Due to biochemical oxygen demand (BOD), the dissolved oxygen (DO) in a water body:

As biochemical oxygen demand rises, dissolved oxygen in the water body decreases.

Microbes consume the dissolved oxygen while decomposing organic matter, lowering it.

It does not increase, since the oxygen is being used up, not added.

It does not stay unchanged, as the microbial demand steadily draws it down.

It does not rise first and then fall, the effect is a decline.

Very low dissolved oxygen makes water unfit for fish and aquatic life.