Dry season conditions present an opportunity for landholders to carry out important maintenance on dams.
Department of Agriculture and Food development officer Tilwin Westrup said low dam levels increased the risk of algal blooms in the south-west over summer.

The release of nutrients stored in silt into the water is part of the reason some dams in the south-west may already be experiencing algal blooms.

“While it’s unfortunate that dam levels are low, it does provide an opportunity for landholders to clear dams of silt,” Mr Westrup said.

“Because the silt has been releasing nutrients into the water and there is less water in dams, nutrients are reaching critical concentrations early this year. Shallow dams also warm up quickly, and this adds to the chance of blooms.”

The most visible algal blooms in fresh-water dams are produced by blue-green algae, and some of these can produce toxins, usually in warm, calm, shallow bodies of water.

“A high concentration of nitrogen and phosphorus is of particular concern,” Mr Westrup said.

“With the dam levels so low, many farmers should be able to get machinery access so they can de-silt the bottom of dams. Just be careful not to compromise the seal of the dam, as this can cause it to leak.

“Not only will removing silt help clear up the algae problems in future seasons, but it will also improve the dam’s ability to retain water, as increased depth reduces evaporation losses.

“Some property owners may also want to make their dams deeper, which can reduce the incidence of algal blooms. If they choose to do so, care should be taken to ensure a good seal is maintained, and they stay well clear of any saline groundwater.”

Silt should be disposed of well clear of the dam and drainage line, otherwise it may end up back in the dam or into the next dam downstream. Silt may be spread out on poor soils (gutless sands) to improve fertility. Have the silt tested for nutrients and contaminants before spreading to see if this is worthwhile.

If farmers are unable to perform this type of maintenance, and the blooms persist, there are some solutions.

“If it’s a water tank, the tank can be covered to exclude light and reduce the temperature,” Mr Westrup said.

“Nutrient levels in dams can be reduced by screening systems, by restricting, or increasing efficiency of fertiliser use in the immediate catchment area.

“There are a range of chemical treatments available for algal blooms. Have the algae tested for toxicity before treating. Sudden death of a toxic algal bloom will release toxins into the water, risking stock poisoning. DAFWA recommends 14 days withholding for stock after toxic blooms are treated.”

Further information can be found on the department’s web site www.agric.wa.gov.au

Acknowledgement: This information is courtesy of the Department of Agriculture Western Australia