Dissolved oxygen, or DO for short, is one of the most important parameters in water analysis. It describes the amount of oxygen gas (O2) dissolved in a liquid and has a direct impact on aquatic ecosystems, water quality, and many industrial processes.
In this article, you will learn what dissolved oxygen is, what factors influence its levels, what values are considered normal, and how DO is measured in practice.
Definition
Oxygen occurs in nature in many different forms. In a water molecule (H₂O), oxygen is chemically bonded to hydrogen. In rocks and minerals, it is bonded to silicon, carbon, or other elements.
Molecular oxygen (O₂), the oxygen we breathe, is not bound to other elements. It is chemically reactive and can be used by plants, animals, and microorganisms to produce energy through respiration.
It is precisely this molecular oxygen that dissolves in water. The amount of dissolved O₂ in a water sample is referred to as the DO content and is typically expressed in milligrams per liter (mg/L) or as a percentage of saturation (% Sat.) [1].
When the dissolved oxygen equals the saturation concentration, this is referred to as 100% saturation. For example, if the dissolved oxygen in freshwater at 25 °C is 5.0 mg/l, this corresponds to a saturation of approximately 60% (5.0 divided by the saturation value of 8.3 mg/l, multiplied by 100%).
How does oxygen get into the water?
Oxygen enters the water through two main pathways:
From the atmosphere: Water absorbs O₂ directly from the air until an equilibrium concentration is reached. Turbulence, wind, and waves significantly accelerate this process because they increase the contact area between water and air [1].
Through photosynthesis: Aquatic plants and algae produce oxygen during photosynthesis, which is released directly into the water. The turbidity of a body of water affects the dissolved oxygen (DO) level because it reduces the amount of light reaching the algae and aquatic plants [1].
At the same time, oxygen is consumed from the water: through the respiration of fish, microorganisms, and other aquatic organisms, as well as through the breakdown of organic matter by bacteria and fungi.
What factors influence the dissolved oxygen content?
The DO content of a body of water depends on several factors. The most important ones are temperature, salinity, and pressure.
Temperature
Temperature is the most significant factor affecting oxygen solubility. Cold water can dissolve significantly more oxygen than warm water. At 0 °C, the saturation concentration in freshwater is approximately 14.6 mg/L; at 30 °C, it drops to approximately 7.6 mg/L [1].
The following diagram shows the difference between freshwater and seawater across various temperature ranges:
Rising water temperatures, caused for example by climate change or industrial discharges, can significantly reduce the dissolved oxygen (DO) levels in a body of water, thereby endangering aquatic life [3].
Salinity
Saltwater can dissolve less oxygen than freshwater at the same temperature. As shown in the diagram, the solubility of oxygen