INFLUENCE OF DAM OPERATION ON MAYFLY ASSEMBLAGE STRUCTURE AND LIFE-HISTORIES IN 2 SOUTH-EASTERN AUSTRALIAN STREAMS

The mayfly fauna, and in particular the life histories of two mayflies , Coloburiscoides sp. and Baetis sp. 3, was studied in two nearby stre ams located in the headwaters of the River Murray, SE Australia. The M itta Mitts River is regulated by the Dartmouth Dam which releases cold water in summer for irrigation. Snowy Creek is a tributary of the Mit ta Mitta River with natural flow and temperature regimes. The structur e of the mayfly assemblage differs in the two streams. In the regulate d Mitta Mitta there are 11 species, of which, caenids (Tasmanocaenis t onnoiri and River Murray sp. C), Coloburiscoides sp. and Baetis sp. 3 dominate. Snowy Creek, with a richer assemblage of 17 species, was dom inated by Coloburiscoides sp., Baetis sp. 3 and two Austrophlebioides species. Monthly collections of mayfly nymphs and adults between Septe mber 1987 and August 1988, in both streams, allowed evaluation of poss ible life history changes in relation to dam operation. Coloburiscoide s sp. and Baetis sp. 3 had synchronous life histories at both regulate d and unregulated sites. Coloburiscoides appeared to be univoltine, an d 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 de lay nymphal hatching in Coloburiscoides. (C) 1998 John Wiley & Sons, L td.