THE CHALLENGE OF MONITORING IMPACTS OF WATER ABSTRACTION ON MACROINVERTEBRATE ASSEMBLAGES IN UNREGULATED STREAMS BRUCE C. CHESSMAN, a * MEREDITH J. ROYAL b and MONIKA MUSCHAL c a New South Wales Department of Environment, Climate Change and Water, PO Box 3720, Parramatta NSW 2124, Australia b New South Wales Department of Environment, Climate Change and Water, University of New England, PO Box U245, Armidale, New South Wales 2351, Australia c New South Wales Department of Environment, Climate Change and Water, PO Box 2213, Dangar, New South Wales 2309, Australia ABSTRACT Monitoring of the ecological impacts of water abstraction from unregulated streams in the state of New South Wales (NSW), Australia, is challenging because water is abstracted by thousands of geographically dispersed users who pump intermittently according to temporally varying needs and the limitations imposed by licences and access rules. Detailed, quantitative monitoring methods are too costly for widespread routine application because of the size of the state (801 000 km 2 ) and the large number of streams affected by abstraction. We therefore tested the possibility of detecting abstraction impacts on aquatic macroinvertebrates with rapid biological assessment (RBA) methods similar to those that are routinely used for biological monitoring of Australian rivers. We sampled 85 sites on unregulated streams in north-eastern NSW during a period of prolonged and recurring drought, 54 of which were designated as reference sites with respect to water abstraction because upstream entitlement for abstraction was less than 1% of their mean annual flow (MAF). The remaining, non-reference sites had an average of 4% of MAF licensed for upstream abstraction (range 1–20%). Sweep and kick samples were collected at each site in two seasons, and invertebrates were picked for 30 min per sample and analysed at genus level. We found a small but statistically significant overall difference in macroinvertebrate assemblages between the reference and non-reference sites, but the amount of upstream entitlement did not affect the degree to which assemblages at individual sites deviated from site-specific reference conditions. We attribute the absence of evident impact mainly to the low proportions of streamflow abstracted, but limitations of RBA methods may also be a factor. We recommend a risk-based approach to future monitoring whereby effort is focussed on those streams where a high proportion of flow is abstracted. Copyright # 2009 John Wiley & Sons, Ltd. key words: freshwater; macroinvertebrate; rapid biological assessment; reference condition; river; water abstraction Received 15 June 2009; Revised 12 October 2009; Accepted 2 November 2009 INTRODUCTION The allocation of freshwater resources to meet utilitarian human needs while conserving aquatic ecosystems is a formidable task for science, policy and management (Jackson et al., 2001; Baron et al., 2002). In Australia, as elsewhere, studies of how human modification of water regimes impacts on freshwater ecosystems have concen- trated on flow regulation downstream of large impound- ments (Arthington and Pusey, 2003). However, Australia has many rivers where flow regimes are not controlled by large dams, but are nevertheless altered by the abstraction of surface or ground water for domestic use, livestock watering, irrigated agriculture and other purposes. The scarcity of information on the impact of water abstraction on the biota of these unregulated rivers is an impediment to the formulation of efficient and effective policy, planning and regulatory controls to protect aquatic ecosystems while providing allocations to abstractive users. In the Australian state of New South Wales (NSW), monitoring of the ecological impact of water abstraction from unregulated streams is challenging because such abstraction involves thousands of geographically dispersed users who pump water intermittently according to temporally varying needs and the limitations imposed by licences and access rules. Moreover, because use is mostly not metered, the actual amounts pumped are generally not known, but only the licensed entitlements. Detailed, quantitative monitoring methods are too expensive for routine application to all potentially affected streams because of the size of the state (801 000 km 2 ) and the large number of streams affected by abstraction. The more affordable alternative is to apply rapid biological assessment (RBA) methods, which are widely used for the biological monitoring of Australian rivers, particularly with macro- invertebrates (e.g. Metzeling et al., 2003; Halse et al., 2007). These methods are cheaper because they use only semi- quantitative sampling gear, subsample a small proportion of RIVER RESEARCH AND APPLICATIONS River Res. Applic. 27: 76–86 (2011) Published online 15 December 2009 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/rra.1340 *Correspondence to: Bruce C. Chessman, New South Wales Department of Environment, Climate Change and Water, PO Box 3720, Parramatta NSW 2124, Australia. E-mail: bruce.chessman@environment.nsw.gov.au Copyright # 2009 John Wiley & Sons, Ltd.