Which stage of self-purification describes the situation where dissolved oxygen increases and fish populations return?

The stage of self-purification characterized by an increase in dissolved oxygen and the return of fish populations is known as the recovery stage. During this phase, the natural processes in the water body begin to heal itself from previous pollution or degradation.

As organic matter is broken down and nutrients are utilized, the levels of dissolved oxygen start to rise again. This improved oxygenation supports the resurgence of aquatic life, including fish that require certain oxygen levels to thrive. In a healthy ecosystem, this stage signifies that the environment is becoming more balanced and resilient.

The presence of fish populations indicates that the aquatic environment is not only recovering from pollution but is also becoming sustainable for various forms of life. A healthy balance of oxygen levels allows for diverse biological communities to re-establish themselves.

In contrast, earlier stages such as the zone of degradation and decomposition are characterized by lower oxygen levels and a decline in aquatic life due to pollution. The clean water stage represents a final stage where the water body has returned to its natural, uncontaminated state, but the recovery phase specifically highlights the process of improvement and the signs of life returning to the ecosystem.