Large river water basins play a critical role in the economic, health, and biodiversity conditions of a region. In some basins, such as the Zambezi River Basin, extreme weather events introduce cycles of drought and heavy rainfall that can have extreme impacts on local communities vulnerable to environmental shifts. Annual flood pulse dynamics drive ecological dynamics in the system. In the dry season, water dependent wildlife in northern Botswana concentrates along the Chobe River- Floodplains. Elephant concentration, in particular, is matched to surface water quality declines. These flood pulse events have been linked to diarrheal disease outbreaks in the local population, the magnitude of which is associated positively with flood height. Modeling these interactions can advance our ability to predict events and develop mitigation and prevention actions. However, many challenges hinder this development including availability of data in regions that lack resources and the difficulties in create models for such large basins that account for overland water movement. This thesis presents work focused on addressing these challenges. Chapter 2 reports the development of a freely available Large Basin Data Portal (LBDP) that can be used to identify and create critical inputs for hydrodynamic models. This portal was used to create a hydrological model of the Upper Zambezi River Basin model (Chapter 3), a hydrodynamic model of the one of the three subbasins of the Zambezi River. The model was used to calculate downstream river discharges entering the Chobe-Zambezi Floodplains based on upstream rain events. The Upper Zambezi River Basin model was integrated with another more detailed model of the Chobe- Zambezi Floodplains (Chapter 4) that is designed to model the Chobe River and flood water movement in the floodplains. The models were created using the set of MIKE modeling software. The models were used to study various scenarios including water reductions that might occur due to climate change or drought and water increase that might be associated with extreme weather events. / Doctor of Philosophy / River water plays a key role in the livelihood of people and wildlife especially in region of the world suffering chronic economic challenges. The areas surrounding the Zambezi River in Africa is home to one of the most diverse ecological systems in the world. Extreme weather conditions bring cycles of drought and flooding especially in the Upper Zambezi region where wildlife, including the largest population of African elephants in the world, move closer to the Chobe River, a tributary of the Zambezi River, seeking water in the dry seasons. This research is focused on building a set of tools and models to enable studying the linkage between these events and aid in predicting the extent of the floods in the Chobe River Floodplain system based on rainfall in the Angolan high lands and other landscape features. Understanding how these dynamics are linked and the outcome in the downstream system provides a lead time for potential action.

Le lac de Tonlé Sap (TSL), Cambodge, est un écosystème lac-rivière de forêt alluviale au régime d'écoulement alternatif. Le lac est un déversoir lors de l'inondation saisonnière du Mékong et sert de réservoir en période de basses eaux. La superficie du lac pendant la saison sèche (février à mai), est d'environ 2 700 km2 pour une profondeur d'environ 1 mètre. Cette superficie est pratiquement multipliée par six quand arrive la saison des pluies, pour atteindre près de 16 000 km2 et une profondeur de 9 mètres, noyant rizières et forêts. C'est le plus grand lac d'eau douce d'Asie du Sud-Est. C'est aussi l'une des zones de pêche d'eau douce les plus importantes et productives du monde avec près de 75% du volume annuel de pêche en eau douce du Cambodge, ce qui permet la survie de près de 2,5 millions de personnes. Les changements saisonniers du cycle hydrologique ont une influence sur la structuration des communautés de poissons à l'échelle temporelle et spatiale, mais aussi sur les comportements trophiques des principales espèces qui n'exploitent alors pas les mêmes habitats. Toutefois, le bassin versant du Mékong est en changement constant avec un développement important des infrastructures en lien avec l'eau : production d'hydro-électricité, besoins important en irrigation, maitrise des inondations, eau potable, ... Les changements climatiques accélèrent les modifications du cycle hydrologique annuel. Il est alors supposé que ces modifications ont des effets forts sur les habitats et les proies disponibles et finalement sur la biodiversité, notamment de l'ichtyofaune et sur l'abondance des poissons disponibles pour les pêcheries.Dans un premier temps, nous avons caractérisé les variations spatio-temporelles de la composition taxonomique des poissons et mis en lumière quels sont les déterminants de ces variations. À cette fin, nous avons estimé la diversité bêta comme la variance totale de la matrice site par communauté d'espèce et l'avons divisée en contribution locale à la diversité bêta (LCBD) et contribution des espèces à la diversité