Le lundi 12 janvier 2026, Sofia MULLER présentera l'examen en vue de l’obtention du grade académique de Docteur en Sciences (Collège de doctorat en Océanographie) sous la direction de Bruno DELILLE et François FRIPIAT.
Cette épreuve consistera en la défense publique d’une dissertation intitulée :
« Fixed Nitrogen Cycling and Nitrous Oxide Production in the Arctic Ocean: Shelf transformation processes and implications for Arctic biogeochemistry ».
Le Jury sera composé de :
Mme M. GREGOIRE (Présidente), Mme et MM. S. ARNDT (ULB), B. DELILLE (Promoteur) (Secrétaire), F. FRIPIAT (ULB) (Promoteur), S. JACCARD (Université de Lausanne).
Nitrous oxide is a potent greenhouse gas and the main chemical responsible for ozone depletion in the atmosphere. Its atmospheric concentrations have risen significantly since the industrialization of agriculture, largely due to the widespread use of nitrogen-based fertilizers, making it the third most important greenhouse gas today. It is produced by microbes transforming nitrogen in soils and in the ocean, particularly in low-oxygen conditions such as in seafloor sediments - where oxygen is rapidly consumed - or in oxygen minimum zones which occur naturally through the breakdown of organic material in the water. The ocean contributes more than a quarter of global nitrous oxide emissions in the atmosphere. Yet, the overall contribution of the Arctic Ocean remains uncertain because it is largely unexplored. It remains unclear whether this region emits nitrous oxide to the atmosphere or removes it, and how important its role is in the global budget. The Arctic Ocean is characterized by extensive continental shelves, that cover nearly half of its seafloor, and it is the most river-influenced ocean system on Earth. Together, shallow shelf environments and river inputs may promote conditions that enhance nitrous oxide production, and could potentially represent a significant source at the global scale. Within the Arctic Ocean, the Kara Sea stands out as it receives over a third of all river discharge entering the Arctic, making it a strategic yet understudied region to investigate river–ocean interactions. This thesis examines the dynamics of nitrous oxide across the Kara Sea shelf environment, focusing on how it is formed, how it accumulates in the water column, and how river-derived nitrogen contributes to the system. Using observations from the recent international MOSAiC and Arctic Century boat expeditions, this work provides new insight into one of the least explored marine regions and clarifies the influence of river discharge on nitrogen inputs and nitrous oxide dynamics across Arctic shelves.
La thèse est intitulée : « Hydrogeomorphological approaches to understanding flood hazards and flood-risk management in northwest Rwanda ».
La thèse est intitulée : « Inferring mechanisms of community assembly from phylogenetic diversity ».
La thèse est intitulée : « Biodiversity under rapid environmental change: trophic interactions and communities’ responses in austral benthic ecosystems ».
