They can adversely affect the appearance, quality and use of the water. The water may be discoloured green, blue-green or greenish brown and several species can produce musty, earthy or grassy odours.
Blooms can also cause foaming on the shoreline which is sometimes confused with sewage pollution.
When blue-green algal blooms die and decay, they use up oxygen in water which can kill fish and other aquatic life.
Some blue-green algae can produce toxins. In high enough concentrations these toxins can kill wild animals, farm livestock and domestic pets. They can also be harmful to human health.