Les rivières sont des écosystèmes dynamiques qui reçoivent, transforment, et exportent de la matière organique comprenant du carbone (C), de l’azote (N), et du phosphore (P). De par leur grande surface de contact entre l’eau et les sédiments, elles offrent un potentiel élevé pour les processus de transformation de ces éléments, dans lesquels ils sont souvent conjointement impliqués. Ces transformations peuvent retirer les éléments de la colonne d’eau et ainsi diminuer leurs concentrations pour améliorer la qualité de l’eau. Par contre, les conditions climatiques (débit, température, luminosité), la configuration du territoire (forêt, urbanisation, agriculture), et la durée des activités humaines sur terre affectent la quantité, composition, et proportion de C, N, et P livrés aux cours d’eau receveurs. Dans un contexte où un surplus de nutriments (N, P) peut surpasser la capacité des rivières à retirer les éléments de l’eau, et où les extrêmes climatiques s’empirent à cause des changements climatiques, cette thèse met en lumière le rôle des rivières dans les dynamiques de C, N, et P pour une meilleure compréhension de la réponse des écosystèmes lotiques aux pressions actuelles et futures. La Rivière du Nord draine séquentiellement des régions couvertes de forêt, d’urbanisation, et d’agriculture, et oscille entre quatre saisons distinctes, l’exposant à des utilisations du territoire et conditions climatiques contrastées. Nous avons échantillonné les formes de C, N, et P à 13 sites le long du tronçon principal (146 km), une fois par saison pour trois ans. De façon générale, les concentrations de N et P totaux ont augmenté d’amont vers l’aval, concordant avec l’activité humaine plus importante dans la deuxième moitié du bassin versant, mais les concentrations de C organique total sont restées constantes peu importe la saison et l’année. La stœchiométrie écosystémique du C : N : P était donc riche en C comparé au N et P en amont, et s’est enrichie en nutriments vers l’aval. L’étendue (2319 : 119 : 1 à 368 : 60 : 1) couvrait presque le continuum terre – océan à l’intérieur d’une seule rivière. Des formes différentes de C, N, et P dominaient la stœchiométrie totale dépendamment des saisons et de l’utilisation du territoire. En été, la composition du N était dominée en amont par sa forme organique dissoute et par le nitrate en aval, tandis qu’en hiver, l’ammonium et le P dissous avaient préséance sur l’entièreté du continuum. Malgré une concentration constante, la proportion des molécules composant le C différait aussi selon la saison et l’utilisation du territoire. L’été était dominé par des formes dégradées par l’action microbienne et l’hiver par des formes bio- et photo-labiles. Ceci fait allusion au potentiel de transformation de la rivière plus élevé dans la saison chaude plutôt que sous la glace, où les formes plus réactives avaient tendance de s’accumuler. La composition du C en amont était aussi distincte de celle en aval, avec un seul changement abrupt ayant lieu entre la section forestière et la section d’utilisation du territoire urbaine et agricole. Ces changements de compositions n’étaient pas présents durant le printemps de crue typique échantillonné, mais dans l’inondation de fréquence historique nous avons observés des apports nouveaux de molécules provenant soit des apports terrestres normalement déconnectés du réseau fluvial ou de surverses d’égouts. L’influence des facteurs naturels et anthropiques s’est aussi reflétée dans les flux historiques riverains de C, N, et P (1980 – 2020). La précipitation explique le plus les flux de C et les flux de N dans la section pristine. Les apports historiques au territoire de N anthropique (nécessaires pour soutenir la population humaine et les activités agricoles) expliquent fortement la tendance temporelle à la hausse des flux riverains de N dans la section urbaine. Durant les quatre dernières décennies, un peu plus du tiers des apports de N au territoire sont livrés à la rivière annuellement, suggérant que la source urbaine de N anthropique est encore peu gérée. Le manque de corrélation entre les flux de P dans la rivière et les précipitations ou les apports au territoire de P anthropique peut être expliqué par les usines de traitement des eaux usées installées dans la région vers la fin des années 1990 qui ont fait diminuer presque de moitié le P livré à la rivière. La variation de ces flux s’est reflétée dans la stœchiométrie écosystémique historique, qui varie de 130 : 23 : 1 en 1980 à 554 : 87 : 1 en 2007-08 après l’effet de l’usine d’épuration et du N qui a augmenté. À travers les axes historiques, spatiaux, et saisonniers, cette thèse contribue à la compréhension du rôle des rivières dans la réception, la transformation, et l’export du C, N, et P. Combinée aux concentrations, l’approche de stœchiométrie écosystémique propose une façon d’intégrer apports et pertes des éléments pour les étudier de pair au niveau du bassin versant. Puis, comme certaines formes de C, N, et P sont associées à des sources terrestres spécifiques, ou à certains types de transformations, les inclure dans un cadre