Assertion: There is a sharp decline in dissolved oxygen downstream from the point of sewage discharge.
Reason: Microorganisms involved in biodegradation of organic matter in the receiving water body consume a lot of oxygen.

Both Assertion (A) and reason (R) are correct and R is the correct explanation of A.

Domestic sewage is rich in biodegradable organic matter and the decomposition of this organic matter by microorganisms requires oxygen. Microorganisms involved in biodegradation of organic matter in the receiving water body consume a lot of oxygen, and as a result there is a sharp decline in dissolved oxygen downstream from the point of sewage discharge. If sewage quantity is large, the whole of dissolved oxygen may be consumed leaving nothing for respiration for fish and other clean water organisms. They, therefore, get killed. However, as sew age 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 discharge.