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The aim of the study is to describe how Inherent Optical Properties vary in perialpine lakes and to understand the impact of their vertical variability on the remote sensing signal.

Les lacs constituent d’importantes ressources en eau potable en Suisse, château d’eau de l’Europe. Pourtant, ils sont souvent négligés dans le cycle hydrologique global. Menacés à la fois à l’échelle globale par les changements climatiques, ainsi qu’à l’échelle locale par divers facteurs anthropogéniques (e. g. déversement de polluants, engrais, pollution thermique, etc.), la réaction de ces écosystèmes doit être comprise et anticipée afin de sécuriser durablement les services écosystémiques essentiels qu’ils fournissent (e. g. captage d’eau potable, pêche, sources/ puits de chaleur [1]). De telles capacités prédictives et de surveillance peuvent uniquement être acquises par l’utilisation de modèles hydrodynamique tridimensionnels exploités en temps réel.

Apart from their contribution in the water balance of Lake Ohrid, Republic of Macedonia, subaquatic springs are also expected to affect the water quality. A simple experiment was developed and applied to subaquatic spring in Kališta region (south-east of Kališta village, in the north-western part of the Lake) and at the spring at Veli Dab (eastern side of the Lake) based on physicochemical spring water properties. Different sampling methods were established with the aim of uncovering a more suitable way of sampling pure subaquatic spring water. The goal was to test these findings and adapt them for further analysis with higher temporal and measurement related resolution. Measurements were also aimed at gathering additional knowledge and methodology for characterizing the hydrogeology of the watersheds. Of interest was information on the general chemical composition of spring water, interactions between aquifer and groundwater and on the origins of groundwater. Integration of this knowledge adds to a better understanding on how and what kind of groundwater is delivered to Lake Ohrid. The obtained results show that the examined springs are rich in nutrients and, comparing to Lake water, exhibit temporal variations in temperature, pH, conductivity and dissolved oxygen. Investigated springs have a very constant flow, as well as water quality.

Lake Nyos, a deep crater lake, located in the north-west of Cameroon, was permanently stratified below 50 m depth due to subaquatic sources supplying warm, salty and CO2- enriched water into the deepest reaches. The high CO2 content in these source waters caused the 1986 limnic eruption. The deep inflowing water is denser than the hypolimnetic water and maintains the stability of the water column, which is double-diffusively stratified. During the dry season in Feb 2002, cooling triggered the formation of a doublediffusive (DD) staircase, a sequence of homogeneously mixed layers separated by distinct stable interfaces. The initiation of the staircase was slightly below the permanent chemocline at ~ 50 m depth, from where the staircase expanded vertically in a diffusion-type manner for ~ 750 days to a maximal vertical extension of ~ 37 m. The staircase pattern caused the upward heat fluxes to increase which depleted the driving temperature gradient. Subsequently, the density ratio increased and reduced the upward heat flux divergence until DD progressively weakened and finally the staircase structure eroded. Based on 39 CTD profiles, we describe the DD phenomenon, explain the three distinct phases of this unique DD event, which lasted for ~ 850 days, and discuss the vertical extension of the DD zone in relation to the rates of new layer formation and layer decay. To our knowledge, this is the only observation over the entire lifespan—“from birth to death”—of a DD event in a natural water body.

Diese Arbeitsanleitung bietet einen Leitfaden zur Analyse der Sauerstoffzehrung und der Netto-Ökosystemproduktion in Seen. Diese beiden Grössen bilden wichtige Grundlagen zur Bestimmung der See-Gesundheit und ermöglichen, deren zeitliche Entwicklung zu untersuchen. Aufbauend auf den beiden in Aqua & Gas erschienenen Artikeln «Sauerstoffzehrung in Seen» (Kiefer et al., 2020) und «Netto-Ökosystemproduktion in Seen» (Kiefer et al., 2021) werden die angewandten Methoden hier schrittweise im Detail vorgestellt und erklärt.

Aquatic physics in inland water is a crucial subject for studying aquatic ecosystems. Transport and mixing are of tremendous importance for the pace at which chemical and biological processes develop. Recent observations allow to distinguish mixing and transport processes in stratified lakes and reservoirs. The surface and bottom boundary layer are turbulent while the lake interior remains comparatively quiescent.

