Algae are a natural part of the ecosystem of rivers and estuaries. They oxygenate the water and contribute to the production of organic matter through photosynthesis.
This is the basis of aquatic food webs and along with macrophytes and organic material that falls into waterways from fringing vegetation supports all other aquatic organisms such as macroinvertebrates, fish and water birds.
In most waterways the composition of the algae species present undergoes seasonal cycles as different species have different requirements in terms of light, salinity, temperature and nutrients. Where the natural balance in a waterway has been altered, generally by an increase in nutrient levels or a decrease in flow, algae may respond by growing rapidly and forming blooms.