bêta (SCBD). Nous avons ensuite effectué plusieurs régressions linéaires pour déterminer si la richesse taxonomique, l'abondance des espèces et le niveau de l'eau expliquaient la variation temporelle de la contribution du site et de l'espèce à la diversité bêta. Nos résultats indiquent une forte variation temporelle de la diversité bêta due aux contributions différentielles des sites et des espèces à la variation spatiale de la composition taxonomique des poissons. Nous avons également constaté que la direction, la forme et l'effet relatif de la richesse spécifique, de l'abondance et du niveau de l'eau sur la variation temporelle des valeurs LCBD et SCBD varient grandement selon les sites. Ces résultats suggèrent ainsi une variation spatiale des processus conduisant à une variation temporelle de la composition de la communauté. Dans l'ensemble, nos résultats suggèrent que la composition taxonomique des poissons n'est pas distribuée de manière homogène dans l'espace et dans le temps et risque d'être affectée à l'avenir si la dynamique saisonnière d'écoulement du système est altérée par les activités humaines et/ou les changements climatiques. Dans un second temps, nous avons cherché à étudier le modèle d'évolution temporel des principales espèces en terme d'occurrence et d'abondance à travers le cycle saisonnier hydrologique. / The Tonle Sap Lake (TSL), Cambodia, is a flood-pulse system. It is the largest natural lake in South- East Asia and constitutes one of the largest fisheries over the world, supporting the livelihood 2.5 million peoples. Seasonal change in annual hydrological cycle appears to have influence on fish community structure, both spatial and temporal variation, particularly on feeding behavior of TSL's fishes. Nonetheless, the Mekong River Basin is changing rapidly due to accelerating water infrastructure development (hydropower, irrigation, flood control, and water supply) and climate change, bringing considerable modifications to the annual flood-pulse of the TSL. Such modifications are expected to have strong impacts on fish biodiversity, abundance, reduced habitat and food availability within the lake. To invest how TSL's fish community structure responds to the seasonal change, how they shift their diet across hydrological cycles and feeding competing for food resource: First, we aim to characterize the spatio-temporal variations of fish taxonomic composition and to highlights the underlying determinants of these variations. For this purpose, we estimated beta diversity as the total variance of the site-by-species community matrix and partitioned it into Local Contribution to Beta Diversity (LCBD) and Species Contribution to Beta Diversity (SCBD). We then performed multiple linear regressions to determine whether species richness, species abundances and water level explained the temporal variation in the contribution of site and species to beta diversity. Our results indicate strong temporal variation of beta diversity due to differential contributions of sites and species to the spatial variation of fish taxonomic composition. We further found that the direction, the shape and the relative effect of species richness, abundances and water level on temporal variation in LCBD and SCBD values greatly varied among sites, thus suggesting spatial variation in the processes leading to temporal variation in community composition. Overall, our results suggest that fish taxonomic composition is not homogeneously distributed over space and time and is likely to be impacted in the future if the flood-pulse dynamic of the system is altered by human activities. Second, we aim to investigate the temporal pattern of the most occurrence and abundance species and how their co-occurrence pattern across hydrological cycles. We found that occurrence and abundance patterns were temporally varied at all water level seasons. Strong temporal variation in species occurrence was occurred with visiting species such as Labiobarbus leptocheilus and Poropuntius deauratus while water level starts to fill into the TSL. We further observed that the abundance of 17 species was strongly varied while other 22 species (mainly TSL's residential species) were stable within the year. Positive species co-occurrence pattern was generally higher than negative species co-occurrence at all water level seasons. Highest positive co-occurrence patterns were found during the period of decrease and low water level seasons while fishes are migrating from flooded areas, competing for resource and habitats during low water season. Study on temporal distribution and species co-occurrence of fish and how community responds to the seasonal change in hydrological cycles provides critical information for fisheries management and conservation in the Tonle Sap Lake (TSL) as well as maintaining fish biodiversity in the Mekong system. Third, the implications of seasonality on food web structure have been notoriously understudied in empirical ecology, particularly in TSL's system. The current study, we focus on seasonal changes in one key attribute of a food web, vertical trophic position of consumers.

