Biofilm structure and bed stability of five contrasting freshwater sediments

Using an annular flume, erosion characteristics of five diverse sediment types (stormwater pond, contaminated lacustrine, fluvial, aquaculture waste and industrial grade kaolin sediment), each with different physical and biological characteristics, were examined for relative erosion resistance and factors contributing to bed sediment stability. Suspended flocs represent primary building blocks of bed sediment with mass settling being independent of suspended solid concentration. Biofilm growth period, depositing floc structure and composition, nutrient supply and sediment properties all played a complex role in dictating the stability of the sediments. The river, lake and stormwater pond sediments were the most resistant to erosion relative to the high nutrient and organic content aquaculture sediment and kaolin. Biofilms developed to varying degrees on all sediments with the greatest growth occurring with the aquaculture sediment and the least with kaolin. While electrochemical properties will provide some attraction and stabilising forces, with no measurable consolidation evident for the examined sediments, it is suggested that active biofilm development was the dominant factor controlling bed stability and erosion potential. Differences in biological mediation of strength between sediments were partially attributed to the structural differences within the biofilms and integration of the extracellular polymeric fibril matrix within the sediment pores.