UNDERSTANDING AND MANAGEMENT OF CYANOBACTERIAL BLOOMS IN SUBTROPICAL RESERVOIRS OF QUEENSLAND, AUSTRALIA

A three year study of two sub-tropical water supply (potable and irrig ation) reservoirs - Baroon Pocket and Leslie - aimed to develop an imp roved understanding and effective management strategy for the control of low level toxic cyanobacterial 'blooms'. The two reservoirs appear to be typical of those elsewhere in tropical and sub-tropical Australi a, being strongly stratified and monomictic if deep, or polymictic to monomictic (depending on inter-annual climatic variation) if shallow. In both reservoirs, thermal stratification appeared to be the key fact or influencing the onset and demise of cyanobacterial blooms. Hypolimn etic deoxygenation was rapid during periods of stratification, and hig h rates of nitrogen and phosphorus release from bottom sediments occur red during anoxia. External nutrient inputs were dominated by episodic storm in-flows following the passing of (sub)tropical depressions, ca rrying massive sediment and nutrient loads into the reservoirs. As a c onsequence of this, and coupled with the high degree of internal nutri ent recycling and the low national targets for cyanobacterial control in Australia, we believe that reduction of external nutrient inputs al one, through improved catchment management, may not be sufficient to p revent cyanobacterial blooms. Therefore, more emphasis should be given to 'in-lake' control strategies such as destratification, biomanipula tion and in situ sediment treatment. (C) 1998 IAWQ. Published by Elsev ier Science Ltd.