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FIGURE 2.1 Schematic classification of what environmental science generally considers as dissolved, colloidal, and particulate materials as defined by size and organic and inorganic components. All of the components to the left of the flocculation wedge can be incorporated into flocculated or aggregated particles with a subsequent increase in effective size. No upper size range for floc size can be determined as it is dependent on a number of physical, chemical, and biological factors, although marine snow has been observed in the order of centimeters. (Reproduced with permission from Droppo (2000).)

(Figure 2.1). A proportion of these particles will be of an organic (living and nonliving) and inorganic nature. All of the components to the left of the flocculation wedge in Figure 2.1 can be incorporated into flocculated or aggregated particles with a subsequent increase in effective size. While the dissolved ionic component of Figure 2.1 may not be considered true particles, they can still influence flocculation through precipitation on and complexation with other components of the floc. Note, however, that there is no static upper size range for floc size as it is dependent on a number of physical, chemical, and biological factors.3

A freshwater floc is defined here as a suspended particulate (in the micrometer to multi-millimeter range) which is (a) derived by freshwater aggregation processes and (b) typically rich in subcomponents whose least dimensions can span the entire colloidal size range and above. Subcomponents (Figure 2.2) include bacteria and other small organisms, extracellular polymeric substances (EPS), aggregated humic

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FIGURE 2.2 The shape and dimensions of some common aquatic colloids: (a) submicrometer eukaryote cell, an alga; (b) prokaryote cell, a Gram-negative bacterium; (c) microfibrillar cell wall fragment from higher plant or alga; (d) frustule fragment from the mineral cell wall of a diatom alga; (e) a clay mineral; (f) amorphous organic debris; (g) mucilaginous aggregate of fibrils; (h) discarded scale from the surface of an alga; (i) refractory wall fragment from Gram-negative bacterium; (j) amorphous iron oxyhydroxyphosphate; (k) individual fibril with associated small colloids; (l) fractal aggregate of humic substance; (m) marine virus; (n) fulvic acid aggregate; and (o) extracellular enzyme. (Reproduced with permission from Leppard and Buffle (1998).)

substances, clay minerals, colloidal iron and manganese oxyhydroxides, biogenic silicates, bacterial envelope fragments, algal cell wall fragments, algal scales, viruses, identifiable cell lysis products, and both mineral and organic nanoscale coatings.28,29 The EPS is frequently packaged by microbiota into nanoscale fibrils,15,30 which cross-connect the various floc subcomponents, and which can be oriented in three dimensions by bacterial secretion processes to establish intra-floc pores, and also densely packed microzones which may represent a structural basis for diffusional gradients.3,16,17,21

A paradoxical description of a floc, which focuses on the structural and behavioral characteristics, was provided by Droppo et al.17 It was paradoxical relative to a much earlier concept of the floc as a "black box." From recent multidisciplinary work, a floc can now be defined as "an individual microecosystem, composed of a matrix of water, inorganic and organic colloidal particles with autonomous and interactive physical, chemical, and biological functions or behaviors operating within the floc matrix."17 The rationale for this definition and the relationships among architecture, biology, chemistry, behavior, and environmental activities are outlined in Droppo3 and elaborated in the following sections.

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