What is the importance of oxygen in water for living organisms?
Consider that most expressive part of the entire biodiversity on Earth (plants, animals, fungus, protozoan and monera) depends on oxygen to survive.
Although there's comparatively very few anaerobic forms of life, they're restricted by its own biochemical ways to obtain energy and to live on physical environments that lacks oxygen.
The water of an ordinary lake, as example, is a complex mixture constituted by water itself (
As most living beings depends on oxygen to breath and live, the quality of water on our planet depends too on
Oxygen itself can stimulate decomposition, and it's widely used to recompose some polluted rivers and lakes by injection of the gas inside water!
So, considering all the aerobic biota living inside water and remembering that aerobics is a metabolic trait which derives itself, evolutionary speaking, from anaerobic biochemical basis, all of us that breathes oxygen depends not only from well provided oxygen environments, but good quality of water in other various above mentioned aspects. That's why we must preserve and take care about natural sources of water, not only plumbed/treated water.