Dissolved oxygen in water is very important for aquatic life. What processes are responsible for the reduction of dissolved oxygen in water?

The large population of bacteria decomposes organic matter present in water. They consume oxygen dissolved in water.If the concentration of dissolved oxygen of water is below 6 ppm, the growth of fish gets inhibited.

Thus, with addition of too many organic substances in water, all the available oxygen is used up. This causes oxygen dependent aquatic life to die. Thus, anaerobic bacteria (which do not require oxygen) begin to break down the organic waste and produce chemicals that have a foul smell and are harmful to human health. Aerobic (oxygen requiring) bacteria degrade these organic wastes and keep depleting the oxygen level in the water.