Dissolved Oxygen- This is how animals
with gills breathe underwater!

The most important gas compound found in natural water sources is oxygen. Humans and other land animals breathe oxygen in the air. Animals with gills, however, obtain the necessary oxygen from compounds dissolved in the water.

When water flows into streams and rivers, it adds oxygen to the water. Mountain streams and large rivers that move rapidly increase the amount of dissolved oxygen available for animals. Water with little movement, or stagnant water, contains much less oxygen. This is why clear mountain streams are important for fish. The rapids in the moving water help to add even more oxygen to it.

The amount of oxygen in the water also depends on the temperature of the water. The colder the water is, the more oxygen it will have. Water can lose some of its dissolved oxygen as it warms up, which can also happen because of eutrophication. Eutrophication occurs due to the nutrients in the water, such as fertilizer or sewage. Extra nutrients cause algae to grow and bloom. When the algae die, they are consumed by bacteria that use up most or all of the dissolved oxygen available.

This can create a hypoxic zone, or low-oxygen zone, an area within a body of water that is deadly to fish and other animals with gills. Keeping water clean and free from pollution benefits not only the people who drink it but also the animals that live in the water.