Question

The graph given below represents changes in different ecological parameters due to effluent mixing in a stream. The three lines A, B and C represent

• A. A : oxygen concentration, B : biological $O_2$ demand, c : pollution resistant species
• B. A : pollutant, B : aerobic process, c : anaerobic process
• C. A : oxygen concentration, b : ${CO}_2$ concentration c : temperature
• D. A : phosphate concentration, B : nitrate concentration, C : rate of photosynthesis.

Answer 1

The correct option is A A : oxygen concentration, B : biological $O_2$ demand, c : pollution resistant species

Effluent discharge mixing into a river will result in rise of BOD because decomposer organisms consume a lot of oxygen. If sewage quantity is large, the whole of dissolved oxygen may be consumed leaving nothing for respiration of fish and other clean water organisms. They, therefore, get killed. However, as sewage is decomposed, there is a gradual rise in dissolved oxygen downstream. Fish and other clean water organisms reappear indicating the recovery of river from sewage pollution. Meanwhile, number of pollution resistant species increases in the stream.

So, the correct answer is 'A : oxygen concentration, B : biological O2 demand, c : pollution resistant species'.