Modelling suppression of cyanobacterial blooms by flow management in a lowland river

Growth and dominance of the cyanobacterium Anabaena circinalis in weir pools of the Barwon–Darling River, Australia, are related to persistent vertical thermal stratification between October and March, when discharge is low. We determined critical velocities and discharges required to suppress bloom formation at three sites, and modelled the occurrence of sub‐critical discharges in order to predict the frequency of blooms under different management scenarios. Our model suggests that the frequency of blooms was about double that expected under near‐natural flows (without major impoundment or water extraction) for 1990–2000. Flow management, through Environmental Water Provisions that limit water extraction when river levels are low, has been in place since July 2000. Our model suggests that these provisions are unlikely to have had an effect on bloom frequency for 2000–2003. In the longer term, however, they could reduce bloom frequency at some sites by up to one‐third. Copyright © 2006 John Wiley & Sons, Ltd.