Dissolved Oxygen in Water Essay example
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 …show more content…
The amount of DO an aquatic organism needs depends upon its species, the temperature of the water, pollutants present, and the state of the organism itself (adult or young, active or dormant).
A trout needs five to six times more DO when the water temperature is 24 degrees C (75 degrees F) as compared to when the water temperature is 4 degrees C (41 degrees F). The increased DO is needed to support an increase in metabolic rates - a phenomenon shared by other cold-blooded aquatic animals.
The generally accepted minimum amount of DO that will support a large population of various fishes is from 4 to 5 mg/l. When the DO drops below 3 mg/l, even the hardy fish die. Keep in mind that even though there may be enough DO to keep an adult alive, reproduction may be hampered by the need for higher DO for eggs and immature stages. Depletion in DO can cause major shifts in the kinds of aquatic organisms found in water bodies. Species that cannot tolerate low levels of DO - mayfly nymphs, stonefly nymphs, and beetle larvae - will be replaced by a few kinds of pollution-tolerant organisms, such as worms and fly larvae. Nuisance algae and anaerobic organisms (that live without oxygen) may also become abundant in waters with low levels of DO.
The following will give you some idea of how