LONGITUDINAL CHANGES IN NEAR-BED FLOWS AND MACROINVERTEBRATE COMMUNITIES IN A WESTERN-AUSTRALIAN STREAM

The responses of macroinvertebrate communities to longitudinal changes in flow and other environmental variables were determined by sampling twelve sites along Carey Brook, in the southwest of Western Australia, at 3-mo intervals over 12 mo. Higher values of mean velocity (UBAR), roughness shear velocity (U*r), Reynolds number (Re), and roughness Reynolds number (Re(r)) and lower values of coarse and fine particulate organic matter were recorded at lowland sites than at upland sites. Shear velocities (U*) and substrate roughness (k) were highest at intermediate sites. Species richness was positively correlated with U*, Re(r), and k. Macroinvertebrate abundance was positively correlated with k and negatively correlated with water surface slope (s). Ordination and classification revealed a longitudinal gradient in community structure; but, unlike the sharp zones of transition that have been postulated by others, the change in the composition of the macroinvertebrate fauna from upland to lowland sites on Carey Brook appeared to be a gradual one. The responses of macroinvertebrates with similar morphological or behavioural adaptations to flow were examined by classifying the fauna into flow exposure groups: obligate, facultative, and avoiders. Abundances of obligates were positively correlated with the majority of flow descriptors; they occurred in highest numbers in turbulent habitats with high shear velocities. Abundances of facultatives were positively correlated with substrate roughness, possibly because rougher substrates provided better refuge from flow. Abundances of flow avoiders were positively correlated with high shear velocities, substrate roughnesses, and turbulence; these conditions may promote higher fluxes of organic matter and oxygen within interstitial sediments. Flow-exposure-group classification provides insight into the response of macroinvertebrates to local flow conditions and may prove to be a useful adjunct to functional-feeding-group analysis in benthic stream ecology.