There are a range of measures that can be used in the prevention management and control of blue–green algal blooms.

Contact your local Regional Algal Coordinating Committee or the NSW Office of Water for more information.

Algal management strategy

In response to the occurrence of the largest recorded blue–green algal bloom in the Darling River in 1991, the NSW Blue–Green Algal Task Force was formed. The Task Force was made up of representatives from a number of key NSW government agencies. In 1992, the Task Force made 30 recommendations to the government which were developed into a comprehensive integrated Algal Management Strategy to minimise the occurrence and impact of algal blooms in New South Wales.

The NSW Algal Management Strategy integrated a large number of measures into five key elements: State Algal Contingency Plan; Management of Blooms; Land and Water Management; Education and Awareness Raising; and Research. The Strategy included Algal Contingency Plans to minimise the effects of blue–green algal blooms, and short to medium term measures to control the factors leading to algal bloom development. It also covered short to long term nutrient and water management measures to minimise nutrient inputs to waterways. These measures were strengthened by education and research, and by increasing community awareness. The Strategy involves Local Land Services (formerly Catchment Management Authorities), NSW Office of Water and other state government agencies, local government, communities, industry, researchers and landholders.

The NSW Algal Management Strategy forms the basis of the work of the Regional Algal Coordinating Committees.

Catchment management

Catchment management is a long–term solution to the minimisation of blue–green algal blooms. Protecting soils from erosion and maintaining vegetation cover within a catchment will ultimately lead to better water quality as less sediments and nutrients will be able to enter waterways. Nutrients encourage the growth of blue–green algae, so reducing the nutrient inputs will reduce the frequency of algal blooms. Measures to improve catchment management will generally not show immediate results, but they will have long term benefits to the environment.

The main ways of reducing the nutrient load of a water body are:

  • Avoiding the excessive use of fertilisers and manures on agricultural land within the catchment
  • Protecting soil from erosion
  • Treating sewage to remove the nutrients nitrogen and phosphorus.

Sediments and nutrients can be prevented from entering a waterway by protecting the strip of land adjacent to the water body. This strip of land is known as the riparian zone and vegetation within the riparian zone, is known as riparian vegetation. Riparian vegetation is important for maintaining and protecting water quality and performs the following tasks:

  • Acts as a buffer zone
  • Filters runoff and prevents pollutants from entering the water body
  • Prevents river bank erosion which can increase turbidity and sedimentation of the water body
  • Shades the water, which reduces the available light and keeps the water temperature lower so algal growth is not encouraged.

Managing algal blooms in water storages

Algal blooms in water storages such as lakes or dams can be dealt with by using a number of management strategies.

Artificial destratification

A water body becomes thermally stratified when two distinct temperature layers form. During spring the sun will warm the surface layers of water. They become less dense, but will be mixed with cooler 'bottom' water by wave action. As heating continues, the wave action will become less able to drive the mixing. When mixing ceases, the warmer surface water will lie over cooler, dense bottom waters. During autumn this process is reversed, and the water body will 'turn over'. During summer, algal blooms often occur in the warm stable conditions of the upper layer. The bottom layer often has very low concentrations of dissolved oxygen that creates favourable conditions for the release of nutrients from the sediments.

Artificial destratification involves increasing the circulation of water that circulates between the shallower and deeper layers of the reservoir. This can be achieved by introducing a plume of bubbles near the bottom of the reservoir or installing a propeller or impeller in or near the dam wall. A circulation pattern is set up that reduces the differences in temperature, oxygen and nutrients between the top and the bottom waters.

Artificial destratification can reduce algal growth by:

  1. Reducing the sediment phosphorus load available to the water column and so starving the algae of nutrients
  2. Mixing algae deeper into the water column and starving them of light.

Examples of aeration systems are Chaffey Dam located on the Peel River, near Tamworth in northern NSW, and Lake Lyell near Lithgow in the Blue Mountains.

Examples of a propeller system are Manly Dam, and Sooley Dam (near Goulburn).

An example of an impeller system is in Medway Dam near Moss Vale on the Southern Highlands.

