Species diversity may simultaneously be a cause and a consequence of variability in population, community and ecosystem properties. Ecology has traditionally focused on elucidating the causes of biodiversity. However, in the last decade and a half ecologists have asked the opposite question: What are the consequences of species diversity? The majority of these studies elucidated the effects of species diversity within single trophic levels. Incorporating trophic complexity is the next step in this research program. In this dissertation I investigated the causes of resource species diversity, as well as the impacts that resource diversity has on rates of consumption and the stability of population, community and ecosystem properties over time in planktonic food webs. The high diversity of phytoplankton found in nature appears to defy the competitive exclusion principle, and elucidating the mechanisms which maintain this diversity continues to be a challenge. In general, variability in limiting factors is required to maintain non-neutral species diversity, but this variability can be generated by forces outside of the competitive community (i.e. exogenous), or may be the outcome of competitive interactions themselves (i.e. endogenous). Using microcosm experiments, I showed that endogenously generated variability in limiting factors was more effective at maintaining phytoplankton species diversity over the long-term, although the strength of this effect depended on the composition of the phytoplankton community. Existing resource diversity has been proposed to generally weaken consumer-resource interaction strengths and limit consumer control of resource biomass. This is because more diverse resource communities are more likely to contain inedible, unpalatable, toxic or non-nutritious species. However, when resource communities contain multiple palatable species, diversity may also accelerate consumption. Using grazing experiments with multiple zooplankton consumer species, I found that the mechanism, direction and magnitude of modulation of consumption depended on the feeding selectivity of the consumer and the composition of the resource community. By altering consumer-resource interaction strengths in the short-term, resource species diversity may impact the stability of consumer-resource dynamics in the long-term. In separate microcosm experiments, I investigated the influence of resource species diversity, community composition and consumer feeding selectivity on population, community, and ecosystem properties over time. Diversity had positive effects on phytoplankton population biomass, resource community biomass, the rate of photosynthesis, the standing stock of particulate nutrients, and the generalist consumer’s population density. It also stabilized resource community biomass and the stocks of particulate nutrients over time. Unexpectedly, diversity did not stabilize either of the consumer populations, regardless of feeding selectivity. This suggests that effects of diversity on resource community properties do not impact consumer dynamics linearly. Resource community composition was generally more important than resource species diversity in determining food web properties. The importance of community composition in determining both the causes and consequences of resource diversity in these experiments points to the importance of species’ traits and the outcomes of their interactions. I suggest that the use of complex adaptive systems theory and trait-based approaches in the future will allow a consideration of the feedbacks between the causes and consequences of species diversity in food webs. / Graduate

Submitted by FABIANA DA SILVA FRANÇA (fabiana21franca@gmail.com) on 2018-02-23T17:33:04Z No. of bitstreams: 1 Arquivo Total.pdf: 2718770 bytes, checksum: 9378d34888dfcacdd356cf58ac71eaa8 (MD5) / Made available in DSpace on 2018-02-23T17:33:04Z (GMT). No. of bitstreams: 1 Arquivo Total.pdf: 2718770 bytes, checksum: 9378d34888dfcacdd356cf58ac71eaa8 (MD5) Previous issue date: 2017-03-22 / The Rufford Foundation / Understanding the ecological role of species within an ecosystem depends largely on knowledge of its trophic relations. Such relations and the position of species within a foodweb are a central step in understanding the dynamics of the communities and the impacts that each species has on the trophic network compartments. The knowledge on the diet of the South American freshwater stingrays has increased considerably over the last years,but still, little is know about their ecological role. Thereby, the aim of this research is to evaluate the importance of the stingray Potamotrygon magdalenae in the structure of a freshwater food web. The feeding habits and the trophic ecology of this species were evaluated using stomach content analysis in combination with stable isotopes of carbon and nitrogen from samples collected in the middle Magdalena river basin, Colombia, and its ecological importance from topological analyzes of trophic networks. Potamotrygon magdalenae showed a specialized diet, feeding primarily on insects. No significant differences of δ13C and δ15N were observed in the muscle of the species between the hydrological periods of high and low waters. The species isotopic niche was intermediate. In terms of ecological importance, it played an intermediary role in the dispersion of indirect effects through the system, due to its intermediate values of centrality and topological importance. These results provide a starting point for further ecological studies about the South American freshwater stingray species, in order to assess their role in the ecosystem. On the other hand, theycomplement the existing studies of Potamotrygon magdalenae. / Compreender o papel ecológico das espécies no ecossistema depende amplamente do conhecimento de suas relações tróficas. Conhecer tais relações e a posição das espécies em umarede é um passo crucial para entender a dinâmica das comunidades e os impactos quecada espécie tem sobre os demais compartimentos. Oconhecimento sobre adieta das espécies de raias de água doce de América do Sul tem aumentado consideravelmente ao longo dos últimos anos, porém, é muito pouco o que se sabe sobre o papel ecológico das espécies nos seus respectivos ecossistemas. Assim, oobjetivo deste trabalho foi avaliar a importânciaecológica da raia Potamotrygon magdalenae na estrutura da rede trófica de um ecossistema de águas continentais. Foram avaliados os hábitos alimentares e a ecologia trófica da espécie através da análise de conteúdos estomacais e isótopos estáveis de carbono e nitrogênio a partir de amostras coletadas na bacia do médio rio Magdalena, Colômbia; e sua importância ecológica a partir de análises topológicas de redes tróficas. A espécie foi considerada um predador especialista de nível trófico intermediário, com preferência pelos insetos.Não foram observadas diferenças significativas de δ13C e δ15N na espécie entre os períodos hidrológicos de cheia e de águas baixas. O seu nicho isotópico foi intermediário. Em relação à importância ecológica, a espécie desempenhou um papel intermediário na dispersão de efeitos indiretos através do sistema, devido aos seus valores intermediários de centralidade e importância topológica. Estes resultadosconstituem um ponto de partida para novos estudos ecológicos das espécies de raias de água doce da América do Sul, que visem avaliar seu papel nos ecossistemas, e complementam os estudos existentes de Potamotrygon magdalenae.

