Influence of dam operation on mayfly assemblage structure and life histories in two south-eastern Australian streams

La Trobe University Faculty of Science, Technology and Engineering Murray Darling Freshwater Research Centre

MDFRC item.

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 Mitts 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.