Understanding lake dynamics is crucial to provide scientifically credible information for ecosystem management. In this context, three-dimensional hydrodynamic models are a key information source to assess critical but often subtle changes in lake dynamics occurring at all spatio-temporal scales. However, those models require timeconsuming calibrations, often carried out by trial-and-error. Through a new coupling of open source software, we present here a flexible and computationally inexpensive automated calibration framework. The method, tailored to the calibration data available to the user, aims at (i) reducing the time spent on calibration, and (ii) making three-dimensional lake modelling accessible to a broader range of users. It is demonstrated for two different lakes (Lake Geneva and Greifensee) with an extensive multi-variable observational dataset. Models mean absolute errors are reduced by up to ~50% over the baseline. Guidelines on heat and momentum transfer parameters are given with their dependence on the observational setup.

Double diffusion in lakes and oceans can transform vertical gradients into staircases of convectively mixed layers separated by thin stable interfaces. Lake Kivu is an outstanding double-diffusive natural laboratory with > 300 such steps over the permanently stratified deep basin. Here, we use 315 microstructure profiles (225 measured in Rwanda and 90 in the DRC) to shed light on the heat and salt balances of Lake Kivu. Comparing profiles from 2011 and 2015 reveals warming of 8.6 mK yr−1 below 80 m depth and negligible changes in salinity. The double-diffusive layering is coherent over horizontal distances of 20–30 km and remained unchanged between 2011 and 2015, indicating little variability. The mean estimated dissipation within mixed layers is 1.5 × 10−10 W kg−1. If unshaped Batchelor microstructure spectra are interpreted as nonturbulent, the rescaled dissipation of 0.44 × 10−10 W kg−1 corresponds to a vertical heat flux of 0.10 W m−2, which agrees with the molecular heat flux through the adjacent stable interfaces. Using estimates of upwelling, temporal changes of temperature and salt, and vertical double-diffusive fluxes, we established heat and salt balances, which require lateral heat and salt inputs. For salt, lateral input of freshwater at the main gradients balances upwelling. For temperature, however, the divergence of the vertical double-diffusive fluxes can only be balanced by horizontal inputs supplying cool water above and warm water below the main gradients. This suggests that lateral inputs of water at various depths are the main drivers for this unique double-diffusive phenomenon in Lake Kivu.

This study presents a novel concept for estimating net ecosystem production (NEP), the export of organic carbon (OC) from the productive surface layer to the deep‐water (hypolimnion) of eleven seasonally stratified lakes, varying in depth and trophic state. As oxygen remineralizes settling OC at a constant ratio, NEP is equivalent to the areal hypolimnetic mineralization rate (AHM) plus burial in the sediment. Two major interferences have to be considered, however. First, OC from terrestrial sources, not originating from primary production, consumes a fraction of oxidants. Second, sediment diagenetic processes of lakes in trophic transition (e.g. undergoing eutrophication or reoligotrophication) that are not in quasi‐steady‐state with actual fluxes of OC from the productive surface layer, bias the NEP estimation. In these cases, the flux of reduced substances diffusing from the sediment must be subtracted. This results in some overestimation for lakes with high allochthonous loads, and slight underestimation in lakes that are not in quasi‐steady‐state, because the actual sediment burial of autochthonous OC is small but not negligible. The presented approach requires data from routinely available monitoring and thus can be applied to historic data. The temporal integration over the productive season makes the estimation of NEP robust. Based on a historic 47 years long data record of Lake Geneva, NEP estimations (∼70 gC m‐2) from AHM rates agree well with P and N export budgets from the productive surface zone, which help to verify and constrain the uncertainty of the estimates.

The assessment of ecological impacts of pumped-storage (PS) hydropower plants on the two connected water bodies is usually based on present climatic conditions. However, significant changes in climate must be expected during their long concession periods. We, therefore, investigate the combined effects of climate change and PS operations on water temperature and quality, as well as extent and duration of stratification and ice cover, using a site in Switzerland. For this purpose, a coupled two-dimensional hydrodynamic and water quality model for the two connected water bodies is run with 150 years long synthetic stochastic meteorological forcing for both current and future climate conditions under two PS and two reference scenarios. The results show relevant synergistic and antagonistic effects of PS operations and climate change. For example, hypolimnion temperatures in September are projected to increase by

Measures to reduce lake phosphorus concentrations have been encouragingly successful in many parts of the world. After significant eutrophication in the twentieth century, nutrient concentrations have declined in many natural settings. In addition to these direct anthropogenic impacts, however, climate change is also altering various processes in lakes. Its effects on lacustrine nutrient budgets remain poorly understood. Here we investigate the total phosphorus (TP) concentrations in the epilimnion of the meromictic Lake Zug under present and future climatic conditions. Results are compared with those of other deep lakes. Data showed that TP transported from the hypolimnion by convective winter mixing was the most important source of TP for the epilimnion, reaching values more than ten times higher than the external input from the catchment. We found a logarithmic relationship between winter mixing depth (WMD) and epilimnetic TP content in spring. Warming climate affects WMD mainly due to its dependence on autumn stratification. Model simulations predict a reduction of average WMD from 78 (current) to 65 m in 2085 assuming IPCC scenario A2. Other scenarios show similar but smaller changes in the future. In scenario A2, climate change is predicted to reduce epilimnetic TP concentrations by up to 24% during warm winters and may consequently introduce significant year-to-year variability in primary productivity.

