MICROINVERTEBRATE DYNAMICS IN RIVERINE SLACKWATER AND MID-CHANNEL HABITATS IN RELATION TO PHYSICO-CHEMICAL PARAMETERS AND FOOD AVAILABILITY NATHAN S. P. NING, a * DARYL L. NIELSEN, b WARREN L. PAUL, a TERRY J. HILLMAN a and PHIL J. SUTER a a Department of Environmental Management and Ecology, La Trobe University, PO Box 821, Wodonga, Victoria 3689, Australia b CSIRO Land and Water, Murray-Darling Freshwater Research Centre, PO Box 991, Wodonga, Victoria 3689, Australia ABSTRACT Microinvertebrates play a critical role in riverine food webs, and recent studies have hypothesized that slackwaters, non-flowing regions associated with the main channel, are important for their reproduction and recruitment. However, little is known regarding the population and community dynamics of microinvertebrate communities in slackwater regions, or how they compare with those in mid-channel regions. This study examined microinvertebrate communities in the epibenthic and pelagic zones of slackwater and mid-channel regions (i.e. four habitats) of an Australian floodplain river in relation to physico-chemical parameters and food availability (as estimated by chlorophyll-a concentration) between September 2005 and November 2006. Results from this study indicate that microinvertebrate abundance and diversity were greater in slackwater habitats than mid- channel habitats overall, corresponding with the slower current velocities associated with the former. Nevertheless, communities in all four habitats were most abundant and diverse in late spring (coinciding with an increase in water temperature), and followed similar seasonal trajectories in terms of density, taxon richness and community structure. These findings support the view that slackwaters are important for in-channel microinvertebrate production, and suggest that animals frequently disperse (either actively or passively) among slackwater and other main channel regions year round. Given the critical importance of microinvertebrates in riverine food webs, rivers should be managed with a view to maintaining a natural variety of accessible slackwater regions in order to support the production and survival of microinvertebrate communities. Copyright # 2009 John Wiley & Sons, Ltd. key words: rotifers; crustaceans; pelagic; epibenthic Received 22 October 2008; Revised 9 March 2009; Accepted 15 March 2009 INTRODUCTION In many riverine systems, the population and community dynamics of microinvertebrate communities are largely constrained by physical conditions, particularly hydrodynamic forces associated with flow (Rzoska, 1978; Saunders and Lewis, 1989; Pace et al., 1992). Hydrodynamic forces often constrain these microinvertebrates, directly or indirectly, by affecting their life history patterns (Rzoska, 1978; Saunders and Lewis, 1989), habitat characteristics (Humphries et al., 2006), dispersal patterns (Speas, 2000), resource acquisition (Guelda et al., 2005) and biotic interactions (Baranyi et al., 2002; Keckeis et al., 2003). As a result, communities occurring in the main channel flow are often of relatively low abundance and/or biomass (Basu and Pick, 1996; Ietswaart et al., 1999), and are typically comprised of small-bodied taxa with short generation times (Shiel et al., 1982; Pace et al., 1992; Kobayashi, 1997). Non-flowing regions in the main channel environment, commonly termed slackwaters (Thorp and Casper, 2003; Casper and Thorp, 2007), function as hydrodynamic refuges, and are critical in supporting microinvertebrate recruitment and reproduction in rivers (Reckendorfer et al., 1999; Schiemer et al., 2001; Nielsen et al., 2005). Slackwaters are usually located on the periphery of the main channel flow, where they are not part of the main flow, RIVER RESEARCH AND APPLICATIONS River. Res. Applic. 26: 279–296 (2010) Published online 29 April 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rra.1266 *Correspondence to: Nathan S. P. Ning, Department of Environmental Management and Ecology, La Trobe University, PO Box 821, Wodonga, Victoria 3689, Australia. E-mail: Nathan.Ning@csiro.au Copyright # 2009 John Wiley & Sons, Ltd.