What is Dissolved Oxygen?

Dissolved Oxygen in a stream may vary from 0 mg/l to 18 mg/l. Readings above 18 mg/l are physically impossible.
Dissolved oxygen gets into the water by diffusion from the atmosphere, aeration of the water as it tumbles over falls and rapids, and as a waste product of photosynthesis. What factors affect the DO level?

Reduced DO levels in stream water may be because the water is too warm. The increased molecular activity of the warm water pushes the oxygen molecules out of the spaces between the moving water molecules. Decreased DO levels may also be indicative of too many bacteria and an excess amount of biological oxygen demand - BOD (untreated sewage, partially treated sewage, organic discharges, anoxic discharges) which use up DO. A third reason for decreased DO may be fertilizer runoff from farm fields and lawns. The same fertilizer which was meant to make land plants grow better now makes the aquatic plants do the same. If the weather becomes cloudy for several days, respiring plants will use much of the DO while failing to photosynthesize. When the increased numbers of aquatic plants eventually die, they support increasing amounts of bacteria which use large amounts of DO.

Water Quality Index Chart- Based on Dissolved Oxygen

Water Quality Index and BOD - Biological Oxygen Demand

Students should be aware that plants, in general, only produce oxygen when light is available for photosynthesis. Rooted aquatic plants are more abundant in lakes and impounded rivers than in rivers with significant current or in streams. Large daily fluctuations in dissolved oxygen are characteristic of bodies of water with extensive plant growth. DO levels rise from morning through the afternoon as a result of photosynthesis, reaching a peak in late afternoon. Photosynthesis stops at night, but plants and animals