Reducing nutrient concentrations in water storages

Once nutrients enter a water storage they are very hard to remove. Therefore the most effective strategy is to prevent nutrients from entering the storage in the first place. There are a number of measures which can be used to reduce the input of nutrients in water storages:

  • Artificial wetlands and/or pre–reservoirs upstream of the water storage act as a nutrient sink and prevent the inflow of nutrients into the storage. These systems often require lot of maintenance and their effectiveness in improving water quality has been varied. Examples of artificial wetlands used for these purposes are Carcoar Wetland and Lake Pillans
  • Planting trees and shrubs around a water body will reduce the input of sediments and therefore nutrients into the water body. The plants also provide shade to the water body which reduces the temperature and solar radiation reaching the water body. This will help in the prevention of blue–green algal blooms
  • Managing the whole catchment above the storage by reducing the use of fertilisers and fencing off waterways to prevent stock access will also reduce the soil erosion and amount of nutrients entering a waterway.

Biomanipulation

Biomanipulation or biological control is a method of altering the ecosystem to reduce the growth of algae. Biomanipulation is not yet a viable control mechanism for algal blooms and is still being actively researched to see whether it will work or not.

Some experimental methods of biomanipulation include:

  • Removing fish that eat zooplankton from the water body by introducing predatory fish that eat the planktivorous fish. This will lead to an increase in zooplankton. As zooplankton eat blue–green algae, this should lead to a decrease in blue–green algae numbers. Unfortunately, not all phytoplankton species are eaten by zooplankton so this method may lead to the dominance of inedible species. The introduction of new species also may cause problems
  • Inhibiting the growth of blue–green algae by introducing aquatic plants that compete with blue–green algae for nutrients and light, and provide refuges for zooplankton.

Algicides and algistats

An algicide is any chemical added to water which is toxic to, and kills algae and/or cyanobacteria (blue–green algae). Examples include copper sulphate, chelated copper–based products such as 'COPTROL', simazine and benzalkonium chloride. The use of algicides to control algal blooms is not recommended by Government Agencies and will only be used in emergency cases. The use of algicides is not an effective long term solution to algal problems.

The Protection of the Environment Operations Act 1997 excludes the application of any chemical to a water body unless a licence has been granted by the Office of Environment and Heritage. The Office of Environment and Heritage should be contacted before adding any chemical to any water body including farm dams.

Copper–based algicides damage and kill algal cells which leads to the release of algal toxins into the surrounding water. Once in the water, toxins can pass more easily through the water treatment filters than the intact cells do. If algicides are used in potable water supply reservoirs the water should not be used until the toxins and odours degrade, or the water needs to be properly treated in a water treatment plant. It is also a lot more difficult to detect algal toxins than whole algal cells. Once algal cells are killed, the only way to determine whether algal toxins are still present in the water is through toxin testing, which can take up to a week and is far more expensive than testing for algal cells. The toxins produced by blue–green algae are generally very stable compounds that are resistant to chemical breakdown and may remain in natural waters for several months. Under natural conditions, sunlight and bacteria may cause the breakdown of some toxins.

Risks

Risks associated with using copper–based algicides include:

  • Mass release of toxins from the algal cells
  • Accumulation of copper in the sediments
  • Growth of species of blue–green algae that are resistant to the algicide may cause greater water quality problems
  • Copper–based products may kill other aquatic flora and fauna. They can also cause the death of fish through reducing the concentration of oxygen in the water when the algae die.

An algistat is any chemical or additive (including plant or animal material), added to water that inhibits or retards the growth of algae, either directly, or by chemical modification of the water column (eg, through precipitation of phosphorus). Examples of algistats include alum, gypsum, lime, coloured dyes, and bacterial, fungal, viral or enzyme–based products that claim algistatic or water quality improving properties, activated clays (eg PhoslockTM). Algistats may alter the chemical composition or physical properties of a water body, which may have an undesirable effect on aquatic biota.

Water treatment

Algae can be removed from water through a number of treatment methods. These include filtration, coagulation using aluminium and ferric iron salts or organic polymers and the use of algicides.

Short–term control techniques for drinking water supplies include changing the position or depth of offtakes so they are away from where algal cells scums accumulate, and the use of barriers to restrict scum movement.

The most reliable method of algal toxin removal is using activated carbon filtration. This approach uses either powdered activated carbon, which can be added intermittently whenever the need arises, or granular activated carbon absorbers, which are used continuously. Accordingly, granular activated carbon may be more expensive than powdered activated carbon when only used intermittently, but it is also generally more effective and more reliable for consistent removal of soluble organic compounds.