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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>

No description available.

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)

Das litorale Makrozoobenthos ist eine bedeutende biotische Komponente in Seen und trägt substantiell zur Biodiversität und Funktion von Seeökosystemen bei. Allerdings unterliegt das Litoral zunehmenden anthropogenen Nutzungen, deren ökologische Auswirkungen jedoch kaum quantifiziert wurden. In dieser Doktorarbeit wurde untersucht, welche Bedeutung maßgebliche Umweltfaktoren auf die Zusammensetzung des litoralen Makrozoobenthos haben, und wie sich anthropogene Nutzungen auf die Zusammensetzung und Funktion des Makrozoobenthos auswirken. Die Zusammensetzung des Makrozoobenthos wurde durch die Uferstruktur, Trophie und das hydrodynamische Regime bestimmt. Die faunistische Ähnlichkeit zwischen Habitaten war jedoch signifikant geringer als zwischen Trophiestufen, so dass die Uferstruktur, und nicht die Trophie, einen größeren Einfluss auf das Makrozoobenthos hat. Strukturelle Degradation führte zu einer Reduktion der Habitatheterogenität, was eine signifikante Verringerung der Diversität und eine signifikant veränderte Artenzusammensetzung verursachte. Infolgedessen war die Komplexität der Makrozoobenthos-Nahrungsnetze an degradierten Ufern signifikant geringer als an natürlichen Ufern. Erhöhte Wasserstandsschwankungen führten zum Ausfall von Wurzelhabitaten und der damit assoziierten Makrozoobenthos-Gemeinschaft. Schiffsinduzierter Wellenschlag führte zur Verdriftung des Makrozoobenthos von ihren Habitaten bereits bei geringen Sohlschubspannungen. Die Effekte von Wasserstandsschwankungen und schiffsinduziertem Wellenschlag wurden jedoch durch Habitate mit hoher struktureller Komplexität verringert. Mit dieser Doktorarbeit konnte ich ein mechanistisches Verständnis darüber erarbeiten, wie anthropogene Nutzungen die Wirkungsbeziehungen zwischen Umweltfaktoren und Artengemeinschaften verändern und welche ökologischen Auswirkungen dies hat. Diese Kenntnisse können als wissenschaftliche Basis zur Bewertung von anthropogenen Beeinträchtigungen des Litorals dienen. / Littoral macroinvertebrates are an important biotic component of lakes by contributing substantially to the biodiversity and functioning of lake ecosystems. Humans alter the littoral and riparian areas for various purposes, but the resulting ecological impacts on littoral macroinvertebrates have not been quantified. In this thesis, I investigated the significance of key environmental factors for littoral macroinvertebrate communities and how human alterations of these environmental factors impact the structure and function of macroinvertebrate communities. Macroinvertebrate community composition was significantly related to littoral structure, trophic state and the hydrodynamic regime. The significantly higher compositional dissimilarities among habitats than among trophic state suggested that littoral structure was the more important driver of community composition. Structural degradation caused a significant reduction of habitat heterogeneity and resulted in a significant reduction of species diversity and a significant altered community composition. This caused a significant reduction of macroinvertebrate food web complexity and substantial alterations of the trophic base of the food webs. Climate-change induced water level fluctuations resulted in the loss of root habitats and the specific community associated with this habitat. Ship-induced waves had substantial direct effects, since macroinvertebrates were detached from their habitats by waves even at moderate shear stress levels. However, the impacts of water level fluctuations and ship-induced waves were mitigated by the presence of habitats with high structural complexities. This thesis provided a mechanistic understanding of how human activities alter relationships between environmental factors and biotic communities. This knowledge can be used to develop scientifically sound approaches to assess the persistent human impacts on lake ecosystems.

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.