In situ observations of biophysical interactions in natural waters typically focus on physical mechanisms influencing biological activity. Yet, biological activity can also drive physical processes in aquatic environments. A community of photoautotrophic, motile and heavy bacteria—Chromatium okenii, which requires light, sulfide, and anoxic conditions to perform anoxygenic photosynthesis, accumulates below the chemocline of the meromictic Lake Cadagno (Switzerland). Upward vertical migration drives bioconvection, which modifies the physical environment of the bacteria-populated water to create a deep and homogeneous mixed layer of up to 1 m thickness. Continuous convection within the mixed layer and diapycnal diffusivity from its adjacent stratified surroundings determine ecologically relevant gradients. The daytime vertical migration that induce convective instabilities is well-established. It consists in bacteria swimming upward towards light and accumulating at the upper part of the anoxic layer, leading to a locally-unstable density excess. However, nocturnal activity has not yet been analyzed. An intensive 48-h survey was conducted in August 2018 using standard and microstructure profilers, as well as a moored high-resolution current meter coupled with temperature and turbidity sensors deployed across the mixed layer depth. This survey revealed a persistent mixed layer also during nighttime hours. Using a mixed layer shape model, vertical velocity observations and turbulent dissipation estimates, we conclude that photoautotrophic bacteria continue their vertical migration at night. This nocturnal activity thereby drives “dark bioconvection” and maintains the subsurface mixed bacterial layer in Lake Cadagno throughout the diel cycle.

Lakes and other confined water bodies are not exposed to tides, and their wind forcing is usually much weaker compared to ocean basins and estuaries. Hence, convective processes are often the dominant drivers for shaping mixing and stratification structures in inland waters. Due to the diverse environments of lakes—defined by local morphological, geochemical, and meteorological conditions, among others—a fascinating variety of convective processes can develop with remarkably unique signatures. Whereas the classical cooling-induced and shear-induced convections are well-known phenomena due to their dominant roles in ocean basins, other convective processes are specific to lakes and often overlooked, for example, sidearm, under-ice, and double-diffusive convection or thermobaric instability and bioconvection. Additionally, the peculiar properties of the density function at low salinities/temperatures leave distinctive traces. In this review, we present these various processes and connect observations with theories and model results.

Les eaux superficielles suisses renferment d’immenses réserves d’énergie thermique renouvelable, dont une fraction peut servir à chauffer et refroidir les infrastructures proches. Cet article présente une estimation du potentiel thermique des principaux lacs et rivières suisses, compare ce potentiel à la demande régionale et résume les considérations et difficultés techniques inhérentes à l’utilisation de cette énergie thermique.

Lake Kivu – a special lake in an active volcanic region.

We report novel results of a numerical experiment designed for examining the basin-scale hydrodynamics that control the mass, momentum, and energy distribution in a daily wind-forced, small thermally-stratified basin. For this purpose, the 3-D Boussinesq equations of motion were numerically solved using large-eddy simulation (LES) in a simplified (trapezoidal) stratified basin to compute the flow driven by a periodic wind shear stress working at the free surface along the principal axis. The domain and flow parameters of the LES experiment were chosen based on the conditions observed during summer in Lake Alpnach, Switzerland. We examine the diurnal circulation once the flow becomes quasi-periodic. First, the LES results show good agreement with available observations of internal seiching, boundary layer currents, vertical distribution of kinetic energy dissipation and effective diffusivity. Second, we investigated the wind-driven baroclinic cross-shore exchange. Results reveal that a near-resonant regime, arising from the coupling of the periodic wind-forcing (T=24 h) and the V2H1 basin-scale internal seiche (TV2H1≈24 h), leads to an active cross-shore circulation that can fully renew near-bottom waters at diurnal scale. Finally, we estimated the bulk mixing efficiency, Γ, of relevant zones, finding high spatial variability both for the turbulence intensity and the rate of mixing (10−3≤Γ≤10−1). In particular, significant temporal variability along the slopes of the basin was controlled by the periodic along-slope currents resulting from the V2H1 internal seiche.

