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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A nondestructive stabilization technique for the characterization of microbial flocs which permits the application of correlative microscopic techniques is described. Flocs embedded in agarose are retained in a porous, resilient medium which allows for the transport, staining, washing, and subsampling of the flocculated material directly within a plankton chamber with minimal or no destructive forces. A single agarose disc can be subdivided into numerous sections for analysis by several microscope types and associated techniques.

Organic colloids from the Adriatic Sea, covering the full colloid size range, were characterized on a per-colloid basis by transmission electron microscopy applied to ultrathin sections of embedded colloids. To detect, assess, and minimize artifacts, a multimethod correlative approach to analysis was used. A colloid classification scheme was developed from the characterizations, with individual colloid species defined in the broad sense established by environmental analytical aquatic chemists. The basic scheme is related to derivative classification schemes in which abundant colloid species are listed according to activities known from the literature, with a focus on activities related to carbon transformations and transport. Morphological criteria show that abundant colloid species occupy broad size ranges, such that colloid fractionation protocols based on sizing alone are likely to be of questionable scientific utility. Image analysis revealed that small colloids, designated as humic substances, represented 91% of the total organic colloid burden of the bulk water during a summer period of high biological productivity. At this time, fibrils were rare in the bulk water while representing the most abundant colloid in marine snow.