Flow and water mixing
CSIRO research has found that algal blooms occur when both water and light are available at the same time. This generally requires a shallow surface mixed layer.
Sunshine creates a surface layer of relatively warmer water, and blue-green algae in the warm layer can harvest the light and grow to plague proportions.
In rivers, shallow surface layers are supported when the river flow decreases below a critical threshold required to completely mix the water column. In reservoirs, the surface layer depth depends on the weather especially by how much the water surface cools during the night.
A number of potential management strategy options have been assessed including:
- maintaining a high level of discharge or flow
- varying the discharge amounts, called pulsing, to maintain a lower overall discharge amount
- using siphons to create an overflow discharge of surface water layers from an underflow weir
- using bottom-mounted bubblers or mechanical mixers to mix the water levels and ensure bottom waters remain oxygenated
- measuring algae concentrations in bottom water layers may provide options for safer withdrawal of water for domestic consumption.
CSIRO scientists and the Western Australian Water and Rivers Commission have developed modified clays that can be applied to bind phosphorous, a nutrient that feeds the algae.
An absorbent clay-based substance, now a commercial product called Phoslock™, can be applied directly to surface waters where it instantly binds dissolved phosphorus as it settles through the water column. This reduces the amount of phosphorus immediately available to algae, in turn reducing their potential for growth.
Phoslock™ also forms a layer capping bottom sediments and preventing the release of nutrients to fuel further algal blooms.
Researchers have investigated the potential for biological controls for blue-green algae by changing fish populations.
Manipulating fish communities, aimed at reducing the quantity of phytoplankton in the habitat through the food chain and through modified nutrient recycling, called biomanipulation, has been successful overseas.
In Australia it has been shown that an excessive quantity of some fish in a habitat can trigger the blooms. Careful management of inland fisheries is required to lower the risk of fish-induced blooms.
Genetic studies into cyanobacteria and toxins
Researchers have undertaken genetic studies of cyanobacteria and the relationship to toxicity. Not all algal blooms result in toxic blooms. However, some blooms produce substances which include hepatoxins (liver-damaging), neurotoxins (nerve-damaging) cytotoxins (cell-damaging) and skin irritants.
Studies discovered that Australian strains show distinct genetic groupings which appear to have a geographic basis. Australian cyanobacterial species are also distinctly different genetically from the same species outside Australia. Molecular tools have been developed which can identify the genetic structure of dominant strains in a bloom over time, therefore potentially tracking toxicity.
Reducing the delivery of nutrients to rivers is one of the long-term management solutions.
Researchers have developed a range of tools for tracing the sources of nutrients in a catchment, so they can be pinpointed and checked. Nutrients can be sourced from fertiliser runoff, sewage effluent, erosion, city stormwater or a range of other causes.
Measuring and collecting water quality data
The difficulties related to reliable in-situ measurements and collection of data on water quality (particularly water chemistry) over extended periods have always posed a significant challenge to water managers, scientists and instrument makers.
Prediction of nutrient impacts on water quality
Catchment management software is also helping water managers to identify the sources of nutrient pollution which can facilitate algal blooms and predict, to some extent, the impact of nutrient management strategies.
Some software examples are Catchment Management Support System (CMSS), SedNet/ANNEX and Source.