The causes of oxygen depletion
All living organisms depend on water supply and many of them live in water. Most of these aquatic animals respire aerobically and so require oxygen from their environment. Any change that alters the amount of oxygen in the water can seriously affect the suitability of the water as a habitat. The two pollutants that most often reduce oxygen in water are:
1. Fertilizers- nitrates and phosphates are added to the soil by farmers. Some of the fertilizer is washed from the soil by rain into the nearest pond, river or lake. This process is called leaching.
2. Sewage- this contains an excellent source of organic food for bacteria, and also contains phosphates from detergents.
How fertilizers and sewage affect oxygen concentration?
Water that contains few nutrients is rich in oxygen and supports a wide variety of living organisms. The oxygen enters the water from the atmosphere by diffusion and from photosynthesizing aquatic plants. Simpler forms of life, such as algae and bacteria, are controlled because the low concentration of nutrients such as nitrate is a limiting factor for their growth. If more nutrients are made available, from fertilizer runoff or from sewage, then:
1. Algae and other surface plants grow rapidly and block out light to plants rooted in the bottom of the river or pond. In result, the rooted plants die, and their bodies provide even more nutrients.
2. The population of bacteria increases expeditiously as they multiply, the bacteria consume oxygen for aerobic respiration. There is now a biological demand in the water because of consumed oxygen by these microbes.
3. Other living creature cannot obtain enough oxygen. They must leave the area if they can, or they will die! Their bodies provide even more food for bacteria, and the situation becomes even worse. This is an example of positive feedback- the greater change from ideal conditions.
Important rivers such as the Indus and its tributaries are affected by the uncontrolled inflow of fertilizers. The process is called eutrophication. The pond or river soon becomes depleted of living organisms. Only a few animals, such as “Tubifex” (sewage worms), can respire at the depressed oxygen concentration that is available.
The solution to this problem is straightforward- do not allow excess nutrients into the water.
1. Unnaturally high levels of nutrients:
2. From leaching of fertilizers
3. From the input of raw sewage
4. From liquid manure (slurry) washed out of farmyards.
Depleted oxygen levels in water cause the death of fish and most invertebrates. Moreover, high nitrate levels can be perilous to human babies.
1. Treat sewage before it enters rivers
2. Prevent farmyard drainage entering rivers and ponds
3. Control use of fertilizers:
a. Apply only when crops are growing
b. Never apply to bare fields
c. Do not apply when rain is forecast
d. Do not dispose of waste fertilizer into rivers and ponds
4. Bubble a stream of air through inadequately polluted ponds
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