Les sources hydrothermales sont localisées le long des dorsales médio-océaniques et des bassins arrière-arcs. Elles abritent une faune spécialisée dont le développement repose sur la chimiosynthèse microbienne, possible grâce aux éléments réduits présents dans le fluide hydrothermal. Alors que les liens entre les assemblages fauniques et les habitats ont relativement bien été décrits autour du globe, notre connaissance de la dynamique temporelle de ces écosystèmes demeure encore limitée, et particulièrement sur ces échelles de temps cohérentes avec la biologie des organismes (de quelques minutes à plusieurs années). Sur l’exemple des communautés hydrothermales inféodées à la dorsale Juan de Fuca, située dans le nord-est de l'océan Pacifique, ce mémoire vise à comprendre le fonctionnement et la dynamique temporelle des communautés de macrofaunes hydrothermales associées aux assemblages de vers Siboglinidae Ridgeia piscesae à travers le couplage de l'échantillonnage et d'une approche observatoire fond de mer. Cette étude a d’abord permis la description quantitative (composition, diversité et biomasse) de six assemblages de siboglinidés sur l’édifice hydrothermal Grotto (champ Main Endeavour, dorsale Juan de Fuca) et d’évaluer la structure trophique de ces communautés à l’aide des isotopes stables. Les résultats obtenus ont permis d’identifier 36 taxons, caractérisés par une importante diversité de polychètes et une forte densité de gastéropodes. Les assemblages de R. piscesae les plus développés présentaient des diversités plus importantes et des réseaux trophiques plus complexes. La coexistence des espèces au sein des structures biogéniques formées par R. piscesae s’expliquerait par un fort partitionnement des niches écologiques et des ressources nutritionnelles. L'analyse des images vidéo recueillies par le module écologique d’observationTEMPO-mini, déployé sur l’édifice Grotto au sein de l’observatoire câblé Ocean Networks Canada, a permis d’étudier la dynamique temporelle de ces espèces en suivant les variations de l'abondance de la macrofaune hydrothermale quotidiennement pendant une année, et toutes les 4 heures pendant un mois d’été et deux mois d’hiver. Les résultats ont permis de mettre en évidence une influence significative du cycle semi-diurne de la marée et des tempêtes de surface hivernale sur la distribution de deux espèces mobiles et non-symbiotiques (pycnogonides Sericosura sp. et polychètes Polynoidae). La dynamique locale des courants océaniques semble affecter périodiquement le mélange entre les émissions de fluides hydrothermaux et l'eau de mer environnante, modifiant les conditions environnementales au sein de l’habitat.Cette étude suggère que les espèces hydrothermales répondent à ces modifications de l'habitat en ajustant leur comportement. Finalement, dans un contexte où l’observation et l’imagerie prennent une place importante dans l’étude des écosystèmes profonds, cette thèse a permis d'optimiser les techniques d'imagerie vidéo dans l'extraction des données écologiques et dans la recherche des cycles naturels. Dans l’ensemble, ces résultats apportent des informations nouvelles et précieuses dans notre compréhension du fonctionnement et de la dynamique des communautés hydrothermales profondes et ouvre de nouvelles voies de recherches pour l'écologie des écosystèmes marins profonds en général. / Hydrothermal vents are distributed along mid-ocean ridges and back-arc basins. These ecosystems harbour a specialized fauna supported by a local chemosynthetic microbial production, made possible through the oxidation of reduced elements present in the hydrothermal fluid. While the relationships between faunal assemblages and habitats have been relatively well described around the globe, our knowledge on the temporal dynamics of these environments is still limited, particularly at time scale relevant to organisms (from few minutes to several years). Based on hydrothermal vent communities occurring on the Juan de Fuca Ridge, located in the northeast Pacific Ocean, this thesis aims at understanding the functioning and temporal dynamics of macrofaunal communities associated with the Siboglinidae Ridgeia piscesae tubeworm assemblages through the coupling of sampling and seafloor observatory approach. This study first allowed the quantitative description (composition, diversity and biomass) of six siboglinid species assemblages on the Grotto hydrothermal edifice (Main Endeavour, Juan de Fuca Ridge) and the assessment of the trophic structure of these communities using stable