Another water quality test that is generally undertaken is dissolved oxygen levels. Dissolved oxygen basically refers to the amount of dissolved oxygen in the water that is available to aquatic life. Testing these levels is important because of its influence on the organisms living within that body of water. When these levels are too high or too low aquatic life can become affected, as can the water quality.
All forms of aquatic life depend on dissolved oxygen, including plants, fish, invertebrates and bacteria. Aquatic life that lives on the ocean floor or in deeper waters need less dissolved oxygen, while those living in shallow waters need more. It might surprise you to learn that organisms responsible for decomposing organic matter at the bottom of the ocean, such as bacteria and fungi, also use dissolved oxygen to help them do their work.
In a stable body of water with no stratification (a system or formation of layers), dissolved oxygen will remain at 100% air saturation. However, the deeper you go, the less dissolved oxygen you will find. This is primarily due to the fact that these deeper waters have not been in contact with air. The actual amount of dissolved oxygen will vary depending on things like water temperature, salinity and pressure.