Background Info
Dissolved oxygen refers to the level of free, non-compound oxygen present in water or other liquids. Dissolved oxygen is what keeps streams and lakes healthy. The dissolved oxygen level can show how polluted the water is and shows how well the water can support aquatic plant and animal life. Typically, a higher dissolved oxygen level means better water quality. If dissolved oxygen levels are too low, some fish and other organisms may not be able to survive. Most of the dissolved oxygen in water comes from oxygen in the air that has dissolved in the water. Other levels of dissolved oxygen in the water are from photosynthesis of aquatic plants. Water temperature also affects dissolved oxygen levels. Colder water can hold more oxygen in it than warmer water. This explains why there is a difference in oxygen levels at different depths of the water because there is a change in water temperature.
Dissolved oxygen levels are measured in parts per million (ppm) but sometimes are given in terms of Percent Saturation. Percent Saturation is the amount of oxygen dissolved in the water sample compared to the maximum amount that could be present at the same temperature.