isotopes. The results enabled the identification of 36 taxa. An elevated polychaete diversities and high densities of gastropods characterized all assemblages. The most developed R. piscesae assemblages exhibited greater diversity and more complex food webs. The species coexistence within the biogenic structures created by R. piscesae could be explained by a strong partitioning of ecological niches and nutritional resources. The analysis of video images gathered by the ecological observatory module TEMPO-mini, deployed on the Grotto edifice on the cabled observatory OceanNetworks Canada, provided information on the temporal dynamics of the assemblage. The abundance variations of four macrofaunal species were tracked at a daily frequency during one year and every 4 hours during one summer and two winter months. Our results highlighted a significant influence of the semi-diurnal tidal cycle and winter surface storms on the distribution of two mobile and non-symbiotic taxa (Sericosura sp. pycnogonids and Polynoidae polychaetes). The local ocean dynamics seemed to periodically affect the mixing between hydrothermal fluid inputs and surrounding seawater, therefore modifying environmental conditions in vent habitats. This study suggests that hydrothermal species respond to these habitat modifications by adjusting their behaviour. Finally, considering the increased importance of observational and imagery approaches in the study of deep-sea ecosystems, this thesis proposes a guideline and methodology in order to optimise video imagery methods both for the extraction of ecological data and the search of natural cycles. Overall, these results bring novel and valuable information to help understand the functioning and dynamics of deep-sea hydrothermal communities and opens new research avenues for deep-sea ecology in general.

Made available in DSpace on 2016-06-02T19:31:50Z (GMT). No. of bitstreams: 1 3025.pdf: 1800244 bytes, checksum: b376383f9bf312a9c49a427e22dcd8c9 (MD5) Previous issue date: 2010-03-08 / Financiadora de Estudos e Projetos / Interactions between endofagous insects in fruits play an important role in ecological communities, since the seed survivorship and hence the successes in recruitment of plant populations are directed related to damage caused by the herbivory from these insects. Because of the specialization of the herbivorous insects, the insect-plant interactions tend to be structured in compartments of insects which use different groups of plant. The aims of this study were: (1) to identify if insect-plant interactions in fruits of the cerrado are compartmented; and (2) to identify if plant species in compartments are related. With two monthly field trips, the fruits of 34 plant species were sampled. The insects that emerged from fruits were classified in morphospecies. A correspondence analysis was used to identify the compartments. About 97 morphospecies of insects were identified. Although there is no consensus if the competitive forces structure the compartmented networks or, if directly or indirectly, chemicals defences of plants, the compartmentation seems to be an overall characteristic of food webs and, thus, of insect-plant networks. There is also evidence of the role of the phylogeny in the structure of compartmented networks. / As interações de insetos endófagos em frutos têm um importante papel nas comunidades ecológicas, porque a sobrevivência das sementes e, consequentemente, o sucesso no recrutamento das populações de plantas estão diretamente relacionados aos danos causados pela herbivoria desses insetos. Devido à especialização dos insetos herbívoros, as interaçãos inseto-planta tendem a estruturar-se em compartimentos de insetos que usam diferentes subgrupos de plantas. O presente trabalho teve como objetivos: (1) identificar se as interações inseto-planta em frutos de cerrado são compartimentadas; e (2) identificar se as espécies de plantas nos compartimentos são relacionadas filogenéticamente. Com duas excursões mensais ao campo durante um ano, foram amostrados os frutos de 34 espécies vegetais. Os insetos que emergiram dos frutos foram identificados em 97 morfoespécies. Análise de correspondência foi usada para identificar os compartimentos. Os resultados indicam que a compartimentação parece ser uma característica geral de redes de alimentação entre herbívoros e plantas e, portanto, das redes entre parasitóides de herbívoros e plantas. Há também evidência de que a compartimentação de redes de interações pode ser filogeneticamente estruturada.

