Dissolved oxygen (DO) is oxygen gas (O2) that is dissolved in water. Gases in the atmosphere, such as oxygen, nitrogen and carbon dioxide, naturally dissolve in water to some degree. Like salt or sugar, these gases are invisible in water once they become dissolved.

The element oxygen exists in many forms in nature. Although most people know that oxygen is part of the water molecule, most would be surprised to hear that oxygen is also the most abundant element in rocks. In these forms, oxygen is bound to other elements such as hydrogen, silicon or carbon. Molecular oxygen (O2), which is in air, is different than other forms because it is not bound to other elements. In nature, the O2 that we breathe is chemically much more reactive than the more abundant forms of oxygen that we come in contact with. This is what allows plants, animals and other organisms to use O2 to metabolize their food through the process of respiration.

Concentration and solubility

The amount (concentration) of O2 dissolved in water is most often expressed in terms of milligrams per liter of water (mg/L). This concentration is referred to as the dissolved oxygen (DO) content of the water. There is a natural tendency for water in contact with air to dissolve O2 until the saturation concentration is reached. For example, the DO in fresh water at 25°C in contact with air is 8.3 mg/L, assuming that equilibrium between water and air is reached and that nothing is removing the O2 from the water. 

DO concentrations are sometimes expressed as % of saturation. If the DO of the water is at the saturation concentration, then it is said to be 100% saturated. If the DO is 5.0 mg/L in fresh water that is at 25°C, for example, then it is 60% saturated (5.0 divided by the saturation level of 8.3 mg/L, multiplied by 100%).

This saturation concentration is known as the solubility of O2, which is the amount of O2 that water can hold. The solubility of O2 changes with temperature, salinity and pressure. The solubility of O2 in water increases as the temperature decreases, meaning that cold water can hold more O2. For example, cold water at 5°C (12.8 mg/L) holds about 55% more dissolved oxygen than warm water at 25°C (8.3 mg/L)[1]. 

Because the temperature of water varies with the seasons, DO levels tend to be higher in the cooler months because the solubility of O2 is higher in cold water. In the summer, water levels tend to be lower and the air is warmer, which leads to warmer water and lower DO levels.

The salinity of water also affects the solubility of O2, such that seawater can hold about 20% less O2 than fresh water[1].

Source: [1]

Dissolved oxygen solubility changes with temperature and salinity.

Pressure also affects the solubility of O2. The water pressure at a certain depth depends on the height of the water column above it, so pressure increases with depth. Water at greater pressure can hold more O2, meaning that the solubility of O2 increases at greater depths. For example, water at 4 m (13.1 ft) depth can hold about 40% more O2 than water at the surface[2]. 

It is possible for water to have a DO level that is higher than the solubility of O2 (more than 100% saturation). This condition is called supersaturation, which can happen under special circumstances (see below).

Sources and sinks of O2 in water

The main source of O2 in water is the atmosphere. Oxygen molecules slowly enter water at the water surface. This process is aided naturally by turbulent flowing water, wind, and waves. Because of this, still water tends to have lower DO values than rapidly moving water. Aeration of water naturally by rapids or waterfalls, or artificially by bubbling air through water, turning waterwheels, or spilling through dams, greatly accelerates the transfer of O2 from air to water. O2 also enters water bodies from tributary streams and groundwater discharge.

O2 in water is also produced through photosynthesis, in which plants and algae convert dissolved carbon dioxide (CO2) into organic matter, releasing O2 into the water. Photosynthesis only takes place at times of day where light is present. The depth at which photosynthesis takes place depends on the clarity of the water. In murky water, light may not reach the bottom of a deep lake.

Aquatic plants, animals and microbes consume O2 by respiration, where organic material used as fuel is converted back into CO2; this is the opposite of photosynthesis. Many people are surprised to learn that plants consume O2 as well as produce it. Plants will actually consume O2 by respiration at night and release O2 through photosynthesis during the day. Because of this, DO in some aquatic environments will tend to decrease at night and increase in the daytime.

Microbes and fungi also consume O2 through the decomposition of dead organic matter. Often, this happens in deeper layers of the water column as dead material sinks toward the bottom. Because of this, deeper layers of water often have lower levels of DO than shallow layers.

DO and aquatic life

Different species of aquatic animals have different DO requirements. Animals that feed on the bottom of a water body, where DO levels tend to be lower, can typically tolerate lower DO levels that animals that dwell near the surface. Most fish are able to survive and grow at DO concentrations of 5 mg/L or higher, although spawning and optimal growth may require higher concentrations[3].

When DO levels are too low for a certain species, the animal can become lethargic or die. Hypoxia is a condition where DO is low enough to threaten aquatic animal species. Hypoxia can cause dead zones in water bodies, where fish and other aquatic life are absent. A DO level of less than 1-2 mg/L is generally considered hypoxic, and a level less than 3 mg/L is a cause for concern. These values are below the requirements for spawning and growth of most fish.

At the opposite extreme, supersaturation of water with O2 can lead to health problems in fish. Supersaturation arises when the solubility of O2 in water rapidly decreases or when O2 is rapidly produced by photosynthesis. The solubility of O2 can decrease when water temperature rises, for example, so a rapid rise in water temperature can lead to supersaturation. Supersaturation with O2 can cause a health condition in fish called gas bubble disease.

Environmental impacts on DO

Because dissolved O2 is needed by most aquatic organisms, the DO of a water body is often used to assess its health. DO levels in water bodies can be impacted by a number of different environmental problems. For example, runoff associated with clearcutting or agricultural wastes can carry excessive organic material into water bodies, which can result in the depletion of O2 as the material is decomposed. 

Another problem is excessive nutrients, which can enter water bodies through runoff associated with fertilizer application on agricultural or recreation land (such as golf courses) or from wastewater treatment plants. Excessive nutrients can result in algal blooms, a process known as eutrophication. Algal blooms can block light from reaching aquatic plants, and dead algae provide a source of organic matter that can deplete DO levels when it decomposes. Because the dead algae sink, this problem especially impacts deeper layers of water and animals that dwell on the floor or bed of the water body.

Riparian vegetation (plants that live along the banks of a stream or river) protects the DO of streams by providing shade that helps keep the water cool. When this vegetation is removed, however, the temperature of the water can increase, causing a corresponding drop in DO levels.

The temperature of water can also be affected by other human activities. When water is withdrawn or stored for drinking water, irrigation, or industrial use, especially during dry months, the water level in streams can decrease, making them especially susceptible to temperature fluctuations and warming. The resulting decrease in DO can harm aquatic life in these water bodies. When water is used for industrial cooling processes and then discharged back into a stream, its temperature is often higher than the water in the stream, resulting in warming of the stream and a decrease in its DO.

Conclusion

Dissolved oxygen is affected by many different factors and processes found in water bodies, and it can fluctuate over short time scales. Fortunately, most aquatic life can tolerate short periods where DO is low. However, persistent problems with low DO levels can lead to poor health of an aquatic environment. This is why routine monitoring of DO is important when there is concern about the health of aquatic life.

References

[1] American Public Health Association (APHA) (2005) Standard methods for examination of water and wastewater, 21st edn. APHA, AWWA, WPCF, Washington.

[2] FAO. (2014). Site selection for aquaculture: Chemical features of water. Washington, DC: Fisheries and Aquaculture Department, www.fao.org.

[3] U.S. Environmental Protection Agency (1986) Ambient water quality criteria for dissolved oxygen. EPA 440/5-86-003.