Jusqu’à présent, la dynamique des fractions végétales n’avait jamais été étudiée dans les estuaires macrotidaux en raison de leur faible quantité, par rapports aux fortes charges en matières en suspension fines, et du manque de protocoles d’étude et d’échantillonnage adéquats. Les débris végétaux sont toutefois à l’origine de perturbations d’activités économiques qui impliquent la filtration de larges volumes d’eau. L’objectif de cette thèse était ainsi de comprendre la dynamique d’apport et de transit de ces débris végétaux dans l’estuaire de la Gironde par la mise en oeuvre d’un suivi spatio-temporel de leur distribution et de techniques nouvelles pour un tel estuaire hyper-turbide (incubations in-situ litter-bag, caractérisation biogéochimique, identification des sources). Les résultats principaux sont la mise en évidence du contrôle du régime hydrologique sur leur distribution et la détermination des échelles de temps de leur persistance dans l’estuaire de la Gironde. / Up to now, vegetal fraction dynamics has not been studied in macrotidal estuaries, due to its low quantity in comparison to the strong charge of suspended particulate matter, and due to the lack of appropriate protocols to sample and examine it. Nevertheless, vegetal debris have been identified as a factor able to disrupt a wide range of stakeholder activities that require huge volumes of water to filter. The objective of this PhD was to understand the input and transfer dynamics of vegetal debris in the Gironde Estuary by the implementation of a spatiotemporal btrack of their distribution and by the development of new techniques for this hyper-turbide estuary (in situ litter-bag incubations, biogeochemical characterization, and identification of the sources). Mean results highlight the influence of the hydrological regime in their distribution and reveal time scales of their persistence in the Gironde Estuary.

Le lac Tonle Sap (TSL) est l'un des plus grands lacs du monde et est connu comme un hot-spot de la biodiversité en Asie du Sud-Est. En raison de la grande diversité ainsi que de la productivité élevée de poissons, le lac contribue à un apport important en protéines pour la population cambodgienne. Cette étude vise principalement à étudier (1) la dynamique spatiale et temporelle de la composition des communautés de poissons, (2) les effets des facteurs environnementaux sur la distribution des espèces et (3) l'effet de l'abolition des lots de pêche sur la biomasse, les communautés de poissions et le structure des poissons. En utilisant des méthodes statistiques multivariées sur les données de poissons et de variables environnementales, je suis en mesure de mettre en évidence les principales conclusions suivantes: * Les communautés de poissons du TSL étaient composées de deux assemblages de poissons: l'assemblage du nord, principalement caractérisé par des poissons noirs, et l'assemblage du sud, principalement lié aux poissons blancs, gris et estuariens. Les assemblages de poissons de la période 1994-1995 étaient représentés par l'abondance de tous les groupes fonctionnels, c'est-à-dire les poissons noirs, blancs et gris, et pour la période de 1996 à 1999, les assemblages étaient liés aux poissons blancs et gris. * Les distributions des abondances espèces de poissons n'étaient pas homogènes dans le TSL. De plus, les aires de distributions des espèces étaient différentes et étaient régies par des combinaisons distinctes de caractéristiques de l'habitat et de facteurs climatiques. * H. lobatus et H. siamensis peuvent coexister ensemble, mais la synchronisation et la migration de H. lobatus conduisent toujours à celles de H. siamensis. Ces résultats suggèrent que la population de H. lobatus est plus sensible aux variations d'impulsion de flux que celles de H. siamensis. Ceci indique que les variations des impulsions d'écoulement sont les principaux déterminants responsables de la dynamique temporelle de chaque espèce.[...] / Tonle Sap Lake (TSL) is one of the world's largest lakes and is a biodiversity hotspot in Southeast Asia. It supports high fish productivity which sustains protein supply for millions of people in the region. This study aims to investigate (1) spatial and temporal dynamics of fish community composition, (2) the effects of environmental factors on fish distribution and (3) effects of fishing lot abolishment on fish biomass, community and structure in TSL. By using multivariate statistical methods on fish and environmental data, the thesis highlights that: * There were two fish assemblages in TSL: the northern assemblage, mostly characterised by black fishes, and the southern assemblage, mainly linked to white, grey and estuarine fishes. Fish assemblages from earlier years (1994 and 1995) were represented by the abundances of all functional groups, i.e. black, white and grey fishes, and from 1996 to 1999, the assemblages were linked only to white and grey fishes. * Fish species distributions were not homogeneous within TSL. In addition, species distribution areas were different and were governed by distinct combinations of the local habitat characteristics and regional climatic factors. * H. lobatus and H. siamensis can co-occur together, but synchronisation and migration of H. lobatus always lead those of H. siamensis. These results suggest that the population of H. lobatus is more responsive to flow pulse variations than those of H. siamensis.[...]