Ce travail de thèse porte principalement sur l'utilisation du calcul moulien et de la technique d'arborification introduits par Jean Ecalle dans les années 70 et leurs applications à l'étude des systèmes dynamiques discrets ou continus.L'une des contributions est une étude systématique des conditions sous lesquelles l'arborification permet de restaurer la convergence de séries formelles via l'introduction d'une notion d'invariance d'un moule sous arborication. Ces résultats permettent de donner une preuve détaillée du théorème de Brjuno de linéarisation analytique des champs de vecteurs telle qu'elle est proposée par Jean Ecalle dans son article "Singularités non abordables par la géométrie". Ces résultats ont été obtenus en collaboration avec Dominique Manchon (Université de Clermont Ferrand) et Jacky Cresson.La puissance du calcul moulien est ensuite illustrée par la résolution presque complète de la conjecture de Jarque-Villadelprat sur les centres isochrones Hamiltoniens. Cette conjecture stipule qu'il n'existe pas de champs de vecteurs polynomiaux du plan de degré pair qui soit hamiltonien. L'examen de la structure algébrique de la correction, introduite dans les années 90 par G. Gallavotti et généralisée ensuite par Jean Ecalle et Bruno Vallet, et son calcul explicite via le calcul moulien, nous ont permis d'obtenir des conditions explicites d'obstructions à l'isochronisme. L'aspect algébrique et combinatoire de ces objets et méthodes conduisent naturellement à une classication des conditions de centre via une notion de complexité. L'arborication quand à elle permet l'unification de nombreuses approches et une simplication de divers travaux, notamment ceux de J.C.Butcher autour de la structure algébrique des méthodes de Runge-Kutta qui a induit ce que les numériciens appellent des B-séries. En étudiant la structure algébrique de l'opérateur de substitution associé à un difféomorphisme, en particulier celui relié à une méthode de Runge-Kutta et celui associé à la solution de l'équation diérentielle sous-jacente, on présente le codage de Butcher comme une traduction particulière de l'arborification directe de l'opérateur de substitution. Notons que ce phénomène est large et permet d'inclure les travaux plus récents sur l'approche par trajectoires rugueuses des solutions d'équations différentielles stochastiques.Une seconde partie de la thèse concerne la recherche des groupes de symétries de Lie des tissus du plan en suivant une approche d'Alain Hénaut (Université de Bordeaux). Ce travail nous a permis de préciser la relation entre la dimension de ces groupes de symétries et le caractère linéarisable ou hexagonale des tissus du plan. Dans le cas des arrangements de droites, on obtient ainsi une relation profonde entre le module de dérivations de Saito associé à l'arrangement et le groupe de symétrie du tissu associé. / This thesis work mainly focuses on the use of the mould calculus and the technic of arborification which had been introduced both by J.Ecalle in the seventies and theirs applications to the study of continuous or discrete systems.One of the contributions is the systematic study of conditions under which the arborification allows to reestablish the convergence of formal series via introduction of a notion of invariance of mould under arborification. These results allow to give a detailed proof of Brjuno Theorem of analytic linearizability of vector fields as it is proposed by J.Ecalle in his article "Singularité non abordable par la géométrie". These results were obtained jointly with Dominique Manchon (University of Clermont Ferrand) and Jacky Cresson.The power of the mould calculus is then illustrated by an almost complete resolution of the Jarque-Villadelprat's conjecture about Hamiltonian Isochronous centers. This conjecture states that there is not existing polynomial vector fields in the plane of odd degree which are Hamiltonian. The study of the algebraic structure of the correction, introduced in the nineties by G.Gallavotti and then generalized by J.Ecalle and B.Vallet and its explicit computation via mould calculus, enables us to obtain explicit conditions of obstruction to isochronicity. The algebraic and combinatoric aspect of these objects and methods brings naturally to the classification of center conditions through a notion of complexity. The arborification allows to the unification of different approaches and a simplicification of different works, especially those of J.C.Butcher about algebraic structures of Runge-kutta methods, who had introduced that is called B-series by numerical mathematicians. Studying the algebraic structure of the substitution operator associated to a diffeomorphism, especially the one related to a Runge-Kutta method and the one which is associated to the solution of the underlying differential equations, we present the Butcher's encoding as a special translation of a direct arborification of the substitution automorphism. We can conclude that this phenomenon is wide and allows to include more recent studies on the approach by rough path of stochastic differential equations.A second part of this thesis involves the research of Lie group of symmetries of planar webs following Hénaut's approach (University of Bordeaux).This work allows to precise the relation between the dimension of the groups of symmetries and the linearizability or hexagonal character of planar webs. In the the case of line arrangement, we obtain a depthful relation between the modulus of derivations of Saito associated to the line arrangement and the group of symmetries of the associated web.

