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DWE Home > Home > Water Management > Water quality > Algal information > Algae: Causes and control

Algae: Causes and control

What causes freshwater algal blooms?

Freshwater algal blooms occur when there is a combination of suitable environmental conditions including:

• Nutrients
• Temperature
• Light
• Turbidity
• Stable conditions

Nutrients

Nutrients encourage the growth of blue–green algae. The process of nutrient enrichment in a waterway is called eutrophication. The main nutrients contributing to eutrophication are phosphorus and nitrogen.

Runoff and erosion from fertilised agricultural areas, erosion from river banks, river beds, land clearing (deforestation), and sewage effluent are the major sources of phosphorus and nitrogen entering water ways.

Phosphate attaches to sediments. When water is low in dissolved oxygen (anoxic), sediments release phosphate into the water column. This encourages the growth of algae.

Blooms of blue–green algae can also occur when the concentration of nutrients is fairly low, but blooms are more frequent when the concentration of nutrients is high.

Temperature

Blue–green algal blooms usually develop during the warmer months of the year or when the water temperature is higher and there is increased light.

Temperatures of 25° C are optimal for the growth of blue–green algae. At this temperature, blue–green algae have a competitive advantage over other types of algae.

In temperate regions, blue–green algal blooms generally do not persist through the winter months due to low water temperatures. Higher water temperatures in tropical regions may cause blue–green algal blooms to persist throughout the year.

Light

Blue–green algae populations are diminished when they are exposed to long periods of high light intensity but have optimal growth when intermittently exposed to high light intensities.

Even under low light conditions, or in turbid water, blue–green algae have higher growth rates than other algae. This ability to adapt to variable light conditions gives blue–green algae a competitive advantage over other algal species.

Blue–green algal cells contain gas vesicles that can be inflated or deflated. By using their gas vesicles blue–green algae are able to regulate their position within the water column and therefore regulate their exposure to light.

Turbidity

Turbidity, or muddiness of water, is caused by the presence of suspended sediments and organic matter in the water column. Low turbidity occurs when there is only a small amount of suspended matter present in the water column. Low turbidity can be due to the influence of the surrounding geological environment and/or slow moving water that allows particulate matter to settle out of the water column. When turbidity is low, more light can penetrate through the water column. This creates optimal growth conditions for blue–green algae.

Stable Conditions

Blue–green algae prefer stable water conditions with low flows, long retention times, light winds and minimal turbulence.

Drought, water extraction for irrigation, human and stock consumption and the regulation of rivers by weirs and dams all contribute to decreased flows of water in our river systems. Water moves more slowly or becomes ponded, which encourages the growth of algae. In some river valleys the environmental flow rules enable water to be released from the storage to control algal growth.

Another consequence of stable conditions is thermal stratification of the water body. Thermal stratification occurs when the top layer of the water column becomes warmer and the lower layer remains cooler. When the two layers stop mixing, the upper layer becomes more stable and the growth of blue–green algal blooms is encouraged. Often water with low levels of oxygen (anoxic) result in bottom waters when a water body is stratified, which may lead to increased nutrient release from the sediments.

What causes toxic marine algal blooms?

Marine algal blooms are a common natural phenomena along the NSW east coast and can result from upwelling of colder nutrient rich water. Studies from a long term coastal station off Sydney found that marine phytoplankton blooms appeared to correspond with upwelling/uplifting or slope water intrusions lasting 2 to 22 days and occurring from September to February.

Nutrient fluctuations as a result of anthropogenic changes can also influence the presence of algal blooms and species succession and may even influence when toxins are generated. Marine blooms may threaten fish resources, human health, ecosystem function and recreational amenity of beaches and bays. Marine algal blooms fall into the classes of:

• BACILLARIOPHYCEAE (diatoms)
• DINOPHYCEAE (dinoflagellates)
• PRYMNESIOPHYCEAE (golden–brown flagellates)
• CHRYSOPHYCEAE (golden–brown algae)
• RAPHIDOPHYCEAE (chloromonads)
• DICTOCHOPYCEAE (silicoflagellates), and
• CYANOPHYCEAE (marine blue–green algae) (Hallegraef,1991).

Marine algal blooms can appear as red water discolourations commonly referred to as ‘red tides’ or a range of other discoloured water, from green, yellow and brownish to an oily or milky appearance. These algal blooms are commonly mistaken by the public for sewage or some other form of pollution. Other blooms can show no discolouration but be highly toxic at low levels. It is important that samples of marine algae are analysed, as relatively harmless algae and potentially toxic algae cannot be differentiated by the naked eye. However, some nuisance red tide blooms can be differentiated in the field by the trained eye or easily identified by microscopy. This includes Noctiluca scintillans, a dinoflagellate, that is easily identified under a crude microscope due to its size and distinct balloon–like shape.

Similar to freshwater blue–green algae, some estuarine and marine algal species produce irritants that can cause respiratory irritation and severe contact dermatitis. The major route for human exposure is through consumption of seafood and shellfish as some species produce potent toxins that can be accumulated in fish and shellfish. Even low densities of toxic algae may be sufficient to cause illness or death in humans, while some species can selectively kill fish by inhibiting their respiration. Not all potentially toxic algal species are toxic in every situation.

Although the distribution of marine and estuarine algae is uncertain in Australia , the number and intensity of marine algal blooms is believed to be increasing world–wide due to:

• Expansion of aquaculture in coastal areas.
• Coastal eutrophication and unusual climatic conditions.
• Movement of shellfish stocks and transport of resting cysts in ballast water.

Unlike freshwater algal species that may be present for extended periods and normally occur where water movement is minimal, marine algal occurrence responds to nutrient enrichment, water circulation such as tides and currents, and wind patterns. As such, their occurrence is often short–lived in a particular area and difficult to predict.

Preventing and controlling blue–green algal blooms

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

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 State 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 Catchment Management Boards, 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.

For more information on destratification see the brochure on Improving Water Quality in Reservoirs from DNR.

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 NSW Department of Environment and Conservation . The Department of Environment and Conservation 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 the intact algal cells. If algicides are used in potable water supply reservoirs the water should not be used until the toxins and odours degrade. This could take several months. 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 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. 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 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.