conceptuel combinant des extrêmes climatiques et des utilisations du territoire différentes offre un aperçu sur le résultat des sources et transformations des éléments. Enfin, les tendances décennales de C, N, et P riverains montrent l’influence des facteurs naturels et anthropiques sur la stœchiométrie écosystémique historique d’une rivière. / Rivers are dynamic ecosystems that receive, transform, and export organic matter that includes carbon (C), nitrogen (N), and phosphorus (P). Their high surface of contact between water and sediments offers important potential for transformation processes to occur, which usually include all three elements together. These transformations can remove elements from the water column, thus decreasing their concentrations and leading to improved water quality. However, climatic conditions (discharge, temperature, light), landscape configuration (forest, urbanisation, agriculture), and length of human activities on land affect the quantity, composition, and proportion of C, N, and P delivered to receiving waters. In a context where a surplus of nutrients (N, P) can surpass a river’s capacity to remove elements from the water column, and where climatic extremes are worsening because of climate change, this thesis shines a light on the role of rivers in the dynamics of C, N, and P for a better understanding of lotic ecosystems’ responses to present and future pressures. The Rivière du Nord sequentially drains forested, urban, and agricultural regions, and oscillates between four distinct seasons, exposing it to contrasting land use changes and climatic conditions. We sampled forms of C, N, and P at 13 sites along the mainstem (146 km), once per season for three years. Overall, concentrations of total N and P increased downstream, concurrent with higher human activities in the lower half of the watershed, but total organic C concentrations remained constant regardless of season or year. As a result, C: N: P ecosystem stoichiometry was rich in C compared to N and P upstream, but became enriched in nutrients downstream. Within a single river, the range spanned by the ratios (2319: 119: 1 to 368: 60: 1) almost covered the land – ocean continuum. Different forms of C, N, and P dominated overall stoichiometry depending on season and land use. In summer, upstream N composition was dominated by dissolved organic N and shifted to nitrate downstream. In winter, ammonium and dissolved P were dominant throughout the continuum. Despite constant concentrations, C composition also differed as a function of season and land use. Summer was dominated by microbial humic-like components and winter by bio- and photolabile ones, hinting at the river’s higher transformation potential in the warm season rather than under the ice, where more reactive forms tended to accumulate. Upstream C composition was also distinct from the downstream one, with a unique sharp change between the forested pristine section and the downstream impacted one. This marked shift in composition was not observed in the typical high spring flow, but in the historical flood sampled, we observed new inputs of molecules coming from either previously disconnected terrestrial sources or sewage overflows. The influence of natural and anthropogenic factors was also reflected in the historical riverine loads of C, N, and P (1980 – 2020). Precipitation explained more than half of C loads, and some N loads from the pristine section. Historical anthropogenic N inputs to land (necessary to sustain human population and agricultural activities) strongly explained the increasing trend in riverine N loads from the urban section. In the last four decades, just over a third of anthropogenic inputs to land are loaded to the river annually, suggesting that the urban source of N is still largely uncontrolled. The lack of correlation between riverine P loads and precipitation or anthropogenic P inputs to land could be explained by the installation of wastewater treatment plants in the region at the end of the 1990s, which reduced almost half the amount of P loaded to the river. The variation in riverine loads was reflected in the historical ecosystem stoichiometry, which varied from 130: 23: 1 in 1980 to 554: 87: 1 in 2007-08 due to both the impact of wastewater treatment and increasing N use. With its historical, spatial, and seasonal axes, this thesis contributes to the understanding of the role of rivers on C, N, and P loading, transformation, and export. In combination with concentrations, ecosystem stoichiometry represents an approach to study elements together and integrate their loadings and losses at the watershed scale. Because certain forms of C, N, and P are associated with specific land use sources or certain types of transformation pathways, including them in a conceptual framework that combines climatic extremes and gradients of land uses allows for insight on the net effect of sources of transformations. Lastly, decadal trends of riverine C, N, and P loads will reveal the influence of natural and anthropogenic factors on the historical ecosystem stoichiometry of a river.