Submitted by Geandra Rodrigues (geandrar@gmail.com) on 2018-01-08T14:49:55Z No. of bitstreams: 1 andressadasilvareis.pdf: 5781298 bytes, checksum: bf00dd44d481610ae8449b6433afb56b (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2018-01-23T11:06:18Z (GMT) No. of bitstreams: 1 andressadasilvareis.pdf: 5781298 bytes, checksum: bf00dd44d481610ae8449b6433afb56b (MD5) / Made available in DSpace on 2018-01-23T11:06:18Z (GMT). No. of bitstreams: 1 andressadasilvareis.pdf: 5781298 bytes, checksum: bf00dd44d481610ae8449b6433afb56b (MD5) Previous issue date: 2016-11-11 / Inúmeros impactos antrópicos vêm afetando os ecossistemas aquáticos e identificar respostas da biota aquática a esses impactos persiste em um grande desafio aos ecólogos. Com o objetivo de determinar essas respostas sobre diferentes níveis de organização biológica e suas interfaces, são abordadas a ecologia trófica dos peixes e a estrutura e dinâmica das teias tróficas em 20 riachos de 3ª ordem de quatro bacias da região de Mata Atlântica (RJ), em gradiente de impactos relacionados ao uso de solo da microbacia e às medidas estruturais e limnológicas locais. Foram realizadas análises do conteúdo estomacal e de isótopos estáveis de carbono (C) e nitrogênio (N) dos peixes e seus recursos alimentares (macroinvertebrados e recursos basais). O gradiente de alguns impactos tanto em escala local (% de cobertura de dossel) quanto de microbacia (uso de solo e input de nutrientes) afetaram a integridade estrutural e funcional das comunidades aquáticas (Capítulos 1, 3 e 4). Foram identificadas variações da proporção de guildas tróficas de peixes e conexões com seus recursos (Capítulo 1), com o aumento das guildas dos Detritívoros e Detritívoros-insetívoros em riachos com alta concentração de nitrogênio total, alta turbidez e baixa temperatura. A métrica H2 (Especialização da teia) foi relacionada a riachos com maior porcentagem de dossel, indicando maior partilha de recursos entre as espécies de peixes (Capítulo 1). Os valores de δ13C do perifíton foram maiores em riachos com menor cobertura vegetal, ao passo que os de δ15N se relacionaram ao conjunto de impactos em escala local e regional (Capítulo 2). Já as alterações isotópicas na biota foram reflexo, em parte, da mudança dos valores do perifíton (Capítulo 2), principal fonte de carbono basal para os consumidores aquáticos (Capítulo 3), e, em parte, da variação da dieta dentro das guildas (Capítulos 3 e 4), influenciando a posição trófica de algumas espécies e guildas de peixes na teia alimentar (Capítulo 4), como os Insetívoros e Detritívoros, e as espécies Gymnotus pantherinus e Astyanax spp. As influências dos gradientes de impactos sobre os vários compartimentos foram sintetizadas em diagramas conceituais (Capítulo 3), demonstrando as variações dos valores isotópicos de todos os consumidores (insetos e peixes), e as mudanças da proporção de insetos consumidos por todas as guildas tróficas de peixes frente aos impactos. Os resultados demonstram que o gradiente de impactos, tanto em escala local quanto regional, atuam em conjunto na determinação da estrutura e dinâmica trófica das comunidades aquáticas, nem sempre sendo possível isolar seus efeitos sobre os diversos parâmetros analisados. Ao mesmo tempo em que demonstrou alguns efeitos idiossincráticos, o presente trabalho permitiu identificar padrões de resposta das comunidades aquáticas em relação a alguns dos principais impactos sofridos por sistemas lóticos em ambientes tropicais. / Numerous anthropic impacts have been affecting aquatic ecosystems. Identifying responses of aquatic biota to these impacts remains a major challenge for ecologists. In order to determine these responses on different levels of biological organization and their interfaces, the trophic ecology of the fish and the food web structure and dynamics were analyzed in 20 streams (3rd order) of four watersheds in the Mata Atlântica (RJ). These streams are arranged in a gradient of impacts related to the watershed land use and the local structural and limnological features. Stomach contents and stable isotopes of carbon (C) and nitrogen (N) of fish and their food resources (macroinvertebrates and basal resources) were analyzed. The gradients of some impacts on the local scale (% canopy cover) and the microbasin (land use and nutrient input) affected the structural and functional integrity of aquatic communities (Chapters 1, 3 and 4). Variations in the proportion of fish trophic guilds and connections with their resources were identified (Chapter 1), with the increase of the guilds of Detritivores and Detritivorousinsectivores in high concentration of total nitrogen, high turbidity and low temperature. The metric H2 (Food web specialization) was related to streams with higher canopy cover, indicating a high resource partitioning among the fish species (Chapter 1). Values of periphyton δ13C were higher in streams with lower canopy cover, while δ15N values were related to the set of impacts at the local and the regional scale (Chapter 2). Isotopic variation in the biota were in part a reflection of the variation of the periphyton (Chapter 2), which is the main source of basal carbon for aquatic consumers (Chapter 3), and in part of the variation of diet within the guilds (Chapters 3 and 4), influencing the trophic position of some fish species and guilds (Chapter 4), such as Insectivores and Detritivores, and the species Gymnotus pantherinus and Astyanax spp.. Influences of the impact gradients upon the biota compartments were summarized in conceptual diagrams (Chapter 3), showing the variations of the isotopic values of all the consumers (insects and fish), and the changes in the proportion of insects consumed by all the fish trophic guilds. The results show that the impact gradients, both local and regional, determine the structure and trophic dynamics of aquatic communities, and it is not always possible to isolate their effects. At the same time that it demonstrated some idiosyncratic effects, the present work allowed to identify patterns of response of aquatic communities related of the main impacts suffered by lotic systems in tropical environments.