Numerical simulations were carried out to investigate gyres within open lacustrine embayments subjected to parallel-to-shore currents. In such embayments, gyre formation occurs due to flow separation at the embayment’s upstream edge. High momentum fluid from the mixing layer between the embayment and offshore flows into the embayment and produces recirculating flow. Systematic numerical experiments using different synthetic embayment configurations were used to examine the impact of embayment geometry. Geometries included embayments with different aspect ratios, depths and embayment corner angles. The magnitudes of the recirculation and turbulent kinetic energy (TKE) in the embayment vary significantly for angles in the range 40° to 55°. Embayments with corner angles less than 50° have much stronger recirculation and TKE, other parameters remaining the same. The numerical findings are consistent with gyre formation observed in two embayments located in Lake Geneva, Switzerland, and thus help explain flow patterns recorded in lacustrine shoreline regions.

Lake productivity is fundamental to biogeochemical budgets as well as estimating ecological state and predicting future development. Combining modelling with Earth Observation data facilitates a new perspective for studying lake primary production. In this study, primary production was modelled in the large Lake Geneva using the MEdium Resolution Imaging Spectrometer (MERIS) image archive for 2002–2012. We used a semi-empirical model that estimates primary production as a function of photosynthetically absorbed radiation and quantum yield of carbon fixation. The necessary input parameters of the model—concentration of chlorophyll a, downwelling irradiance, and the diffuse attenuation coefficient—were obtained from MERIS products. The primary production maps allow us to study decennial temporal (with daily frequency) and spatial changes in this lake that a single sample point cannot provide. Modelled estimates agreed with in situ results (R2 = 0.68) and showed a decreasing trend (∼27%) in production in Lake Geneva for the selected decade. Yet, in situ monitoring measurements missed the general increase of productivity near the incoming Rhône River. We show that the temporal and spatial resolution provided by satellite observations allows estimates of primary production at the basin-scale. The phytoplankton annual primary production was estimated as ∼302 (SD 20) g C m−2 yr−1 for Lake Geneva for 2003 to 2011. This study demonstrates that maps of primary production can be obtained even with reduced resolution (1200 m) MERIS data and relatively simple methods, and thereby calls for deeper integration of remote sensing products into conventional in situ observation approaches.

Remote sensing helps foster our understanding of inland water processes allowing a synoptic view of water quality parameters. In the context of global monitoring of inland waters, we demonstrate the benefit of combining in-situ water analysis, hydrodynamic modelling and remote sensing for investigating biogeochemical processes. This methodology has the potential to be used at global scales. We take the example of four Landsat-8 scenes acquired by the OLI sensor and MODIS-Aqua imagery over Lake Geneva (France—Switzerland) from spring to early summer 2014. Remotely sensed data suggest a strong temporal and spatial variability during this period. We show that combining the complementary spatial, spectral and temporal resolutions of these sensors allows for a comprehensive characterization of estuarine, littoral and pelagic near-surface features. Moreover, by combining in-situ measurements, biogeochemical analysis and hydrodynamic modelling with remote sensing data, we can link these features to river intrusion and calcite precipitation processes, which regularly occur in late spring or early summer. In this context, we propose a procedure that can be used to monitor whiting events in temperate lakes worldwide.

Decline in total phosphorus (TP) during lake reoligotrophication does not apparently immediately influence carbon assimilation or deep-water oxygen levels. Traditional monitoring and interpretation do not typically consider the amount of organic carbon exported from the productive zone into the hypolimnion as a measure of net ecosystem production. This research investigated the carbon-to-phosphorus ratios of suspended particles in the epilimnion, (C:P)epi, as indicators of changing productivity. We report sestonic C:P ratios, phytoplankton biomass, and hypolimnetic oxygen depletion rates in Lake Hallwil, a lake whose recovery from eutrophic conditions has been documented in 35 years of historic water-monitoring data. This study also interpreted long-term (C:P)epi ratios from reoligotrophication occurring in four other lakes. Lake Hallwil exhibited three distinct phases. (i) The (C:P)epi ratio remained low when TP concentrations did not limit production. (ii) (C:P)epi increased steadily when phytoplankton began optimizing the declining P and biomass remained stable. (iii) Below a critical TP threshold of ∼15 to ∼20 mg P m−3, (C:P)epi remained high and the biomass eventually declined. This analysis showed that the (C:P)epi ratio indicates the reduction of productivity prior to classic indicators such as deep-water oxygen depletion.

