How biofilms influence morphology

Abstract

Extracellular Polymeric Substance is found to be responsible for the stabilizing behaviour of biofilms on sediment, but studies disagree on how this stabilizing effect is achieved and what processes are involved in making an EPS biofilm shear off. This study answers the question how the biofilms achieve the stabilizing effect and what processes make a biofilm shear off. For that, it combines the research to the effects of transport on boundary layer turbulence with the study to the stabilizing effects of biofilms under flow. Artificial biofilms were used in order to obtain repeatability in the experiments. The choice for the artificial biofilms was based on previous research done to the properties of biofilms and resulted in the choice for the materials sodium and calcium alginate. To study the effects of transport and biofilms on the turbulence, flume experiments have been performed using a LDA. For studying the effects of sediment transport on turbulence, coarse sediment with a D50 of 2mm and fine sediment with a D50 of 0.5mm have been used. The biofilm experiments have only been performed in combination with the fine sediment. Results showed that the influence of bed load transport on the turbulence intensities depends on the ratio between the particle size and the length scale of the turbulence. Biofilms smoothened the bed decreasing the turbulence intensities and the bed shear stress. It was found that in order to make the biofilm shear off, sediment particles under the biofilm have to be set in motion. Failure was found to occur when sand ripples eroded the sediment in front of the biofilm, allowing the flow to come under the biofilm.