They usually are present in low numbers. Blue-green algae can become very abundant in warm, shallow, undisturbed surface water that receives a lot of sunlight. When this occurs, they can form blooms that discolor the water or produce floating rafts or scum on the surface of the water.

The best way to avoid the problems associated with cyanobacterial blooms is to prevent blooms from forming. This can be done by reducing the input of nutrients, such as phosphates, into the water source or by aerating the water.