The physical environment of natural waters influences biogeochemical processes to generate specific ecological niches, promoting biophysical interactions. Bacteria and phytoplankton communities can form spatial structures, such as layers and patches. The physical characteristics of these structures in lakes, particularly their vertical and horizontal variability are the focus of this PhD thesis. Using temperature microstructure measurements, we aim to characterize turbulent mixing within biological formations and their surroundings in lakes. We start in Lake Cadagno, where unusual bio-convectively-driven mixing takes place. Then we move to Lake Zurich where a thin layer of cyanobacteria persists throughout the stratified season. Finally, we study the thermocline of Lake Geneva, a large and more energetic system, where basin-scale processes are expected to induce lateral variability of algae. While the first two interactions concern vertical structures, we use an autonomous underwater glider, equipped with a turbulence package, to explore lateral variability in this large lake. Bioconvection observed in Lake Cadagno offered a unique environment to analyze the role of microorganism in shaping the water column they inhabit. Within the stratified water column, a highly concentrated layer of heavy, motile and photoautotrophic sulfur bacteria Chromatium okenii migrates upward to form a subsurface convective mixed layer. Field measurements revealed that the mixed layer persists throughout the diel cycle, maintaining a virtually unchanged structure. Direct estimates of turbulent diffusion indicate that without active convective turbulence, the mixed layer would be smoothed in ≈2.5 hours. As this time-scale is much shorter than a night and in principle C. okenii need light, the nighttime mixed layer is not expected. Using intensive and high-resolution measurements throughout two diel cycles, we provide proof that bioconvection occurs also at night and is responsible for the mixed layer persistence. The second interaction was the thin layer of cyanobacteria Planktothrix rubescens forming every spring in the thermocline of Lake Zurich. In this zone, our measurements revealed only tiny overturns, resulting in negligible vertical exchange. This strong stratification inhibits mixing and provides a remarkably stable environment for the P. rubescens thin layer, explaining its persistence. Finally, in Lake Geneva, we first concentrated efforts in the validation of glider-based turbulence estimations, possibly the first of their kind in a large-lake. Although weak turbulence and strong stratification hinder the applicability of state of the art procedures, we demonstrate that our measurements capture the expected variance and spectral shape. We explore the data from repeated transects to assess lateral variability of chlorophyll-a patches. This thesis documents in an exemplary way how vertical turbulent processes interact with bacteria and phytoplankton layers in lakes. Regarding lateral variability, the results presented are a first step for future in-situ studies of phytoplankton patches affected by turbulence and transport processes.

New renewable electricity is nowadays often generated by photovoltaics and wind. Yet, their intermittent nature calls for energy storage, which is today still provided to ~95% by pumped-storage (PS) hydropower plants. However, PS is known to affect abiotic and biotic characteristics of the two connected water bodies. Thus, a two-dimensional laterally-averaged hydrodynamic and water quality model was set up to assess the impacts on water quality and temperature in a first step. Then, this analysis was extended to evaluate the additional effect of climate change, which also modifies abiotic and biotic characteristics. For this purpose, 150-years long synthetic time series of meteorological conditions for current (1998-2012) and future climate (2078-2092) were generated with a weather generator. To assess the robustness of projected impacts on the ice cover of the upper reservoir (Sihlsee) three one-dimensional, vertical hydrodynamic models were additionally set up. To attribute effects to either the PS flows or to water withdrawal from the hypolimnion, two reference scenarios were defined: one with deep-water withdrawal (NoPS) and another one with surface outflow (QNat). While the hypolimnetic temperature differs by

The need for characterizing turbulence in the ocean recently led to new techniques, among which is the use of acoustic Doppler current profilers (ADCP) on various stationary and moving platforms. This study aims to estimate the dissipation of turbulent kinetic energy, ", and the advective oceanic velocities from two 1.2-MHz single ping mode ADCPs mounted on a remotely operated towed vehicle (ROTV). The data consist of an equatorial transect of the Atlantic Ocean on which the ROTV was undulating between the surface and 300 meters depth. " is then estimated from the second and third order structure functions. This work outlines the development and the results of the application of this method on the available dataset. The conclusion is that for this data set, the signal to noise ratio is too low. We recommend acquiring at least an order of magnitude more individual raw pings. This would more likely let the statistics coverage to a reliable estimate of ". Additionally, prior knowledge of the Ozmidov scale, i.e. the size of the largest overturning eddies, is helping to ensure that roughly ten bins are within the inertial subrange. This approach enables continuous measurements of proles for a whole transect, as well as measurements close to the surface by operating the ROTV outside the vessel-induced turbulence zone. This new technique has the advantage to broaden the measurement range of high-frequency ADCP measurements by the undulation of the vehicle.

