Overview of Biological Cycle
The bacteria are the keys to the normal biological cycle. It is the role of the bacteria to convert the soluble organic matter into bacteria cells and inorganic elements. The inorganic elements are taken by the algae and converted into algae cells. The newly formed bacteria and algae become food for the protozoa, rotifers, and crustaceans.
The animal forms and some of the larger algae and bacteria become food for the minnows and tiny fish. The small fish become food for the large fish which become food for man. Man discharges his wastes back into the stream with which a bacteria metabolize the organic matter and completes the cycle. Thus it is that waste charge into streams which is a normal part of the biological cycle. Without the organic waste there would be little fish for man to remove from the streams.
While a certain amount of organic waste is necessary for the normal biological cycle, too many wastes can destroy it. As the organic waste concentrations increase, the bacteria growth is stimulated with a greater and greater demand for oxygen. As the bacteria decrease the oxygen level, the higher forms begin to die off. Sensitive game fish are the first to feel the effect of oxygen depletion, followed by the normal game fish and finally by the crustaceans, rotifers, and higher protozoa. The bacteria remain as the sole form of biological life. The primary problem in river pollution lies in the low solubility of oxygen in the water.
At 20°C, only a little over 8 mg/liter of oxygen will exist in water. As the temperature rises, the oxygen solubility decreases and vice versa. This phenomenon is not good because the rate of the biological activity and hence the rate of demand for oxygen increases with increased temperature and decreases with decreasing temperature. In swiftly moving streams the oxygen used in metabolism is replenished quite rapidly. The water surface at the air-liquid interface is being continuously replaced by the turbulence of the stream movement.
The oxygen saturated layer is overturned and replaced by oxygen-deficient layer. In this way, the oxygen-saturated liquid is rapidly mixed throughout the stream to supply the microorganisms with sufficient oxygen. The microorganisms which exist in any body of water are direct indicators of the condition of that water. The types and numbers of microorganisms are all a direct result of the nutrient concentration. It is impossible to have a high nutrient concentration and a low biological population in nature, or vice versa unless there is a deficiency of a prime element or a toxic condition.