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Dissolved Oxygen


Dissolved oxygen is one of the most important water quality parameters when culturing any species. All stock requires oxygen to live and maintain body functions, although the amount of oxygen required is species dependent to some extent. Dissolved oxygen is a measure of how much oxygen there is in the water and is measured in parts per million (ppm), milligrams per litre (mg/l) or as percentage saturation.

In ponds and Dams oxygen enters the water via two main pathways.

  • Oxygen diffuses into the top surface layer of water from the air, and this rate will be increased by wind and wave action (increasing the surface area for diffusion to take place).
  • Oxygen is also released into the water as a result of photosynthesis, where algae in the water use carbon dioxide, water and sunlight to produce simple sugars and starches. A product of this reaction is the generation of oxygen, which dissolves in the water and increases the dissolved oxygen level.

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During the hours of daylight, plants actively produce oxygen due to photosynthesis. The rate of photosynthesis is directly related to the amount of light energy received. So during the day, oxygen levels will build and reach peaks at about 2pm or 3pm according to the season and weather. After sundown, oxygen levels drop through the night until photosynthesis begins again at day break.
In a sterile pond without plants, this will not happen. In a pond with a heavy plant load, the DO swings will be large. The DO drop will be greater as well because the large plant population will be net users of oxygen, when there is no light.
The danger period for fish, when the DO levels will be at their lowest, will be at day break just before photosynthesis begins.


This effect is not so great in smaller systems such as in aquaponics however depending on the presence of algae there may be a marginal effect. In aquaponics systems it is use of an air pump with an air stone in the fish tank be maintained to ensure sufficient air / oxygen .
OR
Depending on the stocking density in your system it may be as simple as making sure that as water is moved around
the system and sufficient height is provided to enable aeration of the water as it contacts surfaces.



Almost all of the oxygen dissolved into the water from an air bubble occurs when the bubble is being formed. Only a negligible amount occurs during the bubbles transit to the surface of the water. This is why an aeration process that makes many small bubbles is better than one that makes fewer larger ones.

For example In a pond situation a "sheet" type waterfall can provide more dissolved oxygen in a pond than the "cascade" type waterfall whose velocity is low when the water finally enters the pond. Although the cascade type waterfall provides better aeration of the water that is entering the pond, the sheet type provides better aeration of the water that is already in the pond.


The sheet of water tends to shear the larger bubbles of air formed at surface entry into smaller ones below the surface. This action can occur at depths of up to 1m or more and result in oxygen transfer to a much larger amount of water than just that which is entering the pond. For most situations, the amount of water flow is determined by filtration requirements and either type will be more than sufficient to maintain the pond oxygen levels at or near saturation.

A common method of providing additional oxygen to the water is through the use of an eductor type air jet (sometimes called a venturi). An added advantage of this device is that it can simultaneously provide improved circulation of the tank water.

Air stones or similar bubble forming devices driven by an air pump can also be used to provide supplemental air. A single air stone can supply sufficient air for up to a 4000L tank.

Dissolved Oxygen (DO) is temperature dependent -
The graph below shows the dissolved oxygen values in degrees Celsius - note that because DO is temperature dependent, as the temperature rises, the DO decreases.

At 10 oC the water will hold up to 11.3 ppm DO at saturation whilst at 20oC it will hold 9.09 ppm DO and at 25oC it will hold 8.23ppm DO at saturation. Saturation is the maximum DO that water will contain - it will not absorb any further Oxygen no matter how much is injected.



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During periods of oxygen stress, due to low DO’s, fish may be seen congregating near the surface gulping at the surface water. The surface waters will generally contain the highest DO at this time.

Similarly, fish will congregate around recirculating & filter pumps, air stones and the like , when DO’s are low.



Imagine you have a 5 m deep pond that is stratified, the surface I m of the pond is 30 C warmer than the lower water. The pond is stocked with fish that are growing well in the surface 1 m but there is no oxygen in the lower 3 m. One day it is overcast all day and in the afternoon there is a massive thunderstorm with a lot of hail. The area around your pond is carpeted with large hail stones. The hail has cooled the surface water by 5 degrees.

What do you think might happen to your pond water and fish?