Cyanobacterial blooms in the Mildura weir pool 1996-2003

"2003".

Project Number: Blue green algae mitigation in the Mildura weir pool HA27a M/BUS/39.

MDFRC item.

Cyanobacteria are a naturally occurring component of aquatic ecosystems that do not pose a threat to public health or stock until environmental conditions lead to the formation of blooms. The incidence of cyanobacterial blooms has been increasing this century as a likely consequence of flow regulation and land management practices. In the Mildura weir pool this is characterized by reduced flows during the warmer months. A review of data collected between 1996-2003 from the Mildura weir pool revealed that bloom events tended to occur where periods of low flow (<2500 ML.day-1) and high temperatures (>25 oC) coincided and persisted for several days. During such periods, flow induced water column stability facilitated the establishment of thermal and oxygen gradients in the water column. Under these conditions, the ability of cyanobacteria to function anaerobically and to regulate their buoyancy so as to exploit both surface light and benthic nutrient resources allowed them to effectively out-compete other algal taxa and develop into blooms. Existing monitoring programs based on only a single weekly surface sample provide no measure of small-scale spatial or temporal variability, which may mask true environmental variability. This was examined during the summer of 2002-3. Spatial variability was identified as the single largest source of error. The collection of spatially replicate samples per sampling event is highly recommended.