<p> </p> <ol> <li>This dissertation investigates the thermodynamics and transport of vacancies and interstitials of oxygen (O) and thorium (Th) in thorium dioxide (ThO₂) with varying charge states from neutral to maximum, with respect to temperature and oxygen pressure. The study also explores the impact of varying fractions of uranium (U) as a cation (<em>y</em>) on the defect disorder in mixed oxide fuels (Th_1-<em>y</em>U_<em>y</em>O₂). Understanding the properties of point defects in these oxides lays a strong foundation, as defects influence the properties of bulk materials, such as thermal transport. To accomplish the stated objectives of this dissertation, the research is structured into three sections that employ first principles density functional theory (DFT) and phonon calculations. The first section focuses on the structure, internal energy of formation, and vibrational entropy of point defects in ThO₂. The results demonstrate that defect energetics increase with an increase in defect charge for O interstitials and Th vacancies, while the opposite is true for O vacancies and Th interstitials. The lowest internal energy of formation shifts from O vacancies of charge 2+ to O interstitials and Th vacancies at various temperature ranges of 0 to 600 K, 600 to 1300 K, and 1300 to 2000 K. The second section develops a model to calculate the defect disorder and off-stoichiometry in ThO_{2±<em>x</em>} and Th_1-<em>y</em>U_<em>y</em>O_{2±<em>x</em>}. The model shows that ThO₂ exists mainly as a hypo-stoichiometric oxide between 1200 K to 2900 K for oxygen pressures ranging from 10^-30 to 10 atm, with O defects dominating this off-stoichiometric regime. The addition of U increases the thermodynamic window over which Th_1-<em>y</em>U_<em>y</em>O2 is hyper-stoichiometric, with O vacancies dominating in the hypo-stoichiometric regime, and cation vacancies and O interstitials dominating at low and high temperatures, respectively. Specifically, at low U content and low temperatures, U vacancies dominate hyper-stoichiometry, while at high U content and low temperatures, Th vacancies are dominant. This research facilitates the comprehension of the intricate changes in structural and defect equilibria that take place during nuclear fuel irradiation, where the fuel is not in a stoichiometric condition. The third section of the dissertation investigates migration barriers and diffusivities of defects and of O and Th in ThO₂. Results indicate that the migration energy of a point defect is dependent on its charge state. The average diffusivity of O vacancies exceeds that of O interstitials, while the similar is true for Th vacancies and Th interstitials above 1650 K. The self-diffusion coefficient of O and Th increases with temperature and is influenced by oxygen pressure, showing a close agreement with experimental and molecular-dynamics-based computational data. At 1500 K, the self-diffusivity of O and Th in ThO2 is 7.47 x 10^-16 m²s^-1 and 4.48 x 10^-23 m²s^-1 , respectively, while at 2500 K, the values increase to 1.06 x 10^-12 m²s^-1  and 2.28 x 10^-17 m²s^-1 , respectively. The chemical diffusion coefficients of defects decrease initially and then plateau as the hypo-stoichiometry in the oxide increases. These findings serve as a fundamental framework for understanding the diffusion-controlled processes of defects, which affect the radiation tolerance and microstructural evolution of ThO₂ as a nuclear fuel.  </li> </ol>

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Energy-Coupling-Factor (ECF)-Transporter sind Aufnahmesysteme für Vitamine und Übergangsmetallkationen in Prokaryoten. Sie bestehen aus den zwei unverwandten Membranproteinen S und T sowie einem Paar ABC-ATPasen (A). Die S-Einheit vermittelt die Substratspezifität. Die Kombination aus der T- und den A-Einheiten wird als ECF bezeichnet. In dieser Arbeit wurden Fragen zur kontrovers diskutierten Stöchiometrie der Untereinheiten von ECF-Transportern sowie zur zuvor postulierten Substrattransport-Funktion einzelner S-Komponenten auch ohne ECF untersucht. Dazu wurden der ECF-Biotintransporter BioMNY, mehrere natürlicherweise in Organismen ohne ECF existierende biotinspezifische S Einheiten (BioY) sowie zwei Vertreter der metallspezifischen ECF-Systeme genutzt. Die S-Einheit BioY des dreiteiligen Biotinimporters lag in vitro als Monomer und Dimer vor. Oligomeres BioY wurde außerdem in lebenden Bakterienzellen beobachtet. „Pull-down“-Experimente zeigten, dass die T Komponente BioN im BioMNY-Komplex zum Teil als Dimer vorlag. Wachstumsuntersuchungen bestätigten die Transportfunktion von acht solitär vorkommenden BioY. Die in vitro auch für diese BioY-Proteine nachgewiesene Dimerisierung könnte die Transportfunktion von BioY ohne ECF erklären. Die metallspezifischen S Einheiten CbiM/NikM interagieren mit für die Transportfunktion essentiellen, zusätzlichen Transmembranproteinen (N) und zeichnen sich durch eine Topologie mit sieben Transmembranhelices und einem extrem konservierten, weit in das Proteininnere hineinragenden N-Terminus aus. Die Metallbindestelle besteht aus vier Stickstoffatomen von Met1, His2 und His67 und wird durch ein Netz aus Wasserstoffbrückenbindungen stabilisiert. Die Transport¬funktion von CbiMN bzw. Nik(MN) ohne ECF wurde in vivo mittels des nickelabhängigen Enzyms Urease als Indikator für die intrazelluläre Nickelkonzentration verifiziert. Zum gegenwärtigen Zeitpunkt ist die Funktion der für den Transport essentiellen N-Komponente jedoch noch unklar. / Energy-coupling factor (ECF) transporters are uptake systems for vitamins and transition metal cations in prokaryotes. They consist of the two unrelated membrane proteins S and T, and a pair of ABC ATPases (A). The S