Azolla is a small, native, aquatic-floating fern that grows from 1 to 2.5 centimetres wide. It grows in waterways in dense patches, which can make it look like a green or red carpet. In the shade, it is a bright green colour but it changes to deep red when exposed to the sun. There are similar species: Ferny Azolla (Azolla pinnata) and Red azolla (Azolla filiculoides).

Benefits of azolla

Azolla is beneficial to the aquatic environment in many ways. It:

  • is a food source for waterfowl, fish, shrimp, insects, worms, snails and crustaceans
  • provides habitat for many of these small organisms
  • can discourage blue-green algal blooms as its 'mats' restrict the penetration of sunlight into the water and take up nutrients from the water column, limiting the availability of this food source for the algae
  • can be a form of biological mosquito control, as the mats prevent mosquito larvae surfacing for air
  • can restrict the growth of exotic aquatic plants, including salvinia and water hyacinth, as it limits the availability of nutrients to these plants.

Negative effects of azolla

The presence of azolla in a waterway is generally beneficial. But, in circumstances where waterways are extremely rich in nutrients, extensive growth may be a problem.

It also is possible that thick, complete coverings of azolla can cause de-oxygenation of the water. This can affect fish and other aquatic animals and plants. The decay of these can lead to a strong odour. Situations like this are rare because coverage of the waterway by azolla needs to be almost total for it to have a negative impact on the ecosystem.

Managing azolla

Because azolla is a native species with numerous benefits for aquatic ecosystems, we don't automatically remove it in response to aesthetic concerns.

Gold Coast waterways are typical of aquatic systems in sub-tropical environments. This means that aquatic plant growth is a normal occurrence, subject to seasonal fluctuations. In warmer months when sunlight is plentiful, azolla is likely to be more prolific.

The removal of azolla can actually have negative impacts on a waterway. If azolla is no longer there to take up nutrients from the water column, they become available for blue-green algae, which can bloom in nutrient rich environments. Such blooms are potentially toxic.