REGULATED RIVERS: RESEARCH zyxwvuts & MANAGEMENT, VOL. 12, 99-122 (1996) zyxw MACROINVERTEBRATE ASSEMBLAGES OF LITTORAL HABITATS IN THE MACQUARIE AND MERSEY RIVERS, TASMANIA: IMPLICATIONS FOR THE MANAGEMENT OF REGULATED RIVERS PAUL HUMPHRIES*, PETER E. DAVIESt AND MARY E. MULCAHY zyx Inland Fisheries Commission, 127 Davey Street, Hobart, Tasmania, 7000, Australia ABSTRACT Littoral habitats in large rivers are influenced to varying degrees by changes in discharge. Irrigation abstractions can increase the amount of habitat that would naturally be dewatered during low flow periods and therefore it is important to have some knowledge of the potential impact this may have on riverine macroinvertebrates. The macroinvertebrate assemblages of common littoral habitats in riffles, pools and runs in two reaches each of the Macquarie and Mersey Rivers, northern Tasmania, Australia were compared from samples collected during the low flow and irrigation season, between December 1991 and April 1992. The area under water of these habitats, riffle substrata, macrophyte beds and coarse woody debris, responded differently to changes in discharge. Within a reach, the same taxonomic groups often dominated the total number of macroinvertebrates for all habitats, but there were differences in the proportions contributed by these taxa to the different habitats. In general, taxa characteristic of slow-flowing or lentic habitats, such as ostracods and amphipods, were dominant in macrophyte beds in pools and runs, whereas taxa such as larval elmid beetles and hydropsychid caddisflies were dominant in riffles. A substantial component of the fauna from each habitat within a reach was unique to that habitat, but there was always a similar number of taxa common to all habitats. Classification and ordination grouped samples from both rivers firstly by habitat and secondly by month and reach. Total density and family richness of invertebrates differed by reach, habitat and month in both rivers, except for richness in the Mersey River where habitat was not significant. Differences in densities and numbers of invertebrate families among habitats were not consistent between reaches for each river. This study has highlighted the differences in macroinvertebrate assemblages of several littoral habitats in two lowland rivers in Tasmania. Differences in taxonomic composition, density and richness among habitats within reaches strongly imply the uniqueness of these habitats in terms of the invertebrate faunas that occupy them. We suggest that if maintenance of biotic diversity is an aim of instream flow management, water allocations that address low flows should place a high priority on the maintenance of a diversity of habitats. zyxwvutsr KEY WORDS: instream habitat; macrophytes; riffles; coarse woody debris; invertebrates; water levels INTRODUCTION Extended periods of low flow and even the cessation of flow are natural and frequent events in the hydrology of many Australian rivers and the biota of these systems has evolved in the context of such an environment (see Bayly and Williams, 1973; Boulton and Suter, 1986). However, it is during the dry season, when flows are at their lowest, that water is also typically in greatest demand for irrigation. Water abstractions during the natural low flow period can exacerbate problems of high temperature, low oxygen and decreasing habitat already being experienced by stream fauna. Much of the work investigating the effects of low flows or rapid changes in flow on stream invertebrates has focused on the fauna associated with riffles, due to ease of sampling and because small changes in discharge may have a major effect on habitat availability (Minshall and Winger, 1968; Brooker and *Present address: Cooperative Research Centre for Freshwater Ecology, Department of Ecology and Evolutionary Biology, Monash University, Murray-Darling Freshwater Research Centre, PO Box 921, Albury, NSW, 2640, Australia t Present address: Department of Zoology, Universtiy of Tasmania, GPO Box 252C, Hobart, Tasmania, 7001, Australia CCC 0886-9375/96/010099-24 zyxwvuts 0 1996 by John Wiley & Sons, Ltd. Received 5 May 1995 Accepted zyx 8 August 1995