Causes of a blue-green algal bloom
There are several factors that can lead to a bloom:
- Excessive phosphorus and nitrogen in the water promotes rapid algal growth and multiplication. Nutrients come from inflow of water from arable land, animal excreta, decaying organic matter and industrial or sewerage waste.
- Warm water promotes rapid algal growth. Blooms more likely to occur in calm, shallow water during summer and autumn where temperatures exceed 18 degrees Celsius in surface water.
- Insufficient microscopic organisms that feed on algae and keep algal levels under control.
Signs of a blue-green algal bloom
- A scum, looking like green paint, forms on downwind surfaces of water. Algae are vulnerable to the effects of winds or currents so the scum may disappear and reappear on subsequent days.
- An unpleasant odour is released, smelling like foul sewerage.
We recommend checking livestock and water supplies for blue-green algae daily in summer and autumn. Treat all algal blooms as possibly toxic to livestock, and prevent stock access until the algae are identified and the level of toxin determined.
See Blue-green algae poisoning in livestock for signs of livestock poisoning.
Preventing blue-green algal blooms
Prevention, especially with blue-green algae, is much better than cure (see toxin persistence for a few reasons).
- Prevent nutrients and contamination from entering the dam.
- Use barley straw to prevent and reduce algae growth.
- Remove nutrients from the water source.
Prevent nutrients and contamination from entering the dam
Contamination of dams can be reduced by:
- excluding livestock (see good practice on the farm dams page)
- using silt traps (see the farm dams page for more information)
- using roaded catchments instead of paddock grade banks.
Temporary sediment traps can be constructed using straw bales, netting or corrugated iron to help limit inflows of nutrients.
Barley straw inhibition of blue-green algae has been well documented in field and laboratory studies since the 1990s. Note that barley straw is more effective as a preventative rather than a treatment for existing blooms.
Barley straw releases chemicals that are active agents against Microcystis blue-green algae: oat and wheat straw will not work. Barley straw treatment is not harmful to plants, fish or crustaceans.
We recommend getting blue-green algae identified, because barley straw is not effective on all algae (see below).
Adding barley straw to contaminated or potentially contaminated water has two effects:
- Nutrient stripping – the high carbon-to-nitrogen ratio of barley straw means that when it breaks down it uses available nitrogen in the water. This reduces conditions favourable to algae growth.
- Release of chemicals that reduce the growth of harmful blue-green algae – Salcolin A and Salcolin B (both flavonolignans) have been identified as active agents against Microcystis sp.
Barley straw management
- Add barley straw to a dam at 50 grams per square metre of surface area of the dam water. This rate releases enough chemicals to last for up to six months.
- Add the barley straw in spring or early summer, before water conditions favour blue-green algae growth. Barley straw is likely to be more available after harvest, in early summer, and the straw is more effective in warmer water.
- When the surface layer of water is greater than 21°C, adding barley straw to an existing bloom may be effective within two weeks. However, barley straw is more effective as a preventative than a treatment for blooms.
- Place the straw in coarse-weave bags (such as onion bags) and suspend the bags from floats, such as sealed large drink bottles, drums or a pontoon. The floating straw bags will sink as the straw rots. Remove the straw at this stage, and spread it on paddocks away from the dam.
- If there is an incoming flow of water, place the straw net where there is a continuous flow of water over and through the straw. This will help to keep the straw oxygenated and spread the active chemicals throughout the water surface.
Removing nutrients from the water source
Ferric alum can be used to prevent algal growth, by removing phosphorus from the water. Use 50 g of ferric alum per 1000 L of water, by placing a block in a porous bag attached to a float in the water source.
We recommended treating water supplies before warm summer conditions favour algal growth.
Long-term strategies for control
Long-term control strategies for blue-green algae include:
- preventing livestock camping near water supplies
- maintaining full ground cover to prevent movement of soil and nutrients to water supplies
- avoiding excessive use of fertilisers.
Treating blue-green algal blooms
The main treatment method used on farms in Western Australia is with algicides. For reasons explained below, Simazine is the preferred algicide.
Removing the blue-green algae may not remove the toxins.
- Blue-green algae can release toxins into the water, and those toxins can be present for many months after treating the algae.
- When blue-green algae is killed, toxins in the cells are released into the water, increasing the risk of poisoning.
- Sun-dried blue-green algae can remain toxic for up to five months.
Several chemicals can be used to treat blue-green algae growth. Chemical treatment of algae can cause a sudden release of toxin, which can persist for several days.
Follow withholding periods for re-grazing with livestock to prevent residues and any harmful effects from the chemical. Livestock should be excluded from water supplies for the length of the withholding period or 14 days, whichever is longer.
Some options for chemical treatment are listed below.
Only simazine products that are registered for blue-green algae control should be used and the label rates and directions followed. The most convenient method of application of simazine is to premix it at the appropriate rate in a fire-fighting unit, then jet it onto the surface of the dam. The target concentration is 2 parts per million of the active ingredient (2 g per 1000L or 2 kg per 1000 m3).
Simazine is a herbicide – water treated with the product should not be used on plants for at least 14 days.
Cupricide is a chelated copper product that kills algae with fewer toxic effects than copper sulfate and does not precipitate out when there is a high concentration of carbonate. Water treated with cupricide can be used on plants. Despite its reduced toxic effects, it should not be used in hard water containing trout, native fish or crustaceans, or in water used for livestock where the livestock are grazing plants that may cause liver damage (e.g. caltrop, lupins, heliotrope or ragwort).
Copper sulfate is no longer recommended for treatment of water in dams because it can kill crustaceans, fish and aquatic life. Blocks may be used to treat water in troughs, but these can give the water a copper taste that livestock will refuse to drink. It may also cause toxicity in livestock, particularly sheep, and especially if the animals have pre-existing liver damage.
Calcium hypochlorite is no longer recommended for farm dams because large amounts of organic material interfere with the treatment and it can kill crustaceans and fish.