Oxygen is used by organisms in aerobic respiration, where energy is released by the combustion of organic matter. Biological oxygen demand (BOD) is a measure of the amount of oxygen used by aquatic organisms, such as bacteria and other microorganisms, in the aerobic oxidation of sugar.

When leaves fall into the water or aquatic organisms die, they are fed upon by decomposers. Nutrients, such as nitrates and phosphates, are released into the water and stimulate plant growth. Although this process requires oxygen, it is usually replenished by the diffusion of oxygen into the water and the photosynthetic activity of producers.

Often in still waters that collect organic debris, the amount of dissolved oxygen (DO) falls below 90% of its maximum. If the amount of oxygen required by the decomposers is excessive, the amount of DO may even fall below 50% of the maximum. This process can be detrimental to the aquatic ecosystem, as insects and fish may die of suffocation. This often happens when human sources of organic nutrients are discharged into water. Point sources of pollution often include paper mills, meat packing plants, and waste treatment facilities. These are usually easy to identify, as one can test upstream and downstream from a suspected site.

Bacteria thrive in bodies of water with high BOD levels. They limit the survival of other organisms that require substantial amounts of oxygen, such as caddisflies, stoneflies, and mayflies. This reduces the diversity of organisms and increases the number of pollution-tolerant organisms in the watershed.

The BOD is determined by measuring the amount of DO consumed by organisms in a sample of water over a period of five days.

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