Learn about series
A series of information pages on the basics of water.
What is hypoxic water?
Hypoxia means low oxygen. When water sources like rivers or lakes have very low oxygen, we say they have hypoxic water. To be called hypoxic, water must have less than 2-3 milligrams of oxygen per litre of water, or 2-3 parts of oxygen per million of water.
There is usually oxygen in water because water absorbs oxygen from the air. Aquatic plants and algae also produce oxygen during the day during photosynthesis. But at night, oxygen levels lower when plants and algae stop producing oxygen. This can mean water is hypoxic in the morning before oxygen levels increase again.
Why does it matter if water is hypoxic?
Fish and other aquatic animals need oxygen from the water they live in to survive. Some native fish and crustaceans are very vulnerable to oxygen deprivation. If oxygen levels in water drop far enough, aquatic animals can become stressed, or even die.
When does hypoxic water occur?
What is hypoxic water and why does it occur?
This video explores the causes of hypoxic water and what we can do to manage it.
Low or no flow, high flow, or flooding conditions in our rivers can all lead to hypoxic water.
Low or no flow river conditions often occur during dry or hot summer periods, or droughts. In these conditions, rivers may disconnect into pools. When fresh inflows holding oxygen can’t reach these pools, oxygen levels become low.
In deep river and lake pools, sometimes there can be oxygen in the water closest to the surface, but none in the water at the bottom. If the weather changes, wind or rain can mix the low oxygen water at the bottom of the pool into the rest of the pool. This lowers the oxygen in the whole pool.
During times of high river flows or floods, dead leaves and other plant material wash from the floodplain and riverbank and into the river. Bacteria break down this material, using up oxygen in the process. This lowers oxygen levels in the water.
What is hypoxic blackwater?
‘Blackwater’ is a term used when high levels of plant material, such as leaf litter, wash into a river and discolour the water making it appear black. This can then cause the water to become hypoxic.
Most hypoxic blackwater events happen after prolonged dry periods, when temperatures are warm and large amounts of plant material build up on riverbanks and floodplains. When this material is washed into waterways in times of flood or high flows, more oxygen is used by the bacteria breaking it down.
A sudden drop in oxygen may harm aquatic life. But these natural events are an essential and valuable part of river system health. The plant materials which wash into rivers break down into nutrients and become food in the river ecosystem. This helps to support animal breeding cycles, and is very important after long dry periods or drought. In the long term, the river system and its living creatures benefit.
What are the risks to humans?
Risks to human health are low if direct contact with hypoxic blackwater is avoided. Thorough cleansing is advised after any contact with affected water, and discoloured or dead fish should not be eaten because of possible health risks.
Hypoxic blackwater may have social and economic impacts related to the higher costs of treating water for consumption and short-term loss of amenity and recreation opportunities. If suitable town water treatment is not available, boil water alerts may need to be issued for some towns.
Where can it occur?
Water can be hypoxic over large areas. Low or no flow, high flow, or flooding conditions in our rivers can all lead to hypoxic water.
How do we manage hypoxic water?
Because water can be hypoxic over large areas, it is tricky to manage. Diverting river flows carrying large amounts of plant materials away from pools can help prevent water from becoming hypoxic. We can also release available water from upstream storages to dilute the hypoxic water.
In small areas, like weir pools, we can use aerators to increase oxygen nearby. Aerators work by either mixing in more oxygen at the surface, or by spraying or bubbling oxygen into the water, where it dissolves. However, this only affects oxygen levels in a very small area and requires energy to operate.
We also watch rapid increases in algae in case they become an issue.
By maintaining adequate river flows and keeping rivers connected, we reduce the risk of hypoxic water in our rivers. This helps to keep rivers healthy and maintain healthy habitats for all plants and animals.
You can help by reporting changes in water quality, such as a change in colour or odour or the appearance of dead fish, or fish gasping for air, to the Fishers Watch Phoneline on 1800 043 536.