Christine Anlanger

• Biofilms constitute an integral part of freshwater ecosystems and are central to regulating essential stream biogeochemical functions, such as nutrient uptake and metabolism. Under-standing the environmental factors that dictate the composition of biofilm communities and their role in whole-system nutrient cycling remains challenging, given the large spatial and temporal variability of biofilm communities. Pristine mountain streams exhibit a heteroge-neous streambed ranging from boulders to sand, provoking high spatiotemporal flow varia-bility. Our current knowledge of the interactions between flow hydrodynamics and biofilm attributes stems from mesocosm studies, which are inherently limited in environmental real-ism. Moreover, the mechanism linking flow hydrodynamics to microbial biodiversity and ecosystem functioning is currently not studied. My thesis aims to link streambed heteroge-neity and the associated development of the flow field to biofilm attributes and nitrogen uptake based on a multidisciplinary field approach. It integrates several spatial and temporal scales ranging from millimeter-sized spots to stream reaches and from milliseconds to minutes (i.e., the hydraulic scale of velocity fluctuations), up to days, months and years (i.e., the hydrological scale of flow fluctuations). I demonstrate that the spatial niche variability of flow hydrodynamics was an essential driver of biofilm community composition, diversity and morphology, in line with the habitat heterogeneity hypothesis initially formulated for terrestrial ecosystems. Furthermore, hydraulic mass transfer associated to flow diversity and biofilm biomass determined biofilm areal nitrogen uptake at scales ranging from spots to the stream reach. At the whole-ecosystem level, flow diversity determined the quantitative role of biofilms compared to other nitrogen uptake compartments by sorting them according to prevailing flow conditions. The magnitude of effects depended on ambient nutrient back-ground and season, suggesting a hierarchy of the environmental controls on biofilms. In summary, my interdisciplinary research provided a mechanistic understanding of how hy-dromorphological diversity determines the diversity, morphology, and the functional role of biofilms in streams. By improving the understanding of these relationships, my research improves our ability to predict and scale measurements of important stream biogeochemical functions. Moreover, it helps to face the challenges imposed by environmental changes and biodiversity loss.