for the rest of the open-water season through reduced light penetration. The long-lasting effects of turbid storms were related to the inputs and in-lake persistence of very-light glacial suspensoids from the watershed. The occurrence of the observed turbid storms was not related to the wind or rain intensities during the events. Instead, the turbid storms occurred after dry and atypically warm spells, i.e., meteorological conditions expected to be more frequent in this alpine region in the upcoming decades. Consequently, storm events, notwithstanding their intensity, are expected to strongly imprint the future ecological status of alpine lakes under climate warming.

Lake biological parameters show important spatio-temporal heterogeneities. This is why explaining the spatial patchiness of phytoplankton abundance has been a recurrent ecological issue and is an essential prerequisite for objectively assessing, protecting and restoring freshwater ecosystems. The drivers of these heterogeneities can be identified by modeling their dynamics. This approach is useful for theoretical and applied limnology. In this study, a 3D hydrodynamic model of Lake Geneva (France/Switzerland) was created. It is based on the Delft3D suite software and includes the main tributary (Rhône River) and two-dimensional high-resolution meteorological forcing. It provides 3D maps of water temperature and current velocities with a 1 h time step on a 1 km horizontal grid size and with a vertical resolution of 1 m near the surface to 7 m at the bottom of the lake. The dynamics and the drivers of phytoplankton heterogeneities were assessed by combining the outputs of the model and chlorophyll-a concentration (Chl-a) data from MERIS satellite images between 2008 and 2012. Results highlight physical mechanisms responsible for the occurrence of seasonal hot-spots in phytoplankton abundance in the lake. At the beginning of spring, Chl-a heterogeneities are usually caused by an earlier onset of phytoplankton growth in the shallowest and more sheltered areas; spatial differences in the timing of phytoplankton growth can be explained by spatial variability in thermal stratification dynamics. In summer, transient and locally higher phytoplankton abundances are observed in relation to the impact of basin-scale upwelling.

This study deals with the impact of spatio-temporal heterogeneities on the assessment of lake ecological status according to the European water framework directive (WFD). A method, based on threedimensional coupled hydrodynamic and ecological modeling, is presented to assess the variability of lake ecological status, and to locate the most representative sampling station of Lake Geneva (France/ Switzerland). Five variables used in the lake ecological status evaluation were simulated by using the free software Delft3D. The numerical simulation results showed that the simulated ecological status based on chlorophyll a and total phosphorus concentrations measured at the regulatory monitoring station depend on the choice of the sampling date. Results also indicated a strong spatial heterogeneity in ecological status that varies from "poor" to "good" along an East-West gradient. Finally, the numerical simulation results showed that the most representative point of a mean theoretical ecological quality for Lake Geneva would be located in the center of the upper basin, close to the historical sampling station.

Through the artificial manipulation of hydrological residence time, hydropower operations on alpine lakes should not only alter their hydro-biogeochemical characteristics but also modulate the degree of sensitivity of the lake to atmospheric forcing. These hypotheses were addressed by means of in situ observations that were combined with a three-dimensional lake model of a high-altitude pump-storage reservoir, i.e., Lake Corne, during the ice-free period. The fitted model (Pumping scenario) was used to simulate the hydrodynamics and oxygen concentrations of the lake under natural conditions (without pump-storage). Thereafter, the lake response to direct or catchment-mediated meteorological conditions (e.g., changes in inflows during storms) was investigated with a sensitivity analysis of the forcing parameters under both the pumping and natural hydrological scenarios. The pumping operation resulted in significant changes in water temperature, heat content and water mass stability during summer, as compared to a natural scenario. The lake hydrodynamics during the ice-free season were highly responsive to changes in the summer weather conditions, and more sensitive to realistic changes in cloudiness and water transparency than changes in air temperatures for both the pumped and natural scenarios. The spatial and temporal evolution of dissolved oxygen was comparatively less responsive to summer weather conditions, in both scenarios. The pump-storage operation from Lake Corne had a comparatively smaller effect on the lake functioning than the natural meteorological variability in summer. However, the pump-storage operation decreased the sensitivity of the lake hydrodynamics to changes in water transparency and limited water mass stability during summer.

