Blue-green algae in freshwater
Blue-green algae can be considered as simple aquatic plants that occur naturally in habitats such as marine waters, rivers, lakes, damp soil, tree trunks, hot springs and snow. They can vary considerably in shape, colour and size.
Blue-green algae are bacteria
Despite their name, blue-green algae are actually types of bacteria known as Cyanobacteria. In external appearance and requirements for light, nutrients and carbon dioxide, they are similar to algae. They normally look green and sometimes may turn bluish when scums are dying. Taste and odour problems commonly occur with large concentrations of blue-green algae and some species are capable of producing toxins.
Blue-green algae are very small organisms and can be seen with the aid of a microscope as single cells, accumulations of cells (colonies) or filaments of cells (trichomes). Certain types of blue-green algae have tiny gas vesicles in their cells, allowing them to float to the surface or sink to the bottom in response to changing light and nutrient availability. This buoyancy-regulating mechanism gives the blue-green algae a competitive advantage in obtaining light and nutrients.
Blue-green algal blooms
'Bloom' is a common term used to describe an increase in the number of algal cells to a point where they can discolour the water, form scums, produce unpleasant tastes and odours, affect shellfish and fish populations or otherwise create a nuisance and seriously reduce the water quality.
Species of blue-green algae may dominate and increase excessively in water when the:
- nutrient levels, particularly phosphorus and nitrogen are sufficient to support the population growth
- water is still and turbulence is low (lack of mixing)
- weather patterns are stable for a long time
- weather is warm (although blooms can occur in cooler weather too).
Blue-green algal blooms often persist for several weeks, sometimes months, depending mainly on the weather or flow conditions. Cooler, windy weather or increased flow may reduce or prevent blooms from occurring.
As the bloom dies, the cells tend to become 'leaky'. If the bloom contains species that produce toxins, these will be released into the surrounding water. Once released, some toxins may persist for more than three months before sunlight and the natural population of bacteria in the water degrade them.
Fact sheet Blue-green algae (PDF, 127K)*
Fact sheet Managing blue-green algae blooms in farm dams (PDF, 148K)*
* Requires Adobe Reader
Last updated 27 May 2010