Zone of active decomposition
The zone of active decomposition is the pollution zone of the river where the dissolved oxygen is reduced to 0. This zone is characterized by high levels of organic pollution, often caused by the discharge of untreated sewage or industrial effluents into the river.

Factors leading to reduced dissolved oxygen
- In the zone of active decomposition, there is an excessive amount of organic matter present in the water. This organic matter serves as a food source for bacteria, which consume oxygen during the process of decomposition.
- As a result, the bacteria deplete the dissolved oxygen levels in the water, leading to hypoxic (low oxygen) or anoxic (zero oxygen) conditions.
- In the absence of dissolved oxygen, aerobic organisms such as fish are unable to survive in this zone, leading to a decline in biodiversity.

Impact on aquatic life
- The lack of dissolved oxygen in the zone of active decomposition has a detrimental impact on aquatic life. Fish and other aquatic organisms that rely on oxygen for respiration are unable to survive in this oxygen-deprived environment.
- The decomposition of organic matter in the water also produces toxic byproducts, further harming aquatic life in the area.

Importance of monitoring and remediation
- It is crucial to monitor water quality in rivers and identify pollution zones such as the zone of active decomposition to implement remediation measures.
- Remediation strategies may include improving wastewater treatment practices, reducing industrial discharge, and restoring riparian vegetation to absorb excess nutrients and pollutants.
- By addressing the sources of pollution and improving water quality, it is possible to restore the health of the river ecosystem and ensure the survival of aquatic life in the affected areas.