P***** Cakita, j'ai fini ! Bon je sais qu'en tapant ces mots il me reste une dernière(s ?) relecture avant quand même. La thèse ? Un projet qui t'obsède. Un marathon intellectuel et physique. J'ai mis plus de 3 ans à répondre à une question, et encore que partiellement. Mais j'ai eu l'impression de me construire mon meilleur cours, celui qui m'a le plus intéressé ! La mise en application de tes connaissances pour répondre à une question... Une aventure faite de rencontres, d'épreuves et de supers moments. Une véritable étape ! Alors merci un immense merci à toi ma Cakita, pour ton indéfectible soutien et tes petits plats qui ont le pouvoir de garder motivé. L'aventure ne fait que commencer. Encore désolé pour toutes ces nuits de travail où Tortilla m'a veillé. Au passage, merci à cette boule de poils, même si ça n'est pas évident d'écrire au clavier avec un chat dessus. A mes parents pour tout ce qu'ils ont fait pour moi, leur éducation et leur foi en moi. A mes frères Lucas et Yann, partager vos vies est un grand honneur ! Et à tous mes ami.e.s qui m'ont soutenu.e.s à base de soirées, pour s'assurer de ma survie, et de « toujours pas fini ? ». C'est bon maintenant. BIG UP au 225 éternel, au BGSL et au P5 ! Un merci spécial à Florian, qui comprend le sens profond d'abnégation et de patience nécessaire au modélisateur. A l'aventurière éternelle du 93 et à Michou ! Un grand merci à mes directeurs de thèse Marie-Elodie Perga et Damien Bouffard pour leur accompagnement dans cette expérience passionnante. Les contraintes géo-professionnelle n'ont pas rendu la tâche facile et j'apprécie d'autant plus leur investissement. Merci ! Merci au CIH-EDF, et plus particulièrement Vincent Chanudet, Agnès Bariller et Stéphane Descloux pour leurs soutiens scientifiques, amical et leurs confiances, et Segula Engineering (Sébastien Bréteche, Pierre Gervais, Florence Russo et Lionel Verrière) pour leurs soutiens dans le montage de ce projet. Merci à Julien Hacquard et à tout le GU du Cheylas, ainsi que HDF, pour leurs disponibilités et leurs sympathies. Toujours dispo pour un vol en hélico ou pour aller voir ce qu'il se passe là-haut ! Début Juillet, au lever de soleil, c'est le must! A ces heures à porter du matos en claies de portage, parfois en plein orage, à ce temps où tu te retrouves en kayak à 2000m en Août sous la neige, à cette Mike Giver académie qu'est une thèse terrain -Pensée spéciale au Projet survie du bateau du lac Blanc et à l'optimisation de la filtration in situ (merci aux inventeur.e.s du scotch orange), au projet centrifugation in-situ de sédiment (merci Ce projet de thèse a commencé il y a maintenant 5 ans. Déjà en poste au sein de Segula Engineering, j'ai voulu initier une progression de mon travail, autant sur les outils que sur les compétences générales. L'idée de reprendre mes études en parallèle de mon travail m'est alors apparu comme un bon axe pratique (malgré des avertissements contraire d'ami.e.s docteur.e.s, que je saisi bien mieux aujourd'hui). Une discussion s'est donc mis en place sur cette possibilité avec Segula et EDF qui m'ont soutenu et laisser de grande libertés dans la définition de mon sujet. J'ai par la suite monté le projet de thèse (coût, financement, planning et objectifs initiaux) avant de finaliser le tout avec mes directeurs de thèse rencontrés en cours de réflexion. La qualité de l'encadrement scientifique m'a permis de venir à bout de ce projet, malgré des difficultés inhérentes à un éloignement géographique et une activité professionnelle continue (30% du temps). Enfin, ce travail de thèse s'est révélé fructueux dans la progression de mes compétences scientifiques et techniques et m'a ainsi permis de faire évoluer mes missions professionnelles, au travers de nombreuses collaborations scientifiques hors-thèse et liées à ma thèse, tel qu'espéré lors de la mise en place de ce projet. Contributions en co-auteur aux trois articles suivants:

Environmental monitoring is a key component of understanding and managing ecosystems. Given that most monitoring e orts are still expensive and time-consuming, it is essential that monitoring programs are designed to be e cient and e ective. In many situations, the expensive part of monitoring is not sample collection, but instead sample processing, which leads to only a subset of the samples being processed. For example, sediment or ice cores can be quickly obtained in the field, but they require weeks or months of processing in a laboratory setting. Standard sub-sampling approaches often involve equally-spaced sampling. We use simulations to show how many samples, and which types of sampling approaches, are the most e ective in detecting ecosystem change. We test these ideas with a case study of Cladocera community assemblage reconstructed from a sediment core. We demonstrate that standard approaches to sample processing are less e cient than an iterative approach. For our case study, using an optimal sampling approach would have resulted in savings of 195 person-hours-thousands of dollars in labor costs. We also show that, compared with these standard approaches, fewer samples are typically needed to achieve high statistical power. We explain how our approach can be applied to monitoring programs that rely on video records, eDNA, remote sensing, and other common tools that allow re-sampling.

Extreme weather events may be just as important as gradual trends for the long-term trajectories of ecosystems. For alpine lakes, which are exposed to both exacerbated atmospheric warming and intense episodic weather events, future conditions might not be appropriately forecast by only climate change trends, i.e., warming, if extreme events have the potential to deflect their thermal and metabolic states from their seasonal ranges. We used high-frequency monitoring data over three open-water seasons with a one-dimensional hydrodynamic model of the high-altitude Lake Muzelle (France) to show that rainstorms or windstorms, notwithstanding their intensity, did not trigger long-lasting consequences to the lake characteristics when light penetration into the lake was not modified. In contrast, storms associated with high turbidity input from the watershed ("turbid storms") strongly modified the lacustrine hydrodynamics and metabolism for the rest of the open-water season throug...

1. Hysteresis linked to alternative stable states may explain delays in water quality recovery despite reduced nutrient loadings in shallow lakes. Because deep lakes are assumed to be less prone to critical transitions, similar delays are attributed to the confounding effects of additional environmental disturbances, such as climate warming. Herein, we hypothesised that the lack of evidence of nutrient-driven alternative stable states in a deep lake arises from the fact that the nutrient threshold that causes the critical transition is lower than the nutrient threshold in shallow lakes. Thereby, it might have been crossed much earlier in the lake history. 2. To test this hypothesis, we focused on the palaeo-ecological trajectory of Lake Varese, which is a deep, hypereutrophicated peri-alpine lake undergoing restoration. Proxies for drivers of ecological state (i.e. total phosphorus-TP-through diatoms and pigments) and ecological responses (Cladocera), as well as a repeatable analysis, were used to identify transitions and to distinguish hysteretic delays from those of the ecosystems responding to additional constraints over the past century. 3. Our results suggest spatial heterogeneity in the ecological response. The littoral habitats changed abruptly and prematurely for a low TP threshold, causing a shift that met many criteria of a flickering-type critical transition. Soon after the littoral shift, a striking increase in the lake phosphorous concentration was recorded and drove the pelagic assemblage towards a new state. This transition was abrupt, and the pelagic communities exhibited limited evidence of recovery; however, we found no evidence of hysteresis. Therefore, the modern ecological trajectory of the pelagic communities is currently driven by climate warming. 4. This detailed analysis allowed us to go beyond the general pattern that links ecological responses to drivers and suggest that a nonlinear transition following eutrophication can take place in a deep lake synchronously with linear transitions. Instead of triggering a new regime shift, climate warming, to which pelagic habitats are more sensitive than littoral ones, has driven the lake further from its safe operating space.

Short-lived radionuclides are measured in surface sediment to provide a geochronology for the past century. Age-depth models can be produced from 210Pbex activity-derived sedimentation rates and confirmed by 137Cs and 241Am activities that are result of fallout from nuclear weapon tests and the Chernobyl accident. Different methods of age depth modelling using such data require expertise in lake sedimentation processes.Here, we present a package, serac, that allows the user to compute an age-depth model, output a graph and an age model as a text file, and provide metadata using the free open-source statistical software R. serac ensures the reproducibility of age-depth or age-mass depth models and allows testing of several 210Pbex models (CFCS, CIC, CRS) and sedimentation hypotheses (changes in the sedimentation rates, instantaneous deposits, varved sedimentation, etc.). Using several case studies, including lakes and lagoon in different environments, we demonstrate the use of the pr...

Environmental monitoring is a key component of understanding and managing ecosystems. Given that most monitoring efforts are still expensive and time-consuming, it is essential that monitoring programs are designed to be efficient and effective. In many situations, the expensive part of monitoring is not sample collection, but instead sample processing, which leads to only a subset of the samples being processed. For example, sediment or ice cores can be quickly obtained in the field, but they require weeks or months of processing in a laboratory setting. Standard sub-sampling approaches often involve equally-spaced sampling. We use simulations to show how many samples, and which types of sampling approaches, are the most effective in detecting ecosystem change. We test these ideas with a case study of Cladocera community assemblage reconstructed from a sediment core. We demonstrate that standard approaches to sample processing are less efficient than an iterative approach. For our ...

Mountain lakes are especially vulnerable to climate change, but are also increasingly exposed to local anthropogenic development through winter and summer tourism. In this study, we aimed to tease apart the influence of tourism from that of climate in a mountain lake located within one of the largest French ski resorts, by combining paleolimnological and present ecological data. The reconstructed long-term ecological dynamics highlighted an increase in lake biological production from the end of the Little Ice Age up to the 1950s, suggesting a historical dominance of climate control. Afterward, a major drop in pelagic production occurred at the same time as the watershed erosion increased and peaked in the 1990s, concomitant with massive digging for the ski resort expansion. The benthic invertebrates collapsed in the 1980s, concomitantly with the onset of massive salmonid stocking and recent warming. Stable isotope analyses identified benthic invertebrates as the major salmonid diet resource and suggested a possible direct impact of salmonid stocking on benthic invertebrates. However, habitat use may differ among salmonid species as suggested by the way fish DNA was preserved in surficial sediment. The high abundances of macrozooplankton further confirmed the limited reliance of salmonids on pelagic resources. The variable thermal tolerance of benthic invertebrates suggested that the recent warming may mostly affect littoral habitats. Our results indicate that winter and summer tourism may differently affect the biodiversity of mountain lakes and could collectively interfere with the ecological impacts of recent warming, making local management of primary importance to preserve their ecological integrity.