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Institut für Wasser- und Umweltsystemmodellierung - IWS




There is now a consensus that microorganism mediate the erosive response of fine sediments by virtually gluing sediment particles together and cementing the sediment surface. Since long, this fascinating feature “biostabilization” has been recognized in the marine and recently in freshwater habitats, but research in ecohydraulics considers mainly the macroscale; this is despite the fact that microorganisms are the pioneers in settlement with effects on subsequent growth of higher organisms. Moreover, these microbial biofilm function as sediment traps and influence post-entrainment behavior of resuspended sediments with huge impacts on the ETDC cycle of fine sediments. In this presentation, we will present results of the ongoing DFG (Deutsche Forschungsgemeinschaft) project “Ecosystem Engineering: Sediment entrainment and flocculation mediated by microbial produced extracellular polymeric substances (EPS)”. Briefly, the experimental setup will be explained, consisting of six flow channels running under controlled conditions of temperatures, light, nutrients and flow velocity. Biofilm adhesion (as a measure of sediment stability) is determined by MagPI (Magnetic Particle Induction) and critical shear stress by our laboratory flume (SETEG channel). Biofilm growth (by bacterial cell numbers and microalgal biomass) and biofilm glue (determination of proteins and carbohydrates as the main components of the microbial matrix) as well as biofilm community (species composition and diversity for bacteria and microalgae) are monitored. All these parameters are set in relation to the characteristics and features of the eroded flocs to unravel the influence of microbial biostabilization of fine sediments for post entrainment sediment transport and deposition. Moreover, the presentation will reveal particularly interesting aspects of biostabilization and floc characteristics under different regimes of light and flow to show their mutual interaction. In summary, our experiments prove that the critical shear stress of biostabilized sediments exceeds by far the critical shear stress of abiotic sediments and secondly, that flocs of eroded sediments are significantly influenced by their “bed history” (respectively their biological history). Altogether this demonstrates that biostabilization is an important parameter and that the microscale cannot any longer be neglected in morphological modeling. KEYWORDS: biostabilization, adhesion, sediment stability, flocculation, microbial ecology, diatoms, bacteria