In the same way land animals breathe oxygen from the air, aquatic animals such as fish and bugs use dissolved oxygen in the water to “breathe”. When the amount of dissolved oxygen in the water is low it can harm these animals.
What is dissolved oxygen and how is it measured?
Dissolved oxygen (DO) is oxygen held (dissolved) in the water and available to aquatic organisms.
The amount of dissolved oxygen in a river or stream can tell us a lot about its water quality.
Water will naturally contain a certain amount of dissolved oxygen that is absorbed from the air and produced by plants and algae living in the water.
Temperature has a large effect on the amount of oxygen dissolved in water; cold water can hold higher levels of oxygen than warmer water. Higher water temperatures over summer will cause oxygen levels to drop.
Other factors such as river flow, wind, nutrients and bacterial activity can also affect the amount of dissolved oxygen in waterways.
Dissolved oxygen concentrations are measured using scientific equipment and sensors. The sensors can be automated and provide information on dissolved oxygen levels in real time using the mobile network.
The sensors provide information on the amount of dissolved oxygen in the units of Parts Per Million (ppm) or milligrams of oxygen per Litre of water (mg/L). Dissolved oxygen levels typically range between 5 and 14 mg/L (or ppm).
What is blackwater?
'Blackwater' is formed when high levels of organic material such as sticks, leaves, bark, grass and crop residue are washed into the river system, as occurs after flooding.
Carbon and other nutrients, particularly nitrogen and phosphorous, leach out of the organic material (appearing as tannins), which can make the water very dark or black.
A similar process occurs when you make a cup of tea and the tea leaves are covered in hot water.
The carbon and nutrients within blackwater provide crucial food for small water bugs that then provide food for larger fauna like native fish and crayfish. Blackwater therefore plays an important role in a healthy river.
What is low-oxygen blackwater?
Low-oxygen conditions occur when water has become low in dissolved oxygen, typically less than 4mg/L for an extended period of time.
It occurs when bacterial numbers get too high, often in response to large amounts of organic material and nutrients being present in the water.
The bacteria uses up more oxygen in the water than is being produced, causing oxygen concentrations to decline.
In warmer weather, bacteria multiply more quickly, increasing the risk of water becoming low-oxygen. Warmer water also holds less oxygen than cold water.
How does low-oxygen blackwater affect aquatic animals?
If dissolved oxygen levels fall too low, fish and other aquatic animals can die. Native fish and crustaceans are especially vulnerable to low dissolved oxygen concentrations.
The potential response of fish and aquatic animals to different dissolved oxygen levels is shown below.
See the video below for an explanation of the process.
What are the current dissolved oxygen levels?
There are several stations measuring dissolved oxygen in waterways across the Goulburn Broken catchment.
The map below shows the location of the sensors and current level of dissolved oxygen.
Click on a site to bring up a chart showing dissolved oxygen levels for the last 30 days. An example is shown below.
Note: If you are using safari or having difficulty accessing the data, please use this link on a different browser Link
How to read the data and its relationship to water quality is shown below. Further information on the relationship between dissolved oxygen and animal health is described in the image below.
Disclaimer: Data is operational and updated hourly. It may contain errors and is subject to revision. Dissolved oxygen probes are sensitive and may provide inaccurate data following floods, storms or if algae is present on the probe.
What can we do?
River managers, scientists and environmental water holders monitor rivers for water quality issues and work to reduce the impacts of low-oxygen blackwater where possible.
In river systems with water storages, weirs or channels, water with good dissolved oxygen levels can be delivered to dilute low-oxygen water or provide refuge habitat for aquatic animals.
Water for the environment is one source of water that can be used for these purposes.
Up to twice the amount of good quality water is required to dilute low- oxygen blackwater. During times of flood or high flows it may not be possible to achieve this outcome. However small volumes of water may be delivered from irrigation areas to assist with dilution and provide small refuges for aquatic animals. This depends on the availability of good quality water in the right location and flood conditions.
Small numbers of native fish and crayfish can also be moved to waterways with better quality water or to facilities such as hatcheries and returned to waterways once conditions improve.
Aerators have only limited use during low-oxygen events to create small pockets of local refuge and do not have a lasting effect as replenished oxygen is often quickly used up again by bacteria.
Please report fish death to the Victorian Environment Protection Authority on 1300 372 842.
Report Murray River fish dish deaths to the New South Wales Department of Primary Industries Fisheries, Fishers Watch phoneline on 1800 043 536.
Further information on options available to help mitigate low-oxygen blackwater and can be found here Link.