Oxygen is vital to the life cycle common to water. It is essential to keep organisms living, to sustain species reproduction, and for the development of populations. Oxygen is soluble in water in direct proportion to the partial pressure in the gas phase and solubility decreases as temperature increases.
Oxygen enters the water by absorption directly from the atmosphere or by plant photosynthesis.
During respiration and decomposition, animals and plants consume dissolved oxygen and liberate carbon dioxide. Organic waste from municipal, agricultural and industrial sources may overload the natural system causing a serious depletion of the oxygen supply in the water. Waters rich in nutrients produce algae in quantity which upon decomposition deplete the oxygen supply. Fish kills are often associated with the process of eutrophication. Dissolved oxygen determination must be done in the field.