For information on mitigation of low-oxygen blackwater with water for the environment in 2022-23:
Blackwater occurs when organic material, such as sticks, leaves, grass or crops are washed off the floodplain and into rivers and creeks. As the organic matter decays, it releases carbon into waterways, creating ‘blackwater’.
Blackwater provides crucial food for water bugs, native fish, and crays, and plays an important role in a healthy river. The black colour is due to high levels of tannin in the water, like when you make a cup of tea.
What is low-oxygen blackwater?
Low-oxygen (hypoxic) blackwater is water that has become low in dissolved oxygen. It occurs when large amounts of organic material in rivers are broken down by bacteria. These bacteria multiply and use up the oxygen in the water – oxygen that fish need.
As the weather warms up, bacteria multiply more quickly, increasing the risk of water becoming low in oxygen. Warmer water also holds less oxygen than cool water.
If dissolved oxygen levels fall too low, fish and other aquatic animals can die. Native fish and crustaceans are especially vulnerable to low oxygen water.
Low-oxygen blackwater doesn’t last forever. After an initial drop of oxygen levels, the organic material bacteria feed on is used up, bacteria die back and oxygen levels in the water start to return to normal. This could take days to weeks.
What can be done to help native fish when water becomes low in oxygen?
There are limited options to reduce the impact of low-oxygen blackwater on native fish and aquatic life.
In times of flood, the scale of high river flows means it not possible to dilute the effects of low-oxygen blackwater using water for the environment. This is because the volume of water in large floods is too great.
In some cases, oxygenated water can be delivered to small sections of a river to create refuge areas of better-quality water. Native fish and other animals able to move to these areas will improve their chances of survival.
Delivering oxygenated water depends on the availability of good quality water and whether it can be delivered quickly to the affected parts of the river system. This can be difficult during flood conditions.