unit mediates substrate specificity. The combination of the T and the A units is called ECF. In this thesis the controversially discussed stoichiometry of the subunits of ECF transporters and the postulated substrate transport function of solitary S units without ECF were analysed. For this purpose, the biotin-specific ECF transporter BioMNY, several biotin-specific S units (BioY) encoded in organisms lacking any recognizable ECF and two metal-specific ECF transporters were used. The S unit BioY of the tripartite biotin importer existed in vitro as monomer and dimer. Furthermore, oligomeric BioY was observed in living bacterial cells. Oligomerisation of a part of the T unit BioN in the BioMNY complex was shown by “pull-down”- experiments. Growth analyses confirmed the transport function of eight solitary BioY proteins. The dimerisation, also proved for these solitary BioY proteins in vitro, could be an explanation for the transport function of BioY without ECF. The metal-specific S units CbiM/NikM interact with additional and for the transport function essential transmembrane proteins (N). The S units consist of seven transmembrane helices and an extremely conserved N-terminus, which extends deeply into the protein. The metal-binding site consists of four nitrogen atoms from Met1, His2 and His67 and is stabilised by a series of hydrogen bonds. The transport function of CbiMN and Nik(MN) without ECF was verified respectively in vivo using the nickel-depending enzyme urease as an indicator for intracellular nickel concentration, respectively. However, the role of the N component, which is essential for transport activity, is currently under investigation.

Les tourbières stockent des quantités importantes de carbone (C) et jouent un rôle essentiel dans la dynamique globale du C, mais elles sont vulnérables aux changements globaux et aux perturbations humaines, notamment les perturbations linéaires telles que les routes non pavées (en gravier). La construction de ces routes et leur utilisation introduisent des quantités considérables de poussière de route dans les tourbières adjacentes, ce qui, combiné au dépôt de poussière atmosphérique, exerce une influence significative sur les tourbières qui est souvent négligée. L'objectif de cette recherche est d'étudier les effets de la poussière des routes sur la structure et les fonctions des tourbières, et plus particulièrement d'examiner les changements dans les assemblages de plantes et de thécamibiens, la chimie des tissus foliaires, de la tourbe et des eaux souterraines, ainsi que l'accumulation de C le long des gradients de dépôt de poussière. Pour ce faire, trois tourbières ombrotrophes ont été sélectionnées dans l'est du Québec, au Canada, qui sont exposées à des niveaux de dépôt de poussière de route variables et chimiquement distincts. Sur chaque site, un transect aligné sur la direction dominante du vent, commençant au bord de la route et perpendiculaire à celle-ci, a été échantillonné à des intervalles croissants jusqu'à 250 m de la route. Les résultats montrent que les dépôts de poussières de route affectent de manière significative les assemblages de plantes et de thécamibiens notamment en créant de l'ombrage physique et en bloquant les stomates, en apportant des cations basiques et en augmentant la disponibilité en nutriments. Plus précisément, les dépôts de poussières de route tendent à réduire la couverture des sphaignes et, dans le cas des amibes, l'abondance des espèces mixotrophes vivant en symbiose avec les algues et celles construisant des coquilles protéinées. Les dépôts de poussières routières réduisent les rapports stœchiométriques de C, d'azote (N) et de phosphore (P) dans la tourbe de surface et les tissus foliaires en réduisant la teneur nette en C et en fournissant des nutriments jusqu'à 200 m du bord de la route et à 15 cm de profondeur à partir de la surface. L'ampleur de cet impact dépend du niveau de dépôt et de la composition chimique de la poussière de route, ainsi que de la date de construction de la route. L'intensité des dépôts de poussières routières est nettement supérieure à celle des dépôts de poussières atmosphériques historiques, ce qui produit des effets contrastés sur les taux d'accumulation de C. Il a été démontré que les dépôts de poussières atmosphériques favorisent l'accumulation de C, tandis que les dépôts de poussières routières ont l'effet inverse, en réduisant le taux d'accumulation de C. Dans l'ensemble, notre étude met en évidence le rôle essentiel, mais encore méconnu, des dépôts de poussières routières dans la formation de la structure des communautés biologiques et dans la régulation de la fonction des tourbières adjacentes. L'étroite corrélation entre les dépôts de poussières routières et la composition des communautés microbiennes suggère l'utilisation possible des thécamibiens comme bioindicateurs des événements de poussières passés et présents. Les méthodes et les résultats de cette étude, qui ne se limitent pas aux effets des poussières routières sur les tourbières, peuvent être étendus aux effets similaires d'autres dépôts de poussières sur les écosystèmes voisins. / Peatlands store substantial amounts of carbon (C) and are of critical importance in global C dynamics, but are vulnerable to global changes and various forms of human disturbance, including linear disturbances such as unpaved (gravel) roads. The construction and use of these roads introduce considerable amounts of road dust into adjacent peatlands, which, combined with atmospheric dust deposition, exerts a significant influence on peatlands that is often overlooked. The objective of this research is to investigate the effects of road dust on peatland structure and function, specifically