One of the most important measurements in the determination of the health of a body of water is its dissolved oxygen content. The quantity of dissolved oxygen in water is normally expressed in parts per million (ppm) by weight and is due to the solubility of oxygen from the atmosphere around us. The atmosphere consists primarily of nitrogen and oxygen and since both of these gases are soluble in water, they are present in varying amounts in all natural or manmade bodies of water. These amounts are proportional to the solubility and partial pressure of the two gases.
Oxygen is the gas that attracts the greatest amount of interest, simply because of its role in the cycle of all living organisms and, specifically for our interest, those bacteria that feed on organic waste. For this reason, it is the gas that is most commonly measured in aquatic environments. It is important to realize that the term "dissolved oxygen" refers to gaseous oxygen dissolved in water, and it should not be confused with combined oxygen as found in the water molecule, H2O.