Les récifs coralliens de Nouvelle-Calédonie constituent un « hot-spot » de biodiversité marine et sont classés au patrimoine mondial de l’Unesco depuis 2008. Ces récifs sont dans un bon état général, mais ils sont soumis à des pressions anthropiques de plus en plus intenses, engendrées par un développement industriel (mines de nickel) et urbain croissant.Mieux comprendre le fonctionnement des écosystèmes lagonaires calédoniens est donc devenu un enjeu majeur. Ce travail s’inscrit dans cette démarche, avec pour objectifs principaux : (1) de reconstruire l’architecture des réseaux trophiques de ces écosystèmes (méthode des isotopes stables), (2) de déterminer leurs niveaux de contamination en éléments traces métalliques et en polluants organiques, et (3) de décrire l’intégration et le cheminement de certains de ces contaminants en leur sein. L’étude des écosystèmes récifo-lagonaires et de leurs réseaux trophiques a été réalisée au travers du prisme de prédateurs méconnus de hauts rangs trophiques : les poissons anguilliformes (murènes, congres et poissons-serpents). Pour ce faire, différentes sources de matière organique (MO) et divers consommateurs, parmi lesquels les poissons anguilliformes, ont été prélevés dans plusieurs sites répartis sur un gradient côte-large et provenant de deux zones du lagon sud.Quatre réseaux trophiques ont été identifiés au cours de ce travail. La voie benthique basée sur la MO du turf algal est la voie trophique majeure, commune à tous les poissons anguilliformes. L’utilisation complémentaire de la voie benthique sédimentaire et de la voie « pélagique lagonaire », est également mise en évidence avec une importance variable selon les espèces. En revanche, la MO provenant des phanérogames marines n’est intégrée qu’indirectement, par le biais de la voie détritique. Les différentes espèces d’anguilliformes appartiennent donc à des réseaux trophiques en partie divergents selon leur habitat (substrat dur ou meuble) et leur régime alimentaire (micro-, macrocarnivore ou piscivore). Ces prédateurs de hauts niveaux trophiques, consommant principalement des crustacés et des poissons mais également occasionnellement des annélides polychètes et des céphalopodes, se révèlent pour la plupart opportunistes. Une compétition entre certaines espèces est soulignée par le recouvrement de leurs niches trophiques. Les sources de MO et les consommateurs récifaux du lagon calédonien accumulent des concentrations modérées à fortes en éléments traces d’origine agricole, urbaine et minière. La bioaccumulation dépend des propriétés physico-chimiques du contaminant lui-même et des caractéristiques propres à l’organisme (taille, habitat, régime alimentaire, etc.). Ainsi, les contaminants sont répartis différemment entre les compartiments, et seuls Hg et As sont bioamplifiés le long des réseaux trophiques. Une contamination du lagon par les polluants organiques est également soulignée. Bien que les concentrations mesurées chez les poissons anguilliformes restent relativement faibles, l’étendue spatiale de cette contamination, ainsi que la détection de pesticides très toxiques dont l’utilisation est interdite (DDT), attestent de la nécessité à considérer cette pollution avec attention. Toutes ces considérations renforcent l’importance de mener un suivi à long terme des contaminations d’origines diverses en relation avec le fonctionnement trophique des systèmes récifo-lagonaires. / New Caledonian coral reefs constitute a « hot spot » of marine biodiversity and were registered World Heritage by UNESCO in 2008. These reefs are in good health, but they are subject to intense anthropic threats, induced by the increase of industrial (nickel mining) and urban development. A better understanding of the functioning of the Caledonian coral reefs has become a major issue. This work is part of this approach, with as main goals to: (1) reconstruct the architecture of food webs of these ecosystems (stable isotope method), (2) determine their contamination levels in metallic and organic contaminants and (3) describe integration and pathways of some of these contaminants through the food webs. The description of ecosystems and their food webs was carried out by the study of unknown predators: the anguilliform fish (moray, conger and snake eels). Different sources of organic matter (OM) and consumers, including anguilliform fish, were sampled in several sites distributed over a coast to barrier reef gradient within two areas of the south lagoon. Four food webs were identified. The benthic pathway based on the algal turf OM is the main food web, common to all anguilliform fish. The complementary use of benthic sedimentary (SOM) and “lagoon pelagic” (POM) food webs, is also highlighted with variable importance according to species. In addition, OM from seagrasses is included indirectly by the detrital pathway. The numerous anguilliform fish species belong to diverse food webs in part, depending on their habitat (hard and/or soft bottom) and their diet (micro/macro carnivores or piscivores). These predators of high trophic level, consuming mainly crustaceans and fish but also more occasionally annelids and cephalopods, are mostly opportunistic. A competition between some species is underlined by the overlap of their trophic niches. Sources of OM and consumers of the Caledonian coral reefs accumulate moderate to strong concentrations of trace elements issued from agricultural, urban and mining origins. Bioaccumulation depends on both physical and chemical properties of the contaminant and the organism’s own characteristics (size, habitat, diet, etc.). So, contaminants are distributed differently between compartments and only Hg and As are biomagnified along food webs. Contamination of the lagoon by organic pollutants is also pointed out. Even if concentrations measured in anguilliform fish are rather low, widespread contamination, plus the detection of toxic and forbidden pesticides (DDT), confirm the necessity to include this pollution in further studies. All these considerations reinforce the importance of setting up a long-term tracking system of contamination from diverse origins, linked with the trophic functioning of food webs.

