Gas Supersaturation
Nitrogen supersaturation can cause serious fry mortalities and is often a contributing factor to poor performance
Q - What Is Supersaturation?
A - Supersaturation of water is the term used to describe a situation, when the water is holding more gas than it would otherwise naturally contain. 100% saturation (or saturation point) refers to the natural amount of gas that the water contains, most natural surface waters contain around 100% saturation of each gas (variations can be caused by plant life in the water which uses and produces oxygen / carbon dioxide).
If you leave a bowl of clean water standing still it will reach 100%. Water is said to be supersaturated when it holds more than the natural amount of any particular gas. Because nitrogen and oxygen form more than 95% of the gas in water, supersaturation usually occurs with one of these two gasses.
As fish can cope with some oxygen supersaturation, it is nitrogen supersaturation which poses the main danger.
Q - So Why Does Supersaturation Occur?
A - Supersaturation occurs when water is subjected to unnatural forces such as increased pressure and heating. Water can naturally hold more gas at lower temperatures and higher pressures.
For example: When the water is 100% saturated with oxygen at 6°c it contains 11.28mg of oxygen per litre of water. If you then heat the water to 20°c, the water only holds 9.08mg of oxygen per litre when it is at 100% saturated, so the 20°c water now holds more oxygen than it can naturally hold (11.28 mg/l rather than 9.08 mg/l), which results in a supersaturation level of 124%. Given time the extra 24% that the water cannot hold, will diffuse into the air. In extreme cases it can form bubbles on underwater surfaces and then float to the surface (like in fizzy drinks).
The water will eventually return to it's natural 100% level at 20°c. Another example is when air is being sucked into a pump; because of the pressure formed by the pump, the water can hold more gas and so some of the sucked-in air is drawn into solution. This air is mostly nitrogen and causes supersaturation.
