Rethinking what constitutes suspended sediment

Although cohesive suspended sediment is now known to be transported primari ly as flocculated material, there is still a misconception of what constitu tes suspended sediment. Flocs represent a complex matrix of microbial commu nities, organic particles (e.g. detritus, extracellular polymers and cellul ar debris), inorganic particles (e.g. clays and silts) and substantial inte rfloc spaces (pores), which allow for the retention or flow through of wate r. Flocculation results in significant alteration of the hydrodynamics of t he constituent particles (by modifying their effective size, shape, density and porosity). thereby affecting the transport of sediment and associated contaminants. The composition and structure of a flee is in a continuous st ate of change as the medium in which it is transported provides the Roc wit h further building materials, energy, nutrients and chemicals for biologica l growth, chemical reactions and morphological development. As such, a floc 's physical (e.g. transport), chemical (e.g, contaminant adsorption) and bi ological (community development and contaminant biotransformation) behaviou r are also in a continuous state of change, with concomitant effects on the ir aquatic environment as a whole. Although it is recognized that floc form will influence hoc behaviour, there is still a basic lack of knowledge of the complex links between the structural components of a hoc and how their individual properties and behaviours in combination with others will influe nce a Roc's physical, chemical and biological behaviour. This paper provide s a comprehensive conceptual model that links the many interrelated structu ral components of typical flocs and their interrelated behavioural aspects, in order to enhance our understanding of what constitutes suspended sedime nt. Copyright (C) 2001 John Wiley & Sons, Ltd.