Made available in DSpace on 2017-07-10T18:13:13Z (GMT). No. of bitstreams: 1 Carlos Henrique Orsi.pdf: 1050981 bytes, checksum: 04c595bd725a75ff4c7d08c180f10951 (MD5) Previous issue date: 2012-08-23 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / The region of the lower Araguaia River is influenced by a flood pulse that causes changes in the environment. This study aimed to identify the spatial effects (main river and tributaries) on the assembly structure of fish. Samples were taken every two months between March and September 2009, at 14 sites (7 in the Araguaia River and 7 in tributaries), fishes were collected with gill nets exposed for 24 hours, for the analysis effort was standardized and indexed in catch per unit of effort in number and biomass, then the attributes of the assembly and the total abundance were set and spatial distribution patterns were identified through factorial analysis. The beta diversity was estimated as the rate of renewal of species. An ordination analysis indicated the formation of three axes and their scores were retained for a PerManova analysis. Analysis of the indicator species was performed to determine if there were local indicator species. Species richness and abundance of fish were higher in the channel of the Araguaia River during the dry season. The diversity in the channel of the Araguaia River in low water seasons and dry seasons differed from the tax on water and flood in relation to the abundance in number of individuals and beta diversity showed that the Araguaia River and tributaries have their assemblages in a non-random status. The evenness showed no significant differences between sites and seasons. The evaluation of indicator species identified a number of species that stick out among the others, mainly in the main channel in the dry season, these facts may be related to environmental changes between seasons, flooding its banks and several structures along the river, providing new habitats and causing some species seek more favorable environments to their needs during the different seasons. / A região do baixo rio Araguaia sofre a influência de um pulso de inundação que causa modificações no ambiente. O presente trabalho teve por objetivo identificar os efeitos espaciais (calha e tributários) na estrutura da assembleia da ictiofauna. As amostras foram realizadas bimestralmente entre março e setembro de 2009, em 14 locais (7 no rio Araguaia e 7 em tributários), os peixes foram coletados com redes de espera expostas por 24 horas, sendo que para as análises, o esforço foi padronizado e indexado em captura por unidade de esforço em número e biomassa, em seguida os atributos da assembleia e abundância total foram calculados e posteriormente os padrões de distribuição espacial foram identificados através de análises bifatoriais. A diversidade beta foi estimada como a taxa de renovação de espécies. Uma análise de ordenação indicou a formação de 3 eixos e seus escores foram retidos para análise de PerManova. A análise de espécies indicadoras foi realizada para determinar se existiam espécies indicadoras de locais. A riqueza de espécies e abundância de peixes foram maiores na calha do rio Araguaia durante a estação seca. A diversidade na calha do rio Araguaia nas estações vazante e seca divergiu dos tributários nas estações enchente e cheia em relação à abundância em número de indivíduos e a diversidade beta mostrou que o rio Araguaia e os tributários apresentam suas assembleias de forma não aleatórias. A equitabilidade não apresentou diferenças significativas entre os locais e as estações. A avaliação de espécies indicadoras permitiu identificar algumas espécies que sobressaem em relação às outras, principalmente na calha principal na estação seca, tais fatos podem estar relacionados às mudanças no ambiente entre as estações, alagando as margens e várias estruturas ao longo do rio, disponibilizando novos habitats e fazendo que algumas espécies procurem ambientes mais propícios as suas necessidades durante as diferentes estações do ano.

