REGULATED RIVERS: RESEARCH & MANAGEMENT Regul. Riers: Res. Mgmt. 14: 285–295 (1998) INFLUENCE OF DAM OPERATION ON MAYFLY ASSEMBLAGE STRUCTURE AND LIFE HISTORIES IN TWO SOUTH-EASTERN AUSTRALIAN STREAMS I. PARDO a, *, I.C. CAMPBELL b AND J.E. BRITTAIN c a Departamento de Ecologı ´a y Biologı ´a Animal, Uniersidad de Vigo, 36200 Vigo, Spain b Department of Ecology and Eolutionary Biology, Monash Uniersity, Clayton 3168, Australia c Freshwater Ecology and Inland Fisheries Laboratory, Uniersity of Oslo, Sars gate 1, 0562 Oslo, Norway ABSTRACT The mayfly fauna, and in particular the life histories of two mayflies, Coloburiscoides sp. and Baetis sp. 3, was studied in two nearby streams located in the headwaters of the River Murray, SE Australia. The Mitta Mitta River is regulated by the Dartmouth Dam which releases cold water in summer for irrigation. Snowy Creek is a tributary of the Mitta Mitta River with natural flow and temperature regimes. The structure of the mayfly assemblage differs in the two streams. In the regulated Mitta Mitta there are 11 species, of which, caenids (Tasmanocaenis tonnoiri and River Murray sp. C), Coloburiscoides sp. and Baetis sp. 3 dominate. Snowy Creek, with a richer assemblage of 17 species, was dominated by Coloburiscoides sp., Baetis sp. 3 and two Austrophlebioides species. Monthly collections of mayfly nymphs and adults between September 1987 and August 1988, in both streams, allowed evaluation of possible life history changes in relation to dam operation. Coloburiscoides sp. and Baetis sp. 3 had synchronous life histories at both regulated and unregulated sites. Coloburiscoides appeared to be univoltine, and Baetis polyvoltine. At the regulated site, nymphal abundance in both Coloburiscoides and Baetis declined during the summer release, which coincided with the beginning of nymphal recruitment and appeared to delay nymphal hatching in Coloburiscoides. © 1998 John Wiley & Sons, Ltd. KEY WORDS: mayflies; stream regulation; assemblage structure; life histories; Australia INTRODUCTION Downstream effects of dams on stream communities are a consequence of changes in flow patterns, temperature regime, altered habitat, and variation in food type and availability (Ward, 1976; Ward and Stanford, 1979). Following river regulation, benthic organisms inhabiting downstream reaches have to cope with new imposed environmental conditions. Some species are eliminated or reduced in abundance while others increase. One major potential environmental change which may result from river regulation is a change in downstream water temperature, an impact particularly associated with bottom release reservoirs. Changes induced through these impacts are the basis of the serial discontinuity concept of river regulation impact proposed by Ward and Stanford (1983). In much of south-eastern Australia, large reservoirs are used to supply water for irrigation during summer. River channels are used as major water delivery systems, which leads to a reversal of the natural flow regime. Whereas the pre-regulation rivers had minimal discharge and high water temperatures in summer, they now flow bankful and at temperatures substan- tially lower than those which occurred naturally. These patterns have been documented for the Goulburn and Tumut Rivers, and are also a feature of the Mitta Mitta River below Dartmouth Dam (Gippel and Finlayson, 1993; Finlayson et al., 1994). * Correspondence to: Department of Ecology and Animal Biology, Faculty of Science, Apartado 874, University of Vigo, 36200 Vigo, Spain. Contract grant sponsor: Australian Water Research Advisory Council; Murray-Darling Freshwater Research Centre, Albury; The Spanish Government (MEC); Xunta de Galicia CCC 0886–9375/98/030285 – 11$17.50 © 1998 John Wiley & Sons, Ltd. Receied 4 July 1997 Accepted 15 January 1998