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The study of flocs has largely been devoted to the gross (>1 µm) scale so that the behavior of flocs (i.e., transport and settling) can be observed and modeled. With the assistance of a newly developed field kit and correlative microscopy [which includes transmission electron microscopy (TEM), scanning confocal laser microscopy (SCLM), and conventional optical microscopy (COM)], this paper begins to bridge the resolution gap between the gross and fine (submicron) scales in order to better understand the role of floc ultrastructure in outward floc behavior for both natural and engineered systems. Results from both systems have demonstrated that pores which appeared to be devoid of physical structures under the optical microscopic techniques (SCLM and COM) were observed to be composed of complex matrices of polymeric fibrils (4-6 nm diameter) when viewed by high-resolution TEM. These fibrils were found to represent the dominant physical bridging mechanism between organic and inorganic components of the flocs and contributed to the extensive surface area per unit volume of the flocs. In this way, the microbial floc resembles a biofilm and will likely support similar processes with respect to contaminants and the physical-chemical environment.

Abstract. Flocculated fine-grained sediment is a complex matrix of microbial cmnmunities and organic (detritus, cellular debris and extracellular polymers) and inorganic material. Suspended floes within any aquatic system play a significant ecological role as they can regulate the overall water quality through their physical, chemical and/or bioloocal activity. This paper investigates the complex structural matrix of nverine floes 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. adsorbin~transfolqning contaminants and nutrients), and biological (eg. biotransfornmtion and habitat development) behaviour of a floc is investigated. Results suggest that it is the floc's internal structure that bm~; 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 fibnllar material secreted by the active microbial conmmnity 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 colnplex interactive structural and functional properties of a floc are considered to influence a floc's behaviour both physically in how it is transported or senled, chemically in how it adsorbs/transforms contaminants and nutrients, and biologically in how it develops a diverse microhabitat capable of modifying the structural, chelnical and biological makeup of the floe.

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