to examine changes in plant and testate amoeba assemblages; foliar tissue, peat, and groundwater chemistry; and C accumulation along road dust deposition gradients. To achieve this, three ombrotrophic peatlands were selected in eastern Québec, Canada, which are exposed to chemically distinct and varying levels of road dust deposition. At each site, a transect aligned with the dominant wind direction, starting at and perpendicular to the roadside, was sampled at increasing intervals up to 250 m away from the road. The results show that road dust deposition significantly affects plant and testate amoeba assemblages with a diverse array of factors involved, mainly including physical shading and stomatal blockage, base cation inputs, and increased nutrient availability. Specifically, road dust deposition tends to reduce the coverage of Sphagnum mosses, and in the case of amoebae, the abundance of mixotrophic species living in symbiosis with algae and those constructing proteinaceous shells. Road dust deposition reduces the stoichiometric ratios of C, nitrogen (N), and phosphorus (P) in surface peat and foliar tissues by reducing the net C content and supplying nutrients up to 200 m away from the roadside and 15 cm deep from the surface. The extent of this impact depends on the deposition level and the chemical composition of the road dust, as well as the age of the road. The intensity of road dust deposition is markedly greater than that of historical atmospheric dust deposition, yielding contrasting effects on C accumulation rates. Atmospheric dust deposition has been demonstrated to promote C accumulation, while road dust deposition has the opposite effect, reducing the rate of C accumulation. Overall, our study highlights the pivotal yet unacknowledged role of road dust deposition in peatlands on shaping their structure of biological communities and regulating their function. The close correlations between road dust deposition and microbial community composition suggest the possible use of testate amoebae as bioindicators of past and present dust events. The methods and findings of this study, which are not limited to its scope of road dust effects on peatlands, can be extended to similar effects of dust from other proximate sources on neighbouring ecosystems.

The purpose of the study was to investigate grade 11 learners’ problem solving skills and conceptual reasoning on concepts in stoichiometry. Two theoretical frameworks were used in this study namely, cognitive load theory (CLT) and typology of curriculum representation (levels of curriculum alignment). The explanatory sequential mixed method research design were applied where 410 physical sciences learners in their intact classes and eleven of their teachers participated in the study. The participants completed purposefully designed research instruments consisting of an achievement test (LAT), a teacher and a learner questionnaire (TSQ and LCQ), and a teacher lesson plan on stoichiometry-related concepts (TLP). Other instruments used include a semi-structured interview schedule (LIS), classroom observation schedule (COS) and learner work books (LWB). The study was underpinned by five research questions. Pearson correlations showed that the justifications given by learners for choosing right or wrong objective options were not due to chance and suggested a learner choosing the right objective option has the right conceptual reasoning. Findings indicate that there is a positive correlation between problem-solving skills and conceptual reasoning where conceptual reasoning statistically predicted learners’ problem-solving skills using Regression. Problem solving is an important cognitive activity in everyday and professional contexts. Therefore, it requires teachers to know where to focus their teaching and how to assess learners’ work to avoid unnecessary overloading of the working memory of learners which might affect their performance. Conceptual reasoning and problem-solving errors were made during the problem solving, for example, learners could not apply mole ratio, they were unable to do change of subject and they interchanged the meanings of chemical terms such as mole, molecule, atoms and mass. In terms of possible reasons for the errors, and how it linked to the way learners were taught, the curriculum levels were considered. Findings indicate that there is a mismatch between what is expected in stoichiometry from the CAPS curriculum termed intended curriculum and the implemented curriculum which includes teacher lesson plans, classroom observations and learner workbooks. The classroom observations schedule was designed in line with the teacher lesson plan. There were topics planned in the lessons that were not found in the learner workbooks. The analysis of the content of learners’ workbooks for topics treated under stoichiometry, revealed that 75.7% of concepts were being taught while 24.3% were not. The analysis showed that learners experienced difficulties with calculating the concept of limiting reactant. There is a mismatch between implemented and attained curriculum as well. In the TSQ, teachers indicated that the errors were due to misconceptions, misunderstanding, carelessness and misinterpretation. These were found during marking of the learner achievement test. In conclusion, this study did not find a direct link between the way the learners were taught and the errors they committed during problem solving based on the implemented (TLP, LWB and COS) and attained curriculum (LAT, LCQ, LIS and DoBE report) / Science and Technology Education / Ph. D. (Mathematics, Science and Technology Education)