L'objectif principal de cette thèse était d'étudier la dynamique spatiale et temporelle de la structure des communautés de poissons du fleuve Mékong en aval et du grand lac Tonlé Sap au Cambodge. Les deux systèmes sont fortement liés et caractérisés par les systèmes d'inondation pulsés. À l'aide d'approches multivariées sur des bases de données piscicoles, environnementales et des traits fonctionnels des taxons étudiés, j'ai pu mettre en évidence : * les gradients longitudinaux des caractéristiques physicochimiques des eaux et l'importance relative des facteurs environnementaux dans la structuration des communautés de poissons à large échelle ; * la structure temporelle des communautés de poissons qui est fortement liée aux traits fonctionnels des taxons et déclenchée par les crues occasionnées du système * la structure de base du réseau trophique et l'état écologique du système d'inondation pulsé qui est perturbé par la pression anthropique. Ces résultats ont démontré leurs valeurs quant à leurs potentiels de transferts vers la compréhension écologique et la gestion durable des ressources de pêches dans le système d'inondation pulsé en Asie tropicale en relevant que : * la connectivité entre le fleuve Mékong et le lac Tonlé Sap est indispensable pour les poissons pour compléter leurs cycles de vie, et la conservation devrait prioritairement s'effectuer au milieu du Mékong où se trouve la plus forte diversité. * dans le système d'inondation pulsé, la structure temporelle des communautés dépend des dynamiques des crues, donc l'aménagement des cours d'eau est à éviter au maximum. * la compréhension bioécologique complète des communautés et leurs relations trophiques sont primordiales pour la mise en place des plans de gestion et de conservation à long terme. / The main objective of the thesis was to study the spatial and temporal dynamics of fish community structure in Lower Mekong river basin and Tonle Sap Great Lake in Cambodia. These two systems are strongly connected to each other and characterised by flood pulse system. By using the multivariate approaches on the fish, environmental and fish functional traits data sets, I am able to highlight: * the longitudinal gradients of physicochemical characteristics of water and the relative importance of environmental factors in explaining the large scale patterns of fish community; * the temporal structure of fish community, which is strongly linked to functional traits of fish and trigged by the pulse dynamics of the system; * the baseline trophic structure model and ecological health assessment of flood pulse system, which is disturbed by anthropogenic pressure. These results have demonstrated their values as well as their potentials to transfers toward an ecological understanding and sustainable fisheries resource management of the flood pulse system in tropical Asia by suggesting: * the connectivity between Mekong river and Tonle Sap lek is essential for fish to complete their life cycle and also the conservation should be done in the middle part of Lower Mekong river where containing the highest diversity; * in flood pulse system, the temporal structure of fish community strongly depend on the dynamics of pulses; therefore any river development must be strictly prohibited; * complete bio-ecological understanding of communities and their trophic relationships are crucial to the establishment of long-term management and conservation plans.