Low oxygen concentrations remain a global concern for the ecological health of lakes. High nutrient inputs and climate-induced changes in stratification and mixing are anthropogenic threats which largely impact aquatic oxygen budgets and overall ecosystem health. In this thesis, the relevant processes for hypolimnetic oxygen depletion were investigated on different temporal and spatial scales in the deep perialpine Lake Geneva and the corresponding total oxygen budget was estimated. The short-term variability of sediment oxygen uptake (SOU) and its dependency on the bottom boundary layer currents were investigated using microprofile measurements. Sediment core analyses for reduced substances profiles allowed distinguishing between SOU caused by both oxic respiration and the flux of reduced substances out of the sediment. Long-term monitoring data were used to estimate the relative importance of SOU for the total oxygen depletion in the lake. Finally, one-dimensional numerical models were used to reproduce lake temperature and oxygen concentrations and to assess the impact of changing environments on the oxygen budget of the deep-water. The results of the microprofile measurements led to a new parameterization of turbulent diffusion close to the sediment and enabled a similarity scaling of diffusivity as well as oxygen close to the sediment. However, the comparison of microprofile measurements at different lake depths showed that SOU decreased consistently with depth from ~1 g m-2 d-1 at 40 m to ~0.2 g m-2 d-1 at 133 m independently from the small-scale variability due to hydrodynamic forcing. Similar vertical structures of SOU and total oxygen depletion have been found in other Swiss lakes. The decrease of SOU with depth was attributed to the greater amount of easily degradable organic matter available in the upper layers. The comparison between SOU and the reduced substances flux revealed that oxic respiration is by far the dominant pathway of organic matter mineralization. While the long-term monitoring data did not show a decreasing trend in either the areal hypolimnetic mineralization rate (1.34 g m-2 d-1) or the extent of hypoxia, a strong relationship between deep mixing in winter hypoxic conditions was found. Hence, deep-water oxygen concentrations were predominantly controlled by resupply during the unstratified period in winter. To assess the long-term changes of winter mixing in Lake Geneva, the one-dimensional model SIMSTRAT was used to reproduce lake temperature and stratification between 1981 and 2012 and was run afterwards under atmospheric conditions representative for the years 2045–2076 and 2070–2101, according to the IPPC scenario A1B. The simulations predicted (i) a decrease in winter mixing depth from an average of ~172 m to only ~127 m at the end of this century, and (ii) complete homogenization of temperature and oxygen in winter will decrease by ~50%. Hence, changes in mixing may have stronger impact than eutrophication on the deep-water oxygen. A simple oxygen model coupled to SIMSTRAT predicted an increase in hypoxic conditions in the deep part of Lake Geneva by ~25%. Additionally, a detailed oxygen model was developed based on the observational findings of this dissertation which takes the spatial variability of oxygen depletion and its dependency on lake turbulence into account. This model can be generalized to understand and predict climate-induced changes of future oxygen concentrations in other deep lak

A three-dimensional hydrodynamic Z-model was conceived for Lake Zurich (Switzerland, ZH) using the modeling software Delft3D. Calibration was performed on the thermal profile in 2017 at three monitoring stations located in both basins, and was validated on years 2015 and 2016 with the same method. The model reproduced effectively thermal stratification and water temperature’s spatial and temporal variability. Specific hydrodynamic events were correctly simulated: upwellings were detected but their amplitude was underestimated; seiches periods were found accordingly to values reported in the literature. Hypolimneon’s temperature was nicely reproduced but results were less accurate in the epilimneon and especially in the metalimneon. Flow velocities were qualitatively evaluated and exhibited realistic values; the model was able to reproduce clockwise and counterclockwise flow rotation motions. Suggested improvements imply collecting more data about rivers, the importance of the latter on Obersee dynamics having been demonstrated, and the focus on wind drag coefficients in the calibration process.

Freshwater resources play a central role in social and economic development of modern civilisations, yet their value is often underestimated and neglected in developed countries. In fact, freshwater habitats are facing unprecedented threat because of human activities, and it is necessary to provide reliable water quality indicators to monitor the response of aquatic systems. In this context, remote sensing has a great potential to provide a complementary source of data for monitoring and under-standing the processes involved in inland waters around the world at fine temporal and spatial reso-lutions. The scientific approach adopted in this thesis is based on the integration of complementary sources of information provided by state-of-the-art monitoring methods to foster our understanding of freshwater habitats. Specifically, we demonstrate the additional value provided by combining com-plementary sensors with bio-geochemical measurements and hydrodynamic models, using a rare event in Lake Geneva which got a wide public attention in local newspapers: a calcite precipitation event. The principal focus of the remote-sensing community has recently been directed towards very turbid waters in order to address the challenges involved with the retrieval of mixed constituent concentra-tions. In this thesis, I highlight some important challenges relative to clearer waters which also re-quire further attention from the community. Specifically, I provide a solution to account for vertical non-uniformities of water constituent concentrations using simple approximation models in Lake Geneva. Finally, I provide a comprehensive comparison between state-of-the-art atmospheric correction methods, which are presumably relevant for inland water monitoring and applicable to the new con-stellation of remote sensors. The aim is to provide reliable recommendations to help forthcoming studies to apply the most suited procedure to their investigation.

