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Response of an invertebrate community to patch-specific bed disturbance Oldmeadow, David Fenn

Abstract

Geometric bedform structures promote physical stability in streams. They occur in gravel beds in varying degrees of development, and hence may control the spatial distribution of bed disturbance, and the corresponding distribution of invertebrates. Partial stone cell structures and line clusters formed the principal structural elements in East Creek. Bed stabilisation as parameterised by Shields number was at least one and a half times greater where structures existed. They reduced event averaged transport rates by half and resulted in a patchy distribution of bed disturbance. Limited evidence suggests there could be a scaling relation between partial stone cell diameter and area of patch disturbance. Structure development in East Creek took place in an artificially loosened bed. Development was maximised during two moderate floods. These floods were of sufficient magnitude to mobilise only loose sediment. Evidence from this study suggests structural development in a natural system is dependent upon the history of recent flows and external sediment supply. The response of the macroinvertebrate community to bed disturbance was varied. Disturbance mechanisms are taxon specific, and may be related to scour, abrasion from fine bedload and movement of surficial particles. Paradoxically, stabilised areas of the stream bed, such as stone structures, whilst remaining stable during a flood, may not uniformly act as refugia. Population maintenance on stabilised structures during a flood may depend on shear forces and turbulent velocities, and amount of fine bedload.

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