Les phénomènes de diffusion se produisant lors du frittage des oxydes mixtes d’uranium et deplutonium (MOX) dépendent du potentiel d’oxygène de l’atmosphère du four, qui détermine lanature et la concentration des défauts ponctuels dans le matériau. Les travaux de thèse ont porté surune meilleure compréhension de l’influence du potentiel d’oxygène sur la densification, la formationde la solution solide et l’interdiffusion U-Pu lors du frittage des combustibles MOX. Pour cela, unlarge domaine de potentiel d’oxygène a été étudié, entre -600 et -100 kJ.mol-1 à 1700°C, afin demettre en évidence les différents mécanismes diffusionnels et leur impact sur la microstructurelorsqu’on s’éloigne de la composition stoechiométrique i.e. lorsque la concentration en défautsaugmente.Les études ont montré que plus le potentiel d’oxygène augmente, plus la densification du mélange70 % UO2+x + 30 % PuO2 s’effectue à basse température. Lors du chauffage, les oxydes de départ(UO2+x et PuO2-x) densifient dans un premier temps puis la solution solide se forme à une températureplus élevée d’environ 200°C. La solution solide apparaît à plus basse température quand le potentield’oxygène augmente, avec une cinétique de formation plus rapide. L’étude de l’interdiffusion U-Puindique qu’un traitement thermique avec un potentiel d’oxygène supérieur à -150 kJ.mol-1 à 1700°Cpermet d’obtenir un coefficient d’interdiffusion supérieur d’un à deux ordres de grandeur à ceuxobtenus entre -550 et -350 kJ.mol-1 à 1700°C et conduit donc à une homogénéisation U-Pu accrue.Cette étude permet de donner des recommandations sur le choix de l’atmosphère et de proposer uncycle de frittage optimisé en fonction de l’application ou de la caractéristique souhaitée. / Diffusion mechanisms occurring during the sintering of mixed uranium-plutonium oxides (MOX) areaffected by the oxygen potential of the atmosphere, as this latter imposes the nature and theconcentration of point defects in the material. This work is focused on a better knowledge of theinfluence of oxygen potential on densification, solid solution formation and U-Pu interdiffusionduring the sintering of MOX fuels. In this aim, a wide range of oxygen potential was studied, between-600 and -100 kJ.mol-1 at 1700°C, to highlight the various diffusional mechanisms and their impact onthe microstructure as oxygen deviates from stoichiometric composition and the defect concentrationincreases.As oxygen potential increases, the densification of the 70 % UO2+x + 30 % PuO2 mixture occurs at alower temperature. During the heating, the initial oxides (UO2+x et PuO2-x) densify first and then thesolid solution starts forming at about 200°C higher. The solid solution appears at a lowertemperature as the oxygen potential increases, with a faster kinetics of formation. The U-Puinterdiffusion study indicates that a heat treatment with an oxygen potential higher than -150kJ.mol-1 at 1700°C allows to obtain an interdiffusion coefficient higher by one up to two orders ofmagnitude compared with those obtained between -550 and -350 kJ.mol-1 at 1700°C and thereforeleads to an enhanced U-Pu homogenizationThis study enables to make recommendations on the atmosphere choice and to provide an optimizedsintering cycle depending on the desired application or required feature.

With a volcanic origin, Lake Kivu is deep and meromictic, and shows a very particular limnology and some astonishing features. The data available on its limnology and phytoplanktic communities are limited, dispersed or outdated. This is the first deep, long term study (2002-2004) on limnology and phytoplankton ecology of Lake Kivu, combining different techniques: HPLC analysis of marker pigments, flow cytometry, light, epifluorescence and electron microscopy. Lake Kivu combines a relatively shallow euphotic layer (~18m) usually smaller than its mixed layer (20 – 60 m), and with a weak thermal gradient in the mixolimnion. With an annual average chlorophyll a in the mixed layer of 2.2 mg m-3 and low nutrient levels in the euphotic zone, the lake is clearly oligotrophic. Concerning its phytoplanktonic composition, the most common species were the pennate diatoms Nitzschia bacata Hust. and Fragilaria danica (Kütz.) Lange Bert., and the cyanobacteria Planktolyngbya limnetica (Lemm.) Komárková-Legnerová and Cronberg and Synechococcus sp. Diatoms were the dominant group in the lake, particularly during the dry season episodes of deep mixing. During the rainy season, the stratified water column, with high light and lower nutrient availability, favoured dominance of cyanobacteria. Phycoerythrin-rich phototrophic picoplankton and heterotrophic bacteria cell numbers were constantly high, with relatively subtle spatial, seasonal and vertical variations. In open lake waters, where allochthonous carbon inputs are most probably inconsequential, HNA heterotrophic bacteria abundance is strongly correlated with chlorophyll a. Recent investigations revealed an increasing methane production in the deep waters during the past three decades, leading to an accumulation of gas and the subsequent lowering of the energy required to trigger a devastating release of gasses. The role of primary producers and the probable changes on the export ratio of the organic matter into deep waters after the introduction of the planktivorous sardine from Lake Tanganyika Limnothrissa miodon, is discussed. The actual primary production, 0.71 g C m-2 d-1 (~ 260 g C m-2 y-1), doesn’t seem substantially different from some punctual measurements made in the past, which discards the hypothesis of anthropogenic eutrophication. We believe that the export ratio of organic matter into the deep waters was largely affected by biological changes that could explain part of the methane increase in the past 30 years in Lake Kivu.