When lake surface waters above the temperature of maximum density 4◦C cools, the verysurface fluid parcels become denser than their neighbours below. The latter process leads to a gravitationally unstable density distribution which may trigger ‘free convection’, i.e. the upper denser waters look for their equilibrium position somewhere down in the water column. This phenomenon usually occurs at night time, when the atmosphere is colder than surface waters. However, during daytime, it may happen that the lake surface cools—releasing heat to the atmosphere—while shortwave radiation penetrates through upper waters. In this scenario, the near surface waters of the photic zone can host an unstable layer due to the net surface cooling, whereas waters beneath it experience radiative heating and gravitational stabilization. Here, we investigate the competition between shortwave radiation and surface cooling when wind is low or negligible. Thus, depending on the relative intensity of surface cooling and shortwave radiation, the upper water column undergoes different gravitationally unstable density distributions and potentially convective regions. The objective of this Master Project is to investigate, describe, and characterise such convective regimes. To do so, the heat equation controlling temperature evolution of the upper water column subject to surface cooling and penetrative radiative heating is investigated. In parallel with in-situ observations, mathematical and numerical modelling allow to understand the phenomenon taking place. The results allow to infer how the relative importance of surface cooling and penetrating shortwave radiation may affect vertical heat and mass transfer within the lake and between the lake and the atmosphere.

Lake mesoscale processes have no secrets for artists, they have some for scientists and policy-makers. In the introduction to this dissertation, we saw that poets and painters depict the lake with rich variability. This vision conflicts with conventional approaches inferring the lake's bio-physical status from a few in-situ measurements with limited spatio-temporal coverage. Yet it reflects upon true physical processes, capable of disrupting the misleading lentic nature of lakes, the essential ecosystem services they provide, and even citizens' safety. Recent overarching policies now aim at securing those services, using novel approaches like numerical simulations and remote sensing observations. Harnessing the potentiality of in-situ, remote sensing data and model simulations, this thesis developed an end-to-end framework delivering reliable synoptic lake information, at high spatial and temporal resolution. In lakes, three-dimensional hydrodynamic models are the only information source capable of resolving transport and mixing, and forecasting their dynamics. However, they still rely on large observational datasets for their complex parameterizations and for constraining their uncertainties. This thesis addressed such challenges by (i) implementing an automated model calibration framework alleviating the need for expert knowledge, and by (ii) developing a data assimilative scheme capable of reducing and quantifying model uncertainties by incorporating satellite and in-situ data. Both yielded remarkable results: the former returned parametric values diminishing models Root Mean Square Error by up to 47 %, while the latter cut it further down by 54 %. Furthermore, the data assimilation enhanced the spatial coherence and magnitude of imperfectly resolved physical processes, and provided system uncertainties. Finally, this thesis delivered a practical outcome of its findings by developing an online pre-operational three-dimensional lake monitoring and forecasting system: meteolakes.ch. For two years, Meteolakes has been disseminating spatially explicit real-time lake information and data products to more than hundred thousand end-users. This pioneering platform has been featured in numerous media (newspapers, radio, television), public events, museum exhibitions, and benefited scientific, lake professionals, and public communities. A pinnacle of this research has been its early-warning and forecasting capabilities, which anticipated numerous mesoscale physical phenomena in Lake Geneva, such as upwellings, gyres, and strong currents. Two of those events, which impacted public and commercial activities, are illustrated in this dissertation. From observations and models to societal benefit, we created here a long value chain for water management. At the crossroads of scientific, computational and observational advances, this research paves the route for understanding lakes' delicate imbalance. By producing data society can use, it opens novel frontiers for interdisciplinary research on previously elusive lake physical processes, and their implications on the everyday life of people.