L'étude de l'écologie alimentaire des organismes ainsi que l'étude de la structure et du fonctionnement des réseaux trophiques, principale caractéristique de l'organisation des écosystèmes, ne sont pas aisées en environnement marin. Des méthodes indirectes d'étude ont donc été développées récemment, telles que l'utilisation de « traceurs écologiques » qui sont des paramètres (bio)chimiques mesurés dans les tissus biologiques des organismes. Sur la base du postulat « je suis ce que je mange », les traceurs écologiques constituent des indicateurs relativement fiables du niveau trophique des organismes et/ou de leur zone d'alimentation. Dans un premier temps, l'importance de certains facteurs de variations biologiques et environnementaux des traceurs isotopiques et métalliques (mercure Hg et cadmium Cd) a été étudiée dans le Golfe de Gascogne. Dans un tel environnement marin ouvert, supportant une grande diversité biologique et subissant diverses influences, notre hypothèse de travail était effectivement que les sources de variations sont nombreuses. Cette thèse a ainsi mis en évidence qu’il était nécessaire d’intégrer différents facteurs de variations pour utiliser ces traceurs écologiques dans ce type d'écosystème, tels que le changement de la ligne de base sur un gradient côte-large pour un calcul pertinent des positions trophiques. Dans un second temps, ces outils ont été directement appliqués pour l'étude de l'écologie alimentaire de quelques espèces d'intérêt commercial, telles que la sardine, l'anchois et le bar. Ils ont ainsi révélé une relative spécialisation de la sardine sur certaines proies mésozooplanctoniques, alors que l'anchois montre d’une part une plasticité trophique plus importante sur ce compartiment planctonique, et d’autre part une plus grande flexibilité en termes de zones d'alimentation. Dans le car du bar, dont l'écologie alimentaire était jusqu'à présent très peu connue en dehors des zones côtières, le couplage des données isotopiques (révélant des informations sur le régime alimentaire sur le moyen à long terme) avec des données de contenus stomacaux (révélant des informations sur le court terme) a permis de mettre en évidence la sélectivité des individus de bars adultes sur les petits poissons pélagiques à forte valeur énergétique, lorsque ce prédateur fréquente les eaux du large. Enfin, ces traceurs écologiques ont été utilisés pour affiner la compréhension de la structure et du fonctionnement des réseaux trophiques dans le Golfe de Gascogne, et le comportement de certains contaminants tels que le Hg dans ces réseaux. Plus de 140 espèces provenant des différents réseaux trophiques de cet écosystème ont ainsi été analysées en isotopie, et 120 espèces pour leurs concentrations de Hg dans le muscle. Les traceurs isotopiques en particulier ont révélé que dans cet écosystème : 1) les mammifères marins sont un compartiment structurant des réseaux trophiques ; 2) les chaînes alimentaires sont relativement courtes ; 3) les chaînes alimentaires et les relations trophiques sont fortement organisées en "réseau" ; 4) le degré d'omnivorie est plutôt moyen dans l'écosystème, avec une tendance à l'augmentation chez les niveaux trophiques supérieurs. Ce travail de thèse a également mis en évidence une contamination en Hg plus importante dans le muscle des espèces provenant des habitats profonds du Golfe de Gascogne. Dans un contexte de besoin d'outils de suivi de l'état écologique des écosystèmes marins, et des espèces composant les réseaux trophiques en présence, cette thèse a finalement démontré tout l'intérêt d'utiliser les traceurs écologiques tant à des fins de recherches fondamentales qu'à des fins appliquées en écologie marine. / The study of organisms' trophic ecology and those of food webs' structure and functioning are not easy in marine environments, while it is an essential characteristic of ecosystems' organisation. Thus, the use of indirect methods such as « ecological tracers » has dramatically increased recently. These methods consist in measuring (bio)chemical parameters in biological tissues of organisms. Based on the assumption « you are what you eat », ecological tracers constitute reliable indicators of the trophic position and/or of the feeding zone of organisms. First, the importance of some biological and environmental factors of variations on isotopic signatures and metal concentrations (mercury Hg and cadmium Cd) was examined in the Bay of Biscay. Indeed, in such an open marine ecosystem supporting a high biological diversity and undergoing various oceanic and terrigenous influences, we hypothesized that sources of variations are numerous. This work highlighted that different biological and environmental factors of variations have to be taken into account to use these ecological tracers in this type of ecosystem. For instance, we evidenced a change in the baseline along the inshore-offshore gradient that can considerably bias calculations of trophic positions from stable isotopes values, if it is not considered. Secondly, the isotopic tool in particular has been applied to study the trophic ecology of some commercially important species, such as sardine, anchovy and sea bass. Isotopic signatures revealed a relative specialisation of sardine on some mesozooplanktonic prey. On the contrary, anchovy showed a relative trophic plasticity on this type of prey, and a greater flexibility in terms of feeding zones. As for sea bass, the combination of isotopic data (giving an information on diet on the long term) and stomach contents data (giving an information on diet on the short term) highlighted the selectivity of adult individuals on small pelagic fish of high energy content, when foraging in offshore waters. Finally, ecological tracers were used to refine the understanding of food webs' structure and functioning in the Bay of Biscay, as well as Hg behaviour in these food webs. More than 140 species from the different food webs of this ecosystem have been analysed for their isotopic signature, and 120 species for muscle Hg concentrations. Isotopic data in particular revealed that in this ecosystem: 1) marine mammals are a structuring component of food webs; 2) food chains are relatively short; 3) food chains and trophic relationships are strongly "food web like" organised; 3) the degree of omnivory is intermediate in this ecosystem, with a slight trend to increase with increasing trophic position. This work also highlighted an enhanced bioaccumulation of Hg in the muscle of deep-sea fauna from the Bay of Biscay. In the context of the need for tools to monitor the environmental status of marine ecosystems, and of species that structure food webs, this work finally demonstrated the great interest of using ecological tracers for both fundamental and applied marine ecological research.

