The effects of disturbance on the microbial mediation of sediment stability

Abstract

AbstractIn coastal areas, biofilms are often subject to disturbance by hydrodynamic forcing, bioturbation and human activities. These factors affect the influences biofilms have on the sediment. To reveal these effects, we studied laboratory‐incubated and field‐collected biotic sediments reworked by disturbances, and examined their stabilities and three‐dimensional microstructures using laboratory annular flume tests and a wet‐staining X‐ray microcomputed tomography method. We find that, when subject to disturbance, biofilms do not always establish mat‐like matrices that firmly armor the seabed and bio‐stabilize sediments, but instead, have a range of effects on sediment stability, including both bio‐stabilization and destabilization. Disturbance considerably alters microbial influences on sediment stability, but is not the only control. Given equal disturbance, whether or not sediments are bio‐stabilized largely depends on the state of bio‐sediment formation. At a relatively well‐developed state, an organic‐rich, adhesive polymer network tightly interconnects large amounts of sediment particles into aggregates, forms complex internal structures, and enhances sediment stability. By contrast, some bio‐sediment formations only ever reach a less well‐developed state, where scattered organic patches bind relatively few particles into aggregates and reduce sediment stability. Microbial growth likely has two opposing effects on sediment stability, by enhancing either weight/friction or lift/drag on aggregated particles. The former has the positive effect of enhancing sediment stability, whereas the latter can result in greater flow resistance and so have the opposite effect. A conceptual framework is put forward to characterize the different states of bio‐sediment formation and their distinct effects on sediment stability.