Microbiological mediation of sediment structure and behaviour

Almost all natural sediment deposits support the growth of microbial assemblages. On occasions these assemblages become sufficiently developed to be recognisable as microbial mats or biofilms but even before the development of such extensive structure, microbial colonisation can influence the microstructure of the bed and the critical erosion threshold for the resuspension of particles. The entrainment of particles is a critical event in transport of sediments and as yet there is still insufficient knowledge to accurately predict the response of natural sediments to hydrodynamic stress. Early attempts to predict sediment behaviour were for the most part based on the relationship between hydrodynamic stress and particle size (see Miller et al. 1977). This type of relationships, such as the Shield’s curve, have proved to be reasonably reliable for abiotic well-sorted sandy sediments. However, the erosional behaviour of sediment mixtures, small cohesive sediments and, to a large extent, natural sediments in situ defies adequate description. Many workers now recognise that predictions based purely on physical parameters are insufficient to explain even the erosional behaviour of non-cohesive sediments in situ (Paterson and Daborn 1991; Daborn et al. 1993). It has long been suggested that the biological “status” of the sediment may influence transport processes (Carter 1933), but there are still relatively few studies that deal directly with this problem. There is increasing evidence in the literature that as Montague (1986) stated: “The physical and chemical properties of sediments that contain a significant biotic community are radically different from those of abiotic sediments ”.