Author(s): Moritz Thom; Holger Schmidt; Silke Wieprecht; Sabine U. Gerbersdorf
Linked Author(s): Silke Wieprecht
Keywords: Biofilm; Extracellular polymeric substances (EPS); Biostabilization; Morphology; Critical bed shear stress
Abstract: The stabilizing effects of natural biofilm within aquatic sediments have been increasingly recognized in the last decades. In riverine systems these effects do not only influence sediment dynamics but also the remobilization of pollutants and are thus an important feature for ecology, human health and economy. Yet the interactions between biological growth and environmental abiotic parameters as well as morphology are rarely addressed in a comprehensive manner. In the project “Ecosystem Engineering: Sediment entrainment and flocculation mediated by microbial produced extracellular polymeric substances (EPS)”, supported by the “Deutsche Forschungsgemeinschaft” (DFG), the stabilization potential of natural biofilm on riverine sediments will be determined under different environmental conditions. Biofilm are heterogeneous biological structures with a viscoelastic behavior. It influences the stability of sediment in two ways. Firstly, by cementing the grains of the underlying sediment and modifying its resistance to effective shear stress. Secondly, by impacting the surface roughness resulting in local variations of the flow field and bed shear stress. Since the growth of biofilm depends on different environmental conditions (nutrients, hydraulic situation, light etc. ), this results in complex interactions and mutual dependencies. The current project aims at finding a mathematical formulation that is able to express these complex conditions which finally can be implemented and applied in a numerical morphological model. In laboratory tests, biofilm are grown on glass beads in order to evaluate its mechanical stability in specially developed flumes under defined environmental conditions. The critical shear stress is determined with the help of an erosion flume (SETEG) at the Institute for Modeling Hydraulic and Environmental Systems (University of Stuttgart). For the direct measurement of adhesive forces at the biofilm surface, a sensitive measurement device based on the attraction of magnetic particles is constructed and validated. Different biological parameters (describing biomass, cell numbers, diversity and secretion of microbial polymers) are investigated as potential indicators for biofilm stability. Furthermore, the physical process of erosion differs from the classical erosion theory due to biological growth and is described accordingly. It is shown that a) biofilm may increase bed stability depending on the environmental conditions and b) the critical shear stress of biostabilized sediment can be determined with the help of a mathematical formulation. Our experiments prove that the critical shear stress of biostabilized sediments exceeds by far the stability of abiotic sediments which demonstrates that biostabilization is an important parameter and has to be considered in morphological modeling. Moreover, the remobilization of pollutants immobilized within the biofilm matrix is closely related to biostabilization and is one focus of future research.