À l'interface bentho-pélagique, les herbiers marins présentent une multitude de rôles écosystémiques. Historiquement, les scientifiques et décideurs de La Réunion se sont focalisés sur l'écosystème récifal. Néanmoins, des prairies monospécifiques à Syringodium isoetifolium sont présentes de manière pérenne (3 hectares) au sein du complexe récifal de l'Ermitage / La Saline (côte ouest). L'objectif principal de cette étude était d'apporter les premières données sur l'écologie de ces herbiers marins. Ainsi, une approche pluridisciplinaire a été développée : 1. L'étude de la dynamique spatiale du paysage herbier sur 65 ans (dès 1950), et à l'échelle saisonnière (2013-2015) qui a permis de définir le rôle majeur des forçages hydrodynamiques et de l'eutrophisation. 2. L'analyse de la productivité de l'écosystème herbier et sa réponse métabolique face à des apports nutritifs, révèle des bilans métaboliques opposés (autotrophie/hétérotrophie) en fonction des conditions environnementales, ainsi qu'un rôle de tampon sur les flux de nutriments. 3. Des analyses isotopiques (δ13C et δ15N) ont permis d'identifier les sources de matière entrant dans l'écosystème et alimentant les réseaux trophiques associés aux herbiers. L'étude a permis d'établir un lien trophique entre Holothuria leucospilota et les herbiers marins ainsi qu'un rôle essentiel dans le maintien des populations de poisson herbivores récifaux. 4. Des enquêtes locales ont révélé une méconnaissance de cet écosystème malgré leur potentiel économique et écologique. À la lumière de ces travaux de thèse, la prise en considération des herbiers marins dans la gestion du milieu marin réunionnais est nécessaire. / Located at the benthos-pelagos interface, seagrass beds have many ecosystemic roles such as coastal protection or biodiversity. Historically, scientists and decision-makers at Reunion Island have been focusing on reef ecosystems. However, for several decades, monospecific (Syringodium isoetifolium) seagrass beds have been observed (3 hectares – 2013) within Ermitage / La Saline reef (West coast). The main objective of this study was to give the first data on seagrass ecology at Reunion Island. Therefore, a multi-disciplinary study was developed. First, seascape ecology of seagrasses is analyzed at decadal (since 1950) and seasonal scales (2013-2015). Physical settings (swell) and eutrophication were driving factors on the spatial dynamics of seagrass beds. Second, seagrass community metabolism and response to nutrient loads (nitrate and phosphate) show variations in metabolic budget (autotrophy to heterotrophy) and make seagrass beds a nutrient sink following the environmental conditions. Third, stable isotope analysis (δ13C and δ15N) identified sources of particulate matter entering the ecosystem and fueling food webs. The trophic roles of seagrass beds are assessed for echinoderm and herbivorous fish communities. This study highlighted a trophic link between seagrass and Holothuria leucospilota, and proved some species to be heavily dependent on seagrass ecosystem (Leptoscarus vaigiensis, Siganus sutor). Finally, interviews showed a lack of knowledge about this ecosystem despite crucial economic and ecologic roles. Considering their major functions, seagrass beds have to be considered in future management of the coastal marine systems at Reunion Island.

A toute variété algébrique on peut associer différents objets algébrico-géométriques qui rendent compte en particulier des singularités de la variété. Cette thèse traite de l'interaction entre l'étude des singularités, le schéma des arcs et le module des dérivations dans le cadre des courbes algébriques affines planes. Elle démontre que les d-tissus quasi-homogènes incomplets sont linéarisables pour d > 3 en utilisant un théorème d'Alain Hénaut. Enfin, dans un dernier chapitre, cette thèse introduit le formalisme des fonctions zêta motiviques associées à une 1-forme locale. / To any algebraic variety one can associate several algebraic-geometric objets which in particular provide information on the singularities of the variety. This thesis deals with the interaction between the study of singularities, arc spaces and derivations module in the context of affine algebraic plane curves. Using a theorem of Alain Hénaut, we show that quasi-homogeneous incomplete d-webs are linearizable for d > 3. Finally, in the last chapter, this thesis intoduces the formalism of motivic zêta function of a local 1-form.