Algae control in dams

Department of Agriculture and FoodCountryman
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This year is going to be bad for algal growth in dams. The Department of Agriculture and Food (DAFWA) has already received reports of property owners experiencing algal blooms — weeks ahead of usual reports.

Algae occurs most often in a calm body of water with high rates of nitrogen, phosphates, carbonates and organic matter.

Add high temperatures, plenty of light, shallow depth and you have the concoction to produce an algal bloom. Some of these algal blooms are toxic, and blue-green algae species can present a health hazard to humans and stock and can result in unsightly smelly water.

Identification is important because not all algaes are toxic. Knowing the algal species that is present in your dam or soak and its toxicity before treatment are paramount.

Identification of algae samples can be carried out at DAFWA’s office in South Perth. For more details of where else you can get samples identified, contact DAFWA. Samples should be fresh and in a small jar. Treatment option one: Simazine

Simazine is the most popular form of treatment for algae control in dams. It is both effective and will continue to kill algae for several days after application.

A word of caution — it also kills other plants (algae are plants too) and can be harmful to aquaculture, so treated water should not be used to water the garden for at least seven days after applying.

Treatment of algae may also cause a release of toxins into the water.

It’s recommended, therefore, that the water should not be used for stock for at least two weeks.

Simazine also comes in various other trade names, so it is important to check labelling for the chemical strengths (for example, 25 per cent, 50 per cent, 90 per cent).

The amount of chemical required per 1000 litres of water (1 cubic metre) for different strengths are:

Premix the chemical in a drum and then apply with a firefighting unit.

Treatment option two: Barley straw

Limited research papers have highlighted that the presence of rotting barley straw can significantly reduce algae in dams and small reservoirs. Algae control by this method appears to be not related to nutrient removal but more due to:

•Biological activity, by encouraging phytoplankton grazing organisms;

•Chemical activity, by releasing a cocktail of relevant levels of phytotoxic compounds near to the decomposing straw.

Barley straw should be teased out of the bale and spread on the water at a rate of 100g per 1000 litres of water. These rates have been shown to be effective.

Additional top up of barley straw may be required for effective control.

References have also mentioned that barley straw seems to be most effective when it is placed in the water several weeks before the expected algal bloom, and not once the algal bloom has appeared.

That said though, some farmers have reported that they have had success using barley straw to treat present visible algal blooms.

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