Dissolved Oxygen in Water Basics
Every species on our planet depends on water and oxygen. For aquatic species, adequate dissolved oxygen is of prime importance to their continued survival. Since dissolved oxygen levels are directly related to good water quality, the two are highly interdependent. Many factors can affect DO levels, and an understanding of these levels in order to make informed decisions concerning wastewater treatment operations, hypoxic zones, aquaculture facilities or large-scale ecosystems is essential.
Dissolved oxygen refers to the level of free, non-compound oxygen present in water or other liquids. It is one of the most important parameters when assessing water quality because of its influence on the organisms living within a body of water. A dissolved oxygen level that is too high or too low can harm aquatic life and affect water quality.
Non-compound oxygen, or free oxygen (O2), is oxygen that is not bonded to any other element. Dissolved oxygen is the presence of these free O2 molecules within water. The bonded oxygen molecule in water (H2O) is in a compound and does not count toward dissolved oxygen levels. One can imagine that free oxygen molecules dissolve in water much the same way salt or sugar does when it is stirred.
Dissolved oxygen enters water through the air or as a plant byproduct from photosynthesis. From the air, oxygen can slowly diffuse across the water’s surface from the surrounding atmosphere, or be mixed in quickly through aeration, whether natural or man-made which is typical in aquaculture applications. The aeration of water can be caused by wind, rapids, waterfalls, ground water discharge or other forms of running water. Man-made causes of aeration vary from an aquarium air pump to a waterwheel to a dam.
Dissolved oxygen is also produced as a waste product of photosynthesis from phytoplankton, algae, seaweed and other aquatic plants.
While most photosynthesis takes place at the surface, a large portion of the process takes place underwater by seaweed, sub-surface algae and phytoplankton. Light can penetrate water, though the depth that it can reach varies due to dissolved solids and other light-scattering elements present in the water called turbidity.
What is a Dissolved Oxygen Meter and What Does it Report?
DO meters, oxygen meters, measure the amount of oxygen dissolved in an aqueous solution. There are two primary types of dissolved oxygen sensing technologies available: the optical based sensing method which is commonly referred to as luminescent and the Clark electrochemical or membrane-covered electrode. Within these two types of technologies, there are slight variations available. For example, there are two types of optical sensors. Both types of optical sensors measure luminescence as it is affected by the presence of oxygen; however, one sensor measures the lifetime of the luminescence while the other sensor measures the intensity of the luminescence. The two types of electrochemical sensors available are polarographic and galvanic. Oxygen sensors can be selected with many features including the type of sensing technology.
Dissolved oxygen is usually reported in milligrams per liter (mg/L) or as a percent of air saturation. However, some studies will report DO in parts per million (ppm) or in micromoles (umol). 1 mg/L is equal to 1 ppm. The relationship between mg/L and % air saturation has been discussed above, and varies with temperature, pressure and salinity of the water. One micromole of oxygen is equal to 0.022391 milligrams, and this unit is commonly used in oceanic studies. Thus 100 umol/L O2 is equal to 2.2 mg/L O2.
Calculating DO from % Air Saturation
To calculate dissolved oxygen concentrations from air saturation, it is necessary to know the temperature and salinity of the sample. Barometric pressure has already been accounted for as the partial pressure of oxygen contributes to the percent air saturation 7. Salinity and temperature can then be used in Henry’s Law to calculate what the DO concentration would be at 100% air saturation 10. However, it is easier to use an oxygen solubility chart. These charts show the dissolved oxygen concentration at 100% air saturation at varying temperatures, and salinities. This value can then be multiplied by the measured percent air saturation to calculate the dissolved oxygen concentration 7.
O2 mg/L = (Measured % DO)*(DO value from chart at temperature and salinity)
Example:
80% DO measured
30 ppt salinity
10°C
.80 * 9.318 = 7.45 mg/L DO
>>> Review the Oxygen Solubility Table
