Most coastal ecosystems are dominated by marine macrophytes that deliver a range of ecologically and economically important services such as carbon and nitrogen cycling and storage, and habitat provision to a range of associated species. The relative contribution of these services among different vegetated habitats, however, and their alteration due to anthropogenic stressors is little known. In this thesis, I first examined the within and between ecosystem structure and services of eelgrass (Zostera marina) and rockweed (Ascophyllum nodosum) beds in Atlantic Canada. Both habitats significantly enhanced the overall abundance and diversity of associated species, whereas differences in the spe-cies assemblages were attributed to differences in canopy structure within and between habitats. Changes in the canopy structure of the foundation species will affect associated food webs and ecosystem services. Next, I used large-scale field surveys to examine the effects of eutrophication on the structure and services of eelgrass beds. As eutrophication increased, plant dominance shifted from eelgrass to macroalgae and phytoplankton at a regional scale. The faunal community showed increases in filter feeders, detritivores and some herbivores, while sensitive species declined. These faunal changes can be linked to enhanced food availability and predation refuge offered by increased phytoplankton and opportunistic macroalgae. However, the loss of eelgrass and sensitive species highlight the negative consequences of eutrophication on eelgrass ecosystems and the services they provide. I also reviewed the global carbon and nitrogen storage and habitat services of mangroves, salt marshes, seagrass meadows and macroalgal beds. Despite only occupying 0.7% of the ocean area, together these ecosystems make up 12% of the oceanic carbon stock thereby playing an important role in global carbon and nitrogen storage. Moreover, these macrophyte habitats enhanced species richness and abundance of associated fauna and juvenile fishes. Overall, my findings indicate that each macrophyte habitat has its strengths yet all are essential in providing the full range of ecosystem services. Increasing human impacts along the coasts, however, are threatening macrophyte ecosystems worldwide, and their further decline will impair the provision of important services and human well-being. Lastly, I discuss the implications of my work for management and conservation.

La régénération forestière, naturelle ou non, est sujette à de nombreux échecs du fait d’une compétition importante entre les espèces de sous-bois et les jeunes plants d’arbre. L’objectif de ce travail de thèse a été de comprendre les réponses du chêne sessile (Quercus petraea) et d’une poacée, tous deux en compétition dans un contexte de modification de la disponibilité des ressources. Deux espèces de poacées ont été étudiées, la molinie bleue (Molinia caerulea) et la canche cespiteuse (Deschampsia cespitosa). L’accent a été mis sur le rôle des interactions entre différentes ressources (lumière, eau et azote inorganique) sur l’évolution de la compétition (intensité/importance) entre ces espèces et les mécanismes écophysiologiques sous-jacents. Grâce à des expérimentations en conditions semi-contrôlées et à une installation in situ, nous avons pu mettre en évidence une compétition très précoce, dès les premiers mois d’interaction entre le chêne et les poacées voisines. L’abondance de lumière conduit à une compétition plus forte des poacées sur le chêne, amplifiée par l’apport d’azote. Par ailleurs, la nature et l’ampleur de ces interactions varient selon le niveau de stress abiotique appliqué et selon les organes considérés. Nos résultats montrent aussi que le chêne réagit principalement en accumulant les ressources dans son système racinaire, ressources qui sont mobilisées l’année suivante et qui peuvent avoir un arrière effet positif sur le fonctionnement du chêne en absence de stress hydrique. Enfin de la facilitation a également été observée du chêne envers la canche cespiteuse suite à un apport d’azote. Ce dernier améliore la croissance du chêne en compétition, qui pourrait augmenter sa production d’exsudats et son turn-over racinaire au bénéfice de la canche qui valoriserait cette nouvelle source d’azote. Ce travail renforce l’idée d’intégrer l’effet des interactions des différents facteurs abiotiques dans les modèles de compétitions et dans les pratiques de régénération afin d’optimiser la coexistence des espèces forestières, et ce dès la mise en concurrence des jeunes arbres avec les poacées du sous-bois. / Temperate forest ecosystems are prone to regeneration failures because of strong competition between understorey species and tree seedlings. This thesis aimed to improve our understanding of sessile oak seedlings (Quercus petraea) and poacea responses, both in competition among different levels of resource availabilities. Two poacea species were studied, Molinia caerulea and Deschampisa cespitosa. We focused on the role of the different resource combinations (light, water and inorganic nitrogen) on interaction variation (intensity and importance) between those species and underlying ecophysiological mechanisms. Our greenhouse and field experiments allowed us to infer a very early competition, from the first months of interaction between oak seedlings and poacea neighbours. High light level leads to stronger competition from poacea on oak seedlings, increased by nitrogen supply. Secondly nature and extent of these interactions depend on applied abiotic stress level and on considered organs. Thirdly Oak seedlings mainly respond by accumulating resources in coarse roots. These resources are remobilized next year and may have a positive carry-over effect on oak functioning if there is no water stress. Eventually we observed facilitation as well, from oak on D. cespitosa in fertilized environment. Nitrogen supply would foster oak seedling growth, increasing exudate production and root turn-over, to the benefit of D . cespitosa. The poacea would take up this extra nitrogen source for its own development. This study support the idea of including interaction effects of different abiotic factors in competition models. Silvicultural practices would also be enhanced by optimizing species coexistence in temperate forests as soon as tree seedlings and understorey species start to grow together.