Dissolved Oxygen is affected by the presence of organic matter.

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Water absorbs oxygen to the point where partial pressure in air an water are in equilibrium.

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The solubility of oxygen in water depends on atmospheric pressure ( and there on altitude ), and on the temperature and mineralisation of the water ; saturation with O2 decreases as temperature and altitude increase.

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This parameter is used essentially for surface water.

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At sea level, at 20°C, the concentration of oxygen in equilibrium with atmospheric pressure is 8,8 mg/l of O2, or 100 % saturation.

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Highly aerated water ( e.g. mountain streams ) is generally supersaturated with oxygen, whereas water charged with organic matter degradable by micro-organisms is sub-saturated.

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A significant presence of organic matter in a water body, for example, allows micro.organisms to develop and consume the oxygen.

 

Dissolved Oxygen is therefore a useful parameter in the biological diagnosis of the aquatic environment. 

 

Analysis should be done in-situ.