Vam monitoritzar paràmetres físics i químics, macroinvertebrats bentònics, clorofil·la a, productors primaris i matèria orgànica durant un any (2001-2002) per examinar els efectes d'una font puntual sobre la composició taxonòmica, la estructura de la comunitat, l'organització funcional, la utilització de l'habitat i la estoquiometria al riu la Tordera (Catalunya).Aigües avall de la font puntual, concentració de nutrients, cabal i conductivitat eren majors que al tram d'aigües amunt, mentre que oxigen dissolt era menor. La densitat de macroinvertebrats era més elevada al tram d'aigües avall però la biomassa era similar als dos trams. La riquesa taxonòmica al tram de dalt era un 20% més alt que al tram de baix. Els anàlisis d'ordenació separen clarament els dos trams en el primer eix, mentre que els dos trams presentaven una pauta temporal similar en el segon eix. La similaritat entre els dos trams en composició taxonòmica, densitats i biomasses després de les crescudes d'abril i maig de 2002, indiquen que les pertorbacions del cabal poden actuar com a un mecanisme de reinici de la comunitat bentònica i jugar un paper important per a la restauració d'ecosistemes fluvials.Els dos trams presentaven una biomassa de perifiton, plantes vasculars, CPOM i FPOM similars, mentre que clorofil·la a, algues filamentoses, molses i SPOM eren majors al tram d'aigües avall. La densitat relativa de trituradors era menor sota la font puntual mentre que col·lectors i filtradors van ser afavorits. La biomassa relativa de trituradors també era menor sota la font puntual, però la biomassa de col·lectors i depredadors va augmentar. Les relacions entre densitat de grups tròfics i els seus recursos eren rarament significatives. La relació s'explicava millor amb la biomassa de macroinvertebrats. Els dos trams compartien la mateixa relació per raspadors, col·lectors i filtradors però no per trituradors i depredadors.La densitat i la biomassa de macroinvertebrats es trobaven positivament correlacionades amb la quantitat de recursos tròfics i la complexitat d'habitat, mentre que la riquesa taxonòmica es trobava negativament relacionada amb paràmetres hidràulics. La influència dels substrats inorgànics prenia menor rellevància per a la distribució dels macroinvertebrats. Els anàlisis d'ordenació mostren com les variables del microhabitat de major rellevància eren CPOM, clorofil·la a, algues filamentoses i velocitat. La cobertura de sorra només era significativa per al tram d'aigües amunt i les molses, al d'aigües avall. El número de correlacions significatives entre macroinvertebrats i les variables del microhabitat era més elevat per al tram de dalt que per al de baix, bàsicament per diferències en composició taxonòmica. La biomassa de macroinvertebrats va aportar una informació semblant a la obtinguda per la densitat.Perifiton i molses tenien uns continguts de nutrients similars en els dos trams. Els %C i %N d'algues filamentoses també eren similars en els dos trams però el %P sota la font puntual era el doble que al tram de dalt. Les relacions estoquiomètriques en CPOM, FPOM i SPOM eren considerablement menors sota la font puntual. Els continguts elementals i relacions van ser molt variables entre taxons de macroinvertebrats però no van resultar significativament diferents entre els dos trams. Dípters, tricòpters i efemeròpters presentaven una estoquiometria similar, mentre que el C i el N eren inferiors en moluscs i el P en coleòpters. Els depredadors presentaven un contingut en C i N més elevat que la resta de grups tròfics, mentre que el P era major en els filtradors. Els desequilibris elementals entre consumidors i recursos eren menors en el tram d'aigües avall. A la tardor i l'hivern la major font de nutrients va ser la BOM mentre que a la primavera i a l'estiu va ser el perifiton. / We monitored physical and chemical parameters, benthic macroinvertebrates, chlorophyll a, primary producers, and organic matter for one year (2001-2002) to assess the effect of a point source input on the taxa composition, community structure, functional organization, microhabitat distribution, and elemental stoichiometry in La Tordera stream (Catalonia, NE Spain).Below the point source, nutrient concentrations, discharge, and specific conductance were higher than at the upstream reach, while dissolved oxygen decreased. Total macroinvertebrate density was higher at the downstream reach than at the upstream reach. The two reaches did not differ in macroinvertebrate total biomass. On average, taxa richness at the upstream reach was 20% higher than at the downstream reach. Ordination analysis clearly separated the samples of the two reaches in the first axis and corroborated the effect of the point source on the benthic community. The two reaches followed a similar temporal pattern with respect to the distribution of taxa along the second axis. Higher similarities between the two reaches in taxa composition, densities and biomass after the spates of April and May 2002, suggest that flooding events may act as a reset mechanism for benthic communities and play an important role in stream restoration.The two reaches did not differ in standing stocks of periphyton, vascular plants, CPOM and FPOM. Chlorophyll a, filamentous