Dams store water and trap sediment in their reservoirs. Downstream river segments, hence, are affected by a limited sediment dynamics. This becomes obvious by river bed incision, limited geomorphological variability and depletion of hydraulic habitats for in-stream ecological organisms. Replenishment is a measure to counteract the lack of sediment. In segments dams such a sediment replenishment can be combined with the release of an artificial flood, by activating spillways and bottom outlets. Recent laboratory studies revealed the potential of multi-deposit replenishment configuration towards an increase of geomorphological richness in the river. However, this has not yet been tested in the field. The hydro-morphological index of diversity (HMID), is based on the spatial variability flow depth and velocity in a river segment, allowing the quantification of the change in morphology such a measure. Its ease in computation and its objectivity makes the index a useful tool for a large variety of users. However, knowledge concerning data sufficiency and a standardized sampling procedure does not exist. Further, the HMID does not account for temporal variability. This research aims at closing these gaps of knowledge for a sustainable management of complex floodplains downstream of dams. Flow depth and velocity data from 19 river segments were analyzed with the HMID. Structured sub-sampling and analysis of the data revealed different data sufficiency depending on the complexity of the river geomorphology. A representative HMID value is computed from at least 100 datapoints distributed over eight cross-sections, in braided rivers more datapoints are required. The geomorphological changes by an artificial flood event in the Sarine were analyzed with the HMID. In the impact perimeter of the multi-deposit sediment replenishment, the HMID increased by 36 % in the Sarine, in the rest of the river 18 %. A similar study was investigated at the Spöl, pointing out that sediment availability is crucial for the hydraulic habitat diversity. With the help of 489 tracer stones, the movement of the replenished sediment was investigated in detail. The deposition pattern results in repeating sediment clusters on the riverbed, as it was observed in the laboratory experiments. For the two characteristic diameters, the tracers allowed the estimation of transport velocities of 19 cm/min (dm = 57 mm) and 28 cm/min (d90 = 113 mm) during the flood event and consequently the erosion efficiency of the different parts of the flood hydrograph. Macroinvertebrates were less affected by the artificial flood event in the hydropeaking river segment of the Sarine compared with the residual flow segment. The species diversity indices decreased significantly due to the artificial flood event. A direct link between flow regime and EPT richness was observed. A river with a natural flow regime hosts the highest richness of macroinvertebrates followed the residual and finally the hydropeaking flow regime. For other ecological indicators, additional physical river parameters for example the frequency duration curve or the grain size distribution need to be considered as well.

In many countries water quality is a major concern for drinking water management authorities. Freshwater is a limited resource and the demand for good-quality water from natural aquatic systems is therefore high. Additional stress arises from anthropogenic emissions of nutrients, chemicals, pathogens, heat and hydrological alterations as well as from the ongoing global climate change. In this thesis the relevant recent past, present and future factors affecting water quality in the perialpine Lake Biel, used as a source for drinking water, was investigated. Through measurements in the field combined with hydrodynamic logical modelling in one and three dimensions, optimal locations for forthcoming lake water intakes was identified. Water quality properties considered were temperature, oxygen, suspended sediment concentration (SSC) and the risk of subaqueous mass movements. The results identified the forthcoming changes expected in Lake Biel due to the upcoming decommission of the Mühleberg Nuclear Power Plant (MNPP) in 2019. This plant currently release ~700 MW of heat into the upstream Aare River in the form of cooling water. The removal of this anthropogenic thermal source will decrease future lake temperature by ~0.3 °C (volume average). Due to seasonal river discharge patterns, the main impact of the plant closure will occur during winter. Minor effects are expected in summer in the form of moderately weaker lake stratification. Furthermore, the current fate of thermal pollution emitted from MNPP was linked to the hydraulic residence time of Lake Biel. While short-term anthropogenic thermal impacts in Lake Biel can be large, the system is additionally experiencing continuous warming up to ~0.1 °C per decade (volume average) by the ongoing climate change. The climate effect on Lake Biel and its primary tributary, the Aare River, was investigated as well as compared to the much larger Lake Geneva and Rhône River. Climate change causes seasonal river discharge shifts resulting in enhanced river warming in summer and diminished warming in winter, while at the same time SSC increases in winter and decreases in summer. Differences in temperature as well as warming rates between rivers and lakes in turn resulted in a discharge and hydraulic residence time-dependent decrease in climate warming of lakes. Furthermore, deep-water renewal in both lakes is predicted to increase in summer and decreases in winter, possibly influencing the replenishment of deep water oxygen. Additionally, sedimentation patterns in Lake Biel were examined and linked to the historical diversion of the Aare River into the lake. The majority of sediment supplied to Lake Biel (~80 %) came from large SSC events in the Aare. These events resulted in a selective particle settling pattern, which concentrated sedimentation on the shallow shelf area North-East of the Aare delta. Both the large SSC events as well as this selective sedimentation pattern were connected to weather fronts coming in from the Atlantic Ocean. The research performed here opens up for further promising interdisciplinary research between Meteorology, Climatology, Hydrology and Sedimentology. Inland water management would benefit through increased knowledge regarding the propagation of particles and/or anthropogenic heat, from river catchments into downstream lakes and sediments.