The management of rivers, lakes and reservoirs is a difficult task which demands considerable effort to restore deteriorating catchments, water habitats and water quality. The Torrens Lake, in Adelaide South Australia, has similar problems to many lakes worldwide including catchment clearing, nutrient focussing, flow regulation, water extraction for irrigation and cyanobacterial blooms.
It is unlikely that any management strategy will reduce cyanobacterial numbers to zero and therefore the most realistic strategy would be one that controls the cyanobacterial population below the guideline concentration for recreational exposure. Artificial destratification is likely to be ineffective at controlling cyanobacteria in the Torrens, as the system is shallow and mixing inefficient. Nutrient control will constrain cyanobacterial biomass but the Torrens catchment delivers high loads of nutrients to the lake and it will be decades before control measures limit growth to the desired thresholds.
The aim of this study was to investigate whether a controlled upstream release of water could effectively dilute the population and control cyanobacteria numbers below a threshold. For a growth rate of 0.4/day, which is a typical exponential growth rate of cyanobacteria in the Torrens Lake, it can be concluded that a diluting flow of at least 10% per day would be required to have noticeable impact on the cyanobacteria population. With a starting cell concentration of 100 cells/mL, a growth rate of 0.4/day and a diluting flow of 10%, the cell concentration after 20 days would be 74,420 cells/mL, which is below the critical threshold for cell numbers.
A trial was conducted in the summer of 2011/12. Flows were released from Hope Valley Reservoir in response to the level of cyanobacteria present in the Torrens Lake with the aim of keeping cyanobacterial numbers in check and avoiding lake closure. During a period when upstream water was unavailable the cyanobacteria grew exponentially and the population exceeded the threshold concentration. Under similar meteorological conditions later in summer the population was maintained by a combination of controlled amenity flows and rain event inflows.
The study concluded that: