Aquatic organisms require oxygen to survive. Dissolved oxygen is affected by many other parameters, such as water temperature (colder water = more oxygen) and by the presence of nutrients such as nitrogen and phosphorus (found in sewage and fertilizers). These nutrients cause bacteria and algae to grow rapidly, reducing the amount of oxygen available for other aquatic life.
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Water Quality
Dissolved Oxygen (DO) is found in microscopic bubbles of oxygen that are mixed in the water and occur between water molecules. DO is a very important indicator of a water body's ability to support aquatic life. Fish "breathe" by absorbing dissolved oxygen through their gills. Oxygen enters the water by absorption directly from the atmosphere or by aquatic plant and algae photosynthesis. Oxygen is removed from the water by respiration and decomposition of organic matter (ref).
Chemical Oxygen Demand:
A measure of the oxygen-consuming capacity of inorganic and organic matter present in water or wastewater. The COD test is used to determine the degree of pollution in an effluent (ref).