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Is That A Fern On Your Dam?

The surfaces of many local farm dams are sometimes partly or completely covered by a floating native aquatic fern called Azolla.

By Department of Primary Industries - 5th April 2006 - Back to News

Azolla plants have snowflake shaped or triangular fronds up to 10mm wide and 30mm long. They may be red or green in colour depending on the amount of sunlight. During spring or in shaded situations the upper parts of the plants are green, whereas in summer and autumn, when exposed to full sunlight, they become dark red. Fine roots up to 4mm long hang in the water below the floating fronds.

Azolla is not harmful to stock that drink the water or consume the plant. Nor does Azolla have a deleterious effect on domestic house and garden water. It can, however, affect aquatic life in the dam. When Azolla forms a dense surface mat it restricts sunlight and reduces oxygen levels in the water. It can also block pipes and pumps and deter stock from watering.

Azolla is nutrient rich and high in protein. The plant has a bacterium living in its leaves, which allows it to use nitrogen from the water and air for it’s own growth. Overseas, Azolla is used as a fertiliser in rice fields and has been used as a feed source for various domestic animals and plants. It has also been scraped off dams and used on vegetable gardens and house gardens as a free fertiliser.

Aquatic plants such as Azolla are important habitat for a range of aquatic life including invertebrates, fish and birds. Azolla may also help to reduce nuisance algal growth by providing refuge for algae grazers, and by reducing the availability of nutrients and light.

If extensive Azolla mats are causing an immediate problem, some can be removed by manually or mechanically scraping the top of the dam. A home made two-person scraper using fine chicken netting is a cheap, practical method. While there are chemicals registered to kill Azolla, the process may have side effects of directly killing beneficial aquatic life, or indirectly through rapid plant decay leading to oxygen depletion in the dam.

Minimising the amount of available nutrients and sunlight will lower the potential for excessive growth of Azolla, algae and other aquatic plants. Where possible:

• Reduce fertilisers entering the dam by leaving an unfertilised area around the dam.
• Leave a buffer area or filter area of vegetation above the dam to minimise entry of nutrients and sediments.
• Minimise sunlight on the dam by planting vegetation on the north and west sides of the dam. Do not plant trees on the dam embankment.
• Restrict stock access to dam to prevent disturbance to banks and sediments, and to stop stock defecating and urinating in the water. Provide alternative drinking water via troughs.
• Occasionally de-sludge the dam to remove accumulated nutrients.

Source: http://gippsland.com/

Published by: news@gippsland.com

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