The Freshwater Floc: A Functional Relationship of Water and Organic and Inorganic Floc Constituents Affecting Suspended Sediment Properties

Flocculated fine-grained sediment is a complex matrix of microbial communities and organic (detritus, cellular debris and extracellular polymers) and inorganic material. Suspended flocs within any aquatic system play a significant ecological role as they can regulate the overall water quality through their physical, chemical and/or biological activity. This paper investigates the complex structural matrix of riverine flocs over a large range of magnifications using correlative microscopic techniques. The significance of floc structural characteristics [(size, shape, porosity, density, inorganic composition, organic composition (bacteria and fibrils)] on the physical (eg. transport and settling), chemical (eg. adsorbing/transforming contaminants and nutrients), and biological (eg. biotransformation and habitat development) behaviour of a floc is investigated. Results suggest that it is the floc's internal structure that has a significant impact on controlling the above floc behaviours. This internal structure is complex and is often dominated by the existence of a three-dimensional matrix of fibrillar material secreted by the active microbial community within the floc. This matrix, in conjunction with the inorganic and bioorganic (active and inactive) constituents of a floc, provides an intricate pore structure that may result in water being an important bound component of a floc. These complex interactive structural and functional properties of a floc are considered to influence a floc's behaviour both physically in how it is transported or settled, chemically in how it adsorbs/transforms contaminants and nutrients, and biologically in how it develops a diverse microhabitat capable of modifying the structural, chemical and biological makeup of the floc.