algae, mosses, and SPOM concentrations were increased by the point source. Relative density of shredders was reduced at the downstream reach, whereas gatherers and filterers were enhanced. Relative biomass of shredders decreased below the point source as well, but gatherers diminished and predators increased. The relationships between density of functional feeding groups and their presumed food resources were rarely significant. The relation between consumers and their food resources was best explained through macroinvertebrate biomass. The two reaches had the same relationship for scrapers, gatherers, and filterers, but differed for shredders and predators.Macroinvertebrate density and biomass were positively correlated with food resources and complexity of habitat architecture while taxa richness was negatively correlated with hydraulic parameters. Inorganic substrate exerted a lesser influence on macroinvertebrate distribution. Ordination analysis revealed that the microhabitat variables of major relevance at the two reaches were CPOM, chlorophyll a, filamentous algae, and maximum water velocity. Sand coverage was only retained at the upstream reach and moss, at the downstream reach. The number of significant correlations between macroinvertebrates and microhabitat variables was higher at the upstream reach than at the downstream reach mainly because of higher taxa richness. Macroinvertebrate biomass provided similar information to that obtained from density in our study case.Periphyton and mosses had similar nutrient contents in the two reaches. The %C and %N filamentous algae was also similar in the two reaches, but %P below the point source was two times higher than that at the upstream reach. Stoichiometric ratios for CPOM, FPOM, and SPOM decreased considerably below the point source. Elemental contents and ratios were highly variable among macroinvertebrate taxa but did not differ significantly between the two reaches, indicating homeostatic regulation. Dipterans, caddisflies, and mayflies had similar elemental contents and stoichiometry, whereas C and N were lower in mollusks and P in beetles. Predators had the higher C and N contents, while %P was higher in filterers and lower in scrapers. Elemental imbalances between consumers and resources were amended at the downstream reach relative to the upstream reach. Macroinvertebrates represented, on average, relatively low nutrient storage in the ecosystem. At the upstream reach, autochthonous production represented the major input of nutrients into the ecosystem food web, whereas the major source of P for heterotrophic production at the downstream reach was represented by allochthonous inputs.

This investigation is focused on the understanding of mechanical and chemical reaction behaviors of stoichiometric mixtures of nano- and micro-scale aluminum and hematite (Fe2O3) powders dispersed in epoxy. Epoxy-cast Al+Fe2O3 thermite composites are an example of a structural energetic material that can simultaneously release energy while providing structural strength. The structural and energetic response of this material system is investigated by characterizing the mechanical behavior under high-strain rate and shock loading conditions. The mechanical response and reaction behavior are closely interlinked through deformation characteristics. It is, therefore, desirable to understand the deformation behavior up to and beyond failure and establish the necessary stress and strain states required for initiating chemical reactions. The composite s behavior has been altered by changing two main processing parameters; the reactants particle size and the relative volume fraction of the epoxy matrix. This study also establishes processing techniques necessary for incorporating nanometric-scale reactants into energetic material systems. The mechanochemical behavior of epoxy-cast Al+Fe2O3 composites and the influence of epoxy volume fraction have been evaluated for a variety of loading conditions over a broad range of strain rates, which include low-strain rate or quasistatic loading experiments (10-4 to 10-2 1/s), medium-strain rate Charpy and Taylor impacts (103 to 104 1/s), and high-strain rate parallel-plate impacts (105 to 106 1/s). In general, structural strength and toughness have been observed to improve as the volume fraction of epoxy decreases, regardless of the loading strain rate regime explored. Hugoniot experiments show damage occurring at approximately the same critical impact stress for compositions prepared with significantly different volume fractions of the epoxy binder phase. Additionally, Taylor impact experiments have indicated evidence for strain-induced chemical reactions, which subject the composite to large shear accompanied by temperature increase and associated softening, preceding these reactions. Overall, the work aims to establish an understanding of the microstructural influence on mechanical behavior and chemical reactivity exhibited by epoxy-cast Al+Fe2O3 materials when exposed to high stress and high-strain loading conditions. The understanding of fundamental aspects and the results of impact experiment measurements provide information needed for the design of structural energetic materials.