Biostabilization of cohesive sediment beds in a freshwater wave-dominated environment

An assessment of bed sediment stability was carried out over biostabilized and nonbiostabilized cohesive sediment within a laboratory wave flume. Biofilms of 5, 9, and 15 d were grown over kaolinite, and changes in bed shear stress were applied through increasing wave height. Results show that the stability of the bed was highly related to the structural integrity of the overlying and integrated biofilm. Biofilms with substantial algal growth and lifting, due to decay and gas production, were weaker than those in contact with the bed. Generally, biostabilized beds of the same age failed at similar shear stresses (although the mechanism of failure varied), and the 9-d biofilms exhibited the greatest strength. Changes in the microbial diversity and dominant species with time, as measured with phospholipids fatty acid analysis (PLFA) and denaturing gradient gel electrophoresis (DGGE), are believed to have influenced the observed variations in bed strength. Young biofilms were dominated by bacterial species, whereas older biofilms were dominated more by algae (particularly cyanobacteria). Fungal species also played a large role in the biofilm development and structure. Laser confocal microscopy and environmental scanning electron microscopy were used to show structural differences between biofilms and changes with wave energy. Biofilm development and bio-armoring of underlying cohesive sediments are successional processes, with new layers of biofilm integrating into the sediment beneath older decaying biofilm layers. As such, there is a continuous temporal oscillation in bed sediment stability depending on the stage of biofilm development and decay.