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This thesis presents a new template for multidisciplinary habitat mapping that combines the analyses of seafloor geomorphology, oceanographic proxies and modelling of associated biologic features. High resolution swath bathymetry of the Faial and western Pico shelves is used to present the first state-of-the-art geomorphologic assessment of submerged island shelves in the Azores. Solid seafloor structures are described in previously unreported detail together with associated volcanic, tectonic and erosion processes. The large sedimentary expanses identified in the area are also investigated and the large bedforms identified are discussed in view of new data on the local hydrodynamic conditions. Coarse-sediment zones of types hitherto unreported for volcanic island shelves are described using swath data and in situ imagery together with sub-bottom profiles and grainsize information. The hydrodynamic and geological processes producing these features are discussed. New oceanographic information extracted from satellite imagery is presented including yearly and seasonal sea surface temperature and chlorophyll-a concentration fields. These are used as proxies to understand the spatio-temporal variability of water temperature and primary productivity in the immediate island vicinity. The patterns observed are discussed, including onshore-offshore gradients and the prevalence of colder/more productive waters in the Faial-Pico passage and shelf areas in general. Furthermore, oceanographic proxies for swell exposure and tidal currents are derived from GIS analyses and shallow-water hydrographic modelling. Finally, environmental variables that potentially regulate the distribution of benthic organisms (seafloor nature, depth, slope, sea surface temperature, chlorophyll-a concentration, swell exposure and maximum tidal currents) are brought together and used to develop innovative statistical models of the distribution of six macroalgae taxa dominant in the infralittoral (articulated Corallinaceae, Codium elisabethae, Dictyota spp., Halopteris filicina, Padina pavonica and Zonaria tournefortii). Predictive distributions of these macroalgae are spatialized around Faial island using ordered logistic regression equations and raster fields of the explanatory variables found to be statistically significant. This new approach represents a potentially highly significant step forward in modelling benthic communities not only in the Azores but also in other oceanic island shelves where the management of benthic species and biotopes is critical to preserve ecosystem health.

Dans cette étude, nous cherchons à dresser le bilan de l’altération à l’échelle de l’île de Mayotte (en surface et en souterrain), à contraindre l’impact des facteurs de contrôle dominant l’altération, puis à caractériser les processus impliqués dans les fractionnements des isotopes du silicium à l’échelle du profil d’altération.Deux campagnes d’échantillonnage incluant des eaux de rivières et des eaux souterraines ont été réalisées en saison humide et sèche, complétées par le prélèvement mensuel de 5 rivières. La prise en compte des crues (3 % de l’année) dans le calcul des taux d’altération moyens annuels en rivière a entraîné une augmentation de ≈32 % du bilan annuel. Le taux d’altération chimique global de l’île de Mayotte s’élève à 94 t/km²/an (81 t/km²/an en surface & 131 t/km²/an en souterrain). Nos résultats mettent en avant le rôle prépondérant des écoulements souterrains dans le transport de matériel dissous directement à l’océan. La contribution du domaine souterrain au bilan de l’altération diminue avec l’âge des formations, soulignant l’implication de l’âge des roches du bassin versant comme paramètre clé dans le contrôle des taux d’altération.Les isotopes du Si ont été analysés sur des roches totales le long de profils d’altération associés à différentes conditions d'altération du régolite : météorique (basse température) ou hydrothermale (haute température). À l’échelle du profil, les deux types de régimes ont montré un appauvrissement en 30Si en fonction du degré d’altération. À l’échelle du minéral, le fractionnement des isotopes du Si s’est révélé plus négatif pendant la précipitation de kaolinite secondaire à haute température qu’à basse température. / In this study, we seek to establish the weathering budget at the scale of the island of Mayotte (rivers and groundwater), to constrain the impact of dominant control factors to the weathering both locally and at a global scale, and then to characterize the processes involved in the fractionation of silicon isotopes across the weathering profile.Two field campaigns, in order to sample river water and groundwater, were carried out during wet and dry season, supplemented by the monthly monitoring of 5 rivers. By taking account of river floods (3% of the year) in the calculation of average annual weathering rates in the river has increased the annual weathering budget by ≈32%. The overall rate of Mayotte Island chemical weathering is 94 t/km²/yr (81 t/km²/yr from surface & 131 t/km²/yr from underground). Our results highlight the important role of groundwater flow to the dissolved material export directly to the ocean. The contribution of groundwater to the weathering budget decreases with the age of the geological formations, highlighting the involvement of the age of the rocks of the watershed as a key parameter in the weathering rates control.Si isotopes were analyzed for whole rock along two weathering profiles associated with different alteration conditions of the regolith: meteoric (low temperature) or hydrothermal (high temperature). At the weathering profile scale, the two types of alteration regimes showed 30Si depletion as a function of the degree of weathering. At the mineral scale, Si isotope fractionation was more negative during the secondary kaolinite precipitation at high temperatures than at low temperatures.