Analyse et modélisation de l'eutrophisation de la Loire

This thesis aims at studying the physical and chemical causes and consequences that result from excessivephytoplankton growth in the Loire River. The analysis identified some parameters characterizing rivereutrophication and was conducted on several spatial and temporal scales. This was based on long-termwater quality time-series (1980-2012), a daily survey carried out during this work (2012-2014), and thenumerical modeling of the river biogeochemical functioning with an hourly resolution.Phytoplankton development in summer was reduced 2.5-fold in the Loire River and in the main tributaries,synchronously with the generalized reduction 3-fold of bioavailable phosphorus as a result of controllingand limiting phosphorus point sources.However, the Loire River remains sensitive to eutrophication, with a significant urban and agriculturalpressure, low water levels in summer, and its multiple channels morphology slowing down the watervelocity. All these factors combined favor phytoplankton development. When hydrological conditionsare favorable, phytoplankton grows and significantly affects the ecosystem functioning, with an impacton nutrients, carbon and oxygen biogeochemical cycles.Ce travail a pour objectif d’étudier les conditions physicochimiques qui causent dans la Loire desdéveloppements excessifs de phytoplancton, et de mettre en évidence les modifications des équilibresbiogéochimiques qui en résultent. L’analyse des variables de l’eutrophisation a été conduite sur plusieursniveaux d’échelle temporelle et spatiale, sur la base de longues chroniques de qualité de l’eau (1980-2012), de données acquises à l’échelle journalière pendant la thèse (2012-2014), et d’un modèle numériquebiogéochimique à résolution horaire.Depuis les conditions hypereutrophes des années 1980, la biomasse phytoplanctonique qui se développe enpériode estivale a été réduite d’un facteur 2,5 dans la Loire et dans ses affluents, de manière concomitanteavec la baisse généralisée d’un facteur 3 des teneurs en phosphore biodisponible, liée au contrôle desrejets ponctuels urbains et industriels.Le fleuve de la Loire (France) reste cependant sensible à l’eutrophisation, du fait d’une forte pressionagricole et urbaine, des étiages prononcés, et une morphologie fluviale à chenaux multiples qui ralentitl’écoulement, laissant pleinement au phytoplancton le temps de se développer. Lorsque les conditionshydrologiques le permettent, le développement du phytoplancton impacte fortement le fonctionnementde l’écosystème, en agissant significativement sur les cycles biogéochimiques des nutriments, du carboneet de l’oxygène

Non-photochemical quenching estimates from in situ spectroradiometer measurements: implications on remote sensing of sun-induced chlorophyll fluorescence in lakes

Quantum yield of fluorescence (ϕF) is key to interpret remote measurements of sun-induced fluorescence (SIF), and whether the SIF signal is governed by photochemical quenching (PQ) or non-photochemical quenching (NPQ). Disentangling PQ from NPQ allows using SIF estimates in various applications in aquatic optics. However, obtaining ϕF is challenging due to its high temporal and physiological variability, and the combined measurements needed to enclose all relevant optical paths. In inland waters, this type of data is scarce and information on diurnal and seasonal ϕF dynamics are almost unknown. Using an autonomous hyperspectral Thetis profiler in Lake Geneva, we demonstrate how to estimate ϕF using an ensemble of in-situ measurements acquired between 2018 to 2021. We use vertical and temporal changes in retrieved ϕF to determine NPQ and PQ conditions. We observed NPQ in 36% of the total daytime profiles used in the ϕF analysis. While downwelling irradiance is a significant contributor to ϕF, its role cannot be easily interpreted. Other factors such as phytoplankton photoregulation and assemblages also likely play significant roles in quenching mechanisms. We conclude that an adapted approach exploiting in-situ data is suitable to determine diurnal and seasonal NPQ occurrence, and helps develop future remote sensing algorithms

The imprint of primary production on high-frequency profiles of lake optical properties

Water inherent optical properties (IOPs) contain integrative information on the optical constituents of surface waters. In lakes, IOP measurements have not been traditionally collected. This study describes how high-frequency IOP profiles can be used to document short-term physical and biogeochemical processes that ultimately influence the long-term trajectory of lake ecosystems. Between October 2018 and May 2020, we collected 1373 high-resolution hyperspectral IOP profiles in the uppermost 50 m of the large mesotrophic Lake Geneva (Switzerland-France), using an autonomous profiler. A data set of this size and content does not exist for any other lake. Results showed seasonal variations in the IOPs, following the expected dynamic of phytoplankton. We found systematic diel patterns in the IOPs. Phases of these diel cycles were consistent year-round, and amplitudes correlated to the diurnal variations of dissolved oxygen, clarifying the link between IOPs and phytoplankton metabolism. Diel amplitudes were largest in spring and summer under low wind condition. Wind-driven changes in thermal stratification impacted the dynamic of the IOPs, illustrating the potential of high-frequency profiles of water optical properties to increase our understanding of carbon cycling in lake ecosystems

Distribution of Landscape Units Within Catchments Influences Nutrient Export Dynamics

Excess nutrients from agriculture have caused persistent eutrophication in aquatic ecosystems worldwide. Here, we present a conceptual framework for landscape management to achieve one or several water quality targets along the river continuum from headwaters to estuaries. Based on monitoring of representative headwaters and downstream reaches, we divide catchments into elementary landscape units defined by ecosystem properties and anthropogenic land use. We use a theoretical simulation to evaluate our hypothesis that the water-quality responses of redistributing these elementary units within the catchment will vary depending on the water quality targets (e.g., reduction in concentration or load). This landscape unit distribution (LUD) framework can efficiently assess the current ecohydrological functioning of a catchment and provide simple but robust predictions of its response to landscape management changes. Using simulated data, we show that different scenarios of landscape redistribution can allow attainment of one or several, but often not all desired water quality targets. Therefore, we recommend that water quality targets must be clearly defined and prioritized prior to designing landscape management strategies

A Bayesian data assimilation framework for lake 3D hydrodynamic models with a physics-preserving particle filtering method using SPUX-MITgcm v1

We present a Bayesian inference for a three-dimensional hydrodynamic model of Lake Geneva with stochastic weather forcing and high-frequency observational datasets. This is achieved by coupling a Bayesian inference package, SPUX, with a hydrodynamics package, MITgcm, into a single framework, SPUX-MITgcm. To mitigate uncertainty in the atmospheric forcing, we use a smoothed particle Markov chain Monte Carlo method, where the intermediate model state posteriors are resampled in accordance with their respective observational likelihoods. To improve the uncertainty quantification in the particle filter, we develop a bi-directional long short-term memory (BiLSTM) neural network to estimate lake skin temperature from a history of hydrodynamic bulk temperature predictions and atmospheric data. This study analyzes the benefit and costs of such a state-of-the-art computationally expensive calibration and assimilation method for lakes.</p

Nonlinear empirical modeling to estimate phosphorus exports using continuous records of turbidity and discharge

Special section: Continuous nutrient sensing in research and management: applications and lessons learned across aquatic environments and watershedsInternational audienceWe tested an empirical modeling approach using relatively low‐cost continuous records of turbidity and discharge as proxies to estimate phosphorus (P) concentrations at a subhourly time step for estimating loads. The method takes into account nonlinearity and hysteresis effects during storm events, and hydrological conditions variability. High‐frequency records of total P and reactive P originating from four contrasting European agricultural catchments in terms of P loads were used to test the method. The models were calibrated on weekly grab sampling data combined with 10 storms surveyed subhourly per year (weekly+ survey) and then used to reconstruct P concentrations during all storm events for computing annual loads. For total P, results showed that this modeling approach allowed the estimation of annual loads with limited uncertainties (≈ −10% ± 15%), more reliable than estimations based on simple linear regressions using turbidity, based on interpolated weekly+ data without storm event reconstruction, or on discharge weighted calculations from weekly series or monthly series. For reactive P, load uncertainties based on the nonlinear model were similar to uncertainties based on storm event reconstruction using simple linear regression (≈ 20% ± 30%), and remained lower than uncertainties obtained without storm reconstruction on weekly or monthly series, but larger than uncertainties based on interpolated weekly+ data (≈ −15% ± 20%). These empirical models showed we could estimate reliable P exports from noncontinuous P time series when using continuous proxies, and this could potentially be very useful for completing time‐series data sets in high‐frequency surveys, even over extended periods

Human domination of the global water cycle absent from depictions and perceptions

International audienceHuman water use, climate change and land conversion have created a water crisis for billions of individuals and many ecosystems worldwide. Global water stocks and fluxes are estimated empirically and with computer models, but this information is conveyed to policymakers and researchers through water cycle diagrams. Here we compiled a synthesis of the global water cycle, which we compared with 464 water cycle diagrams from around the world. Although human freshwater appropriation now equals half of global river discharge, only 15% of the water cycle diagrams depicted human interaction with water. Only 2% of the diagrams showed climate change or water pollution—two of the central causes of the global water crisis—which effectively conveys a false sense of water security. A single catchment was depicted in 95% of the diagrams, which precludes the representation of teleconnections such as ocean–land interactions and continental moisture recycling. These inaccuracies correspond with specific dimensions of water mismanagement, which suggest that flaws in water diagrams reflect and reinforce the misunderstanding of global hydrology by policymakers, researchers and the public. Correct depictions of the water cycle will not solve the global water crisis, but reconceiving this symbol is an important step towards equitable water governance, sustainable development and planetary thinking in the Anthropocene

A water cycle for the Anthropocene

International audienceHumor us for a minute and do an online image search of the water cycle. How many diagrams do you have to scroll through before seeing any sign of humans? What about water pollution or climate change—two of the main drivers of the global water crisis? In a recent analysis of more than 450 water cycle diagrams, we found that 85% showed no human interaction with the water cycle and 98% omitted any sign of climate change or waterpollution (Abbott et al., 2019). Additionally, 92% of diagrams depicted verdant, temperate ecosystems with abundant freshwater and 95% showed only a single river basin. It did not matter if the diagrams came from textbooks, scientific articles, or the internet, nor if they were old or new; most showed an undisturbed water cycle, free from human interference. These depictions contrast starkly with the state of the water cycle in the Anthropocene, when land conversion, human water use, and climate change affect nearly every water pool and flux (Wurtsbaugh et al., 2017; Falkenmark et al., 2019; Wine and Davison, 2019). The dimensions and scale of human interference with water are manifest in failing fossil aquifersin the world’s great agricultural regions (Famiglietti, 2014), accelerating ice discharge from the Arctic (Box et al., 2018), and instability in atmospheric rivers that support continental rainfall (Paul et al., 2016).We believe that incorrect water cycle diagrams are a symptom of a much deeper and widespread problem about how humanity relates to water on Earth. Society does not understand how the water cycle works nor how humans fit into it (Attari, 2014; Linton, 2014; Abbott et al., 2019). In response to this crisis of understanding, we call on researchers, educators, journalists, lawyers, and policy makers to change how we conceptualize and present the global water cycle. Specifically, we must teach where water comes from, what determines its availability, and how many individuals and ecosystems are in crisis because of water mismanagement, climate change, and land conversion. Because the drivers of the global water crisis are truly global, ensuring adequate water for humans and ecosystems will require coordinated efforts that extend beyond geopolitical borders and outlast the tenure of individual administrations (Keys et al., 2017; Adler, 2019). This level of coordination and holistic thinking requires widespread understanding of the water cycle and the global water crisis. Making the causes and consequences of the water crisis visible in our diagrams is atractable and important step towards the goal of a sustainable relationship with water that includes ecosystems and society

Analysing and modelling eutrophisation in the Loire river

Ce travail a pour objectif d’étudier les conditions physicochimiques qui causent dans la Loire des développements excessifs de phytoplancton, et de mettre en évidence les modifications des équilibres biogéochimiques qui en résultent. L’analyse des variables de l’eutrophisation a été conduite sur plusieurs niveaux d’échelle temporelle et spatiale, sur la base de longues chroniques de qualité de l’eau (1980- 2012), de données acquises à l’échelle journalière pendant la thèse (2012-2014), et d’un modèle numérique biogéochimique à résolution horaire. Depuis les conditions hypereutrophes des années 1980, la biomasse phytoplanctonique qui se développe en période estivale a été réduite d’un facteur 2,5 dans la Loire et dans ses affluents, de manière concomitante avec la baisse généralisée d’un facteur 3 des teneurs en phosphore biodisponible, liée au contrôle des rejets ponctuels urbains et industriels. Le fleuve de la Loire (France) reste cependant sensible à l’eutrophisation, du fait d’une forte pression agricole et urbaine, des étiages prononcés, et une morphologie fluviale à chenaux multiples qui ralentit l’écoulement, laissant pleinement au phytoplancton le temps de se développer. Lorsque les conditions hydrologiques le permettent, le développement du phytoplancton impacte fortement le fonctionnement de l’écosystème, en agissant significativement sur les cycles biogéochimiques des nutriments, du carbone et de l’oxygène.This thesis aims at studying the physical and chemical causes and consequences that result from excessive phytoplankton growth in the Loire River. The analysis identified some parameters characterizing river eutrophication and was conducted on several spatial and temporal scales. This was based on long-term water quality time-series (1980-2012), a daily survey carried out during this work (2012-2014), and the numerical modeling of the river biogeochemical functioning with an hourly resolution. Phytoplankton development in summer was reduced 2.5-fold in the Loire River and in the main tributaries, synchronously with the generalized reduction 3-fold of bioavailable phosphorus as a result of controlling and limiting phosphorus point sources. However, the Loire River remains sensitive to eutrophication, with a significant urban and agricultural pressure, low water levels in summer, and its multiple channels morphology slowing down the water velocity. All these factors combined favor phytoplankton development. When hydrological conditions are favorable, phytoplankton grows and significantly affects the ecosystem functioning, with an impact on nutrients, carbon and oxygen biogeochemical cycles

Distribution of landscape units within catchments influences nutrient export dynamics

Excess nutrients from agriculture have caused persistent eutrophication in aquatic ecosystems worldwide. Here, we present a conceptual framework for landscape management to achieve one or several water quality targets along the river continuum from headwaters to estuaries. Based on monitoring of representative headwaters and downstream reaches, we divide catchments into elementary landscape units defined by ecosystem properties and anthropogenic land use. We use a theoretical simulation to evaluate our hypothesis that the water-quality responses of redistributing these elementary units within the catchment will vary depending on the water quality targets (e.g., reduction in concentration or load). This landscape unit distribution (LUD) framework can efficiently assess the current ecohydrological functioning of a catchment and provide simple but robust predictions of its response to landscape management changes. Using simulated data, we show that different scenarios of landscape redistribution can allow attainment of one or several, but often not all desired water quality targets. Therefore, we recommend that water quality targets must be clearly defined and prioritized prior to designing landscape management strategies