Oxygen gas dissolves freely in freshwater. Thus, oxygen from the atmosphere as well as that produced as a by-product of photosynthesis may increase the dissolved oxygen concentration in water. Its distribution is dependent upon atmospheric contact, biological activity, wave and current actions, thermal phenomena, waste inputs, and other characteristics of a lake or stream. High levels of oxygen are likely in surface water on windy days. Dissolved oxygen levels are also temperature and pressure dependent.
Cold water holds more oxygen than warm water.
There are biological processes in water that consume oxygen such as respiration by organisms and decomposition of organic matter by microorganisms. When demand for oxygen is high and oxygen production from photosynthesis is not occurring such as before sunrise, dissolved oxygen readings can be low. As photosynthesis contributes to an increase in dissolved oxygen levels during the day, deep areas of a lake would be expected to yield low dissolved oxygen readings. Since an adequate supply of oxygen is necessary to support life in a body of water, a determination of the amount of oxygen provides a means of assessing the quality of the water with respect to sustaining life. A standard chemical method to determine the amount of oxygen dissolved in a water sample is a type of titration, the Azide Modification of the Winkler Method.