During which stage of self-purification does oxygen depletion occur due to the action of anaerobic bacteria?

The stage of self-purification where oxygen depletion occurs due to the action of anaerobic bacteria is known as Decomposition. In this phase, organic matter is broken down by bacteria, including anaerobic organisms that thrive in the absence of oxygen. As these bacteria metabolize the organic material, they consume dissolved oxygen in the water, which can lead to significant oxygen depletion.

This depletion is critical because it often indicates that the water is being negatively affected by high levels of organic pollution. When anaerobic bacteria are prevalent, it typically signifies that the conditions have become poor for aerobic bacteria, which require oxygen for their metabolic processes. Thus, during the Decomposition stage, the environment shifts from aerobic to anaerobic conditions, highlighting the challenges in water quality and the necessity for effective waste management practices to restore oxygen levels and support aquatic life.

In contrast, the other stages such as Zone of Degradation, Recovery, and Clean Water each involve different processes and conditions where oxygen levels may stabilize or improve, rather than diminish significantly due to bacterial activity.