Dissolved oxygen (DO) is important for

Dissolved oxygen matters most for river water, where it sustains aquatic life and self-purification.

Drinking water quality is judged more by microbes and chemical safety than by dissolved oxygen.

Rain water is naturally well aerated and not assessed on this parameter.

Underground water sits low in oxygen and is not evaluated on it.

In rivers, a healthy dissolved oxygen level supports fish and breaks down organic waste.

Falling dissolved oxygen is a classic sign of organic pollution in flowing water.