Interactions between flow hydrodynamics and biofilm attributes and functioning in streams

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Biofilms constitute an integral part of aquatic ecosystems and are controlled by many factors such as light, grazing, resource availability, water chemistry and flow conditions. Natural mountainous streams exhibit a heterogeneous river bed ranging from boulders to small sediment grains like sand and thus provoking high spatio-temporal variability of the flow field. Here, flow hydrodynamics become a dominant factor shaping biofilm morphology such as biomass and architecture and controlling biofilm community composition through drag forces on the one side and supply of resources through mass transfer processes on the other. Current knowledge of the interactions between flow hydrodynamics and biofilm attributes is based on a few idealized case studies and is lacking a link to the consequences for stream ecosystem functioning like, e.g., nitrogen uptake. Previous studies have been restricted to flume experiments where the highly complex flow field of natural streams can not be reconstructed to the full extent. In a novel approach we are aiming at linking detailed investigations on river bed heterogeneity and associated development of flow fields to biofilm characteristics and the consequences for nitrogen uptake. In order to avoid artefacts of mesocosm studies we will conduct experiments and measurements in both, field and mesocosm environments, thereby providing the advantages of real stream scale results and detailed flume studies on basic processes of mass transfer. Furthermore, we will integrate not only results from the river patch scale but also the role of biofilms on the river reach scale. This highly interdisciplinary research will provide mechanistic understanding of nitrogen uptake of biofilms under varying flow conditions and will thereby extend current knowledge on the regulation of whole stream ecosystems processes in mountainous streams.