Effects of drying on biofilm-mediated sediment stability in intertidal mudflats: transition from biostabilization to biodestabilization

Abstract

The presence and growth of biofilms can considerably enhance sediment stability, a phenomenon known as biostabilization. Intertidal mudflats are periodically exposed to air due to tidal cycles. During exposure, they are affected by drying, which may alter the properties of biofilms and then their sediment-stabilizing ability. However, to date, little information is available on this topic. For this, the effects of a biofilm on sediment stability were investigated over a 15-day drying period. The biofilm was obtained by culturing microorganisms isolated from intertidal sediment. Previous studies demonstrate that biofilms enhances sediment stability in two ways. On one hand, biofilms coating on the surface of sediment bed can protect surface sediment from erosion (surface protection), evidenced by an increase in the critical shear stress of sediment. On the other hand, after the detachment of surface biofilms, biofilms growing between sediment particles can hinder the erosion of bottom sediment (hindered erosion), indicated by less amount of sediment eroded following a same erosion event. Results of this study showed that during the drying process, surface protection was initially enhanced but later weakened, while hindered erosion was consistently weakened. By the end of experimental drying trials, the biofilm no longer stabilized sediment; instead, it reduced the stability of surface sediment and promoted the erosion of bottom sediment, leading to biodestabilization. After drying, the structure of the biofilm covering sediment bed became more compact, similar to that of soil crust, whereas the crust-like biofilm was gradually broken towards the end of experimental drying trials. This alteration explains the observed variations in surface protection. The weakened hindered erosion is associated with the loss of adhesion and aggregation of sediment particles as drying progressed. Moreover, many gas bubbles were generated and trapped within dried biofilm and between sediment particles, which possibly increase the buoyancy of biofilm and weaken the inter-particle bonding of sediment particles, thus leading to biodestabilization.