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How is dissolved oxygen (DO) measured?

Since, the amount of oxygen dissolved in water is small compared to the weight of water, it isn't appropriate to describe the level of dissolved oxygen in terms of a percentage.  Dissolved oxygen is measured in milligrams per liter (mg/L).  One milligram (mg) is 1/1000 (one thousandth) of a gram or 1/1000000 (one millionth) of a kilogram.  1 kilogram of water weighs 1 kilogram and occupies a volume of 1 liter (L).  Therefore, expressing dissolved oxygen in mg/L is the same as using units of parts per million (ppm).  Twelve parts per million (12 mg/L) is the highest amount of oxygen that can be dissolved in water under standard barometric pressures (sea level); 12 mg/L is known as the saturation point.  Zero parts per million (0 mg/L) is the lowest amount of dissolved oxygen in water.

How much dissolved oxygen do organisms need?

It depends on what kind of organism it is.  Generally, aquatic organisms can be divided into two types, cold water and warm water organisms.

Cold water organisms:

These would include fish, such as salmon and trout, and aquatic insects, such as stoneflies and mayfly nymphs.  Generally, these species require a minimum DO (dissolved oxygen) level of at least 6.0 mg/L.

Additionally, these cold water organisms require special conditions when spawning (laying eggs).  Eggs laid by salmon and trout are especially delicate, and the fry (baby fish) that hatch are sensitive as well.  For these fish to successfully reproduce, a DO level above 7.0 mg/L is required.

Warm water organisms: 

These would include fish, such as bass, carp, and catfish, and aquatic insects, such as blackflies and midge larvae.  Generally, these species require less dissolved oxygen than cold water organisms.  If the DO level drops below 4.0-5.0 mg/L, the organisms will become stressed.  In an environment with a low level of dissolved oxygen, the fish will not feed and their behavior will become erratic.  They will seek out water that has a DO level high enough for their requirements.  If they cannot escape the level of low DO, they will eventually suffocate and die.

If dissolved oxygen drops below 1.0-2.0 mg/L, it will result in a fish kill, where large amounts of fish die, and float to the surface.


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