River Torrens Water Quality Improvement Trial Summer 2011/2012

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Project Partners:

The University of Adelaide and SARDI

Research Theme:

N/A

Status:

Project Overview

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.

Progress Update and Key Findings

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.

Project Impacts

The study concluded that:

  • The simple model of growth and dilution is a reasonable predictor of biomass change at low cell density, however population decrease is underestimated by the predictive model.
  • Dilution is possibly greater than predicted because high concentrations of cells near the surface are washed out of the lake with the overflow of water over the weir.
  • Nutrient or light limitation possibly contributed to the observed decrease in growth rates; nutrient limitation as the population expands and consumes the available resources during periods of rapid growth, and light limitation following rain events as coloured material is washed into the lake.
  • The results of the trial suggest that if dilution flows are released early enough, the size of the cyanobacterial population can be controlled. However, there is a reliance on rain events to flush the system and dilute the resident cyanobacterial population. On average, the flow return interval analysis suggests that rain events occur frequently enough in summer for this strategy to be effective. However, in a variable climate like that observed in Adelaide, there may be incidences of very long periods between significant rainfall events. This may reduce the confidence in rain events to reset the population.
  • The dilution flow was not observed to have any impact on the freshwater fish community.

Research Outputs

ELCOM Hydrodynamic Model

River Torrens Water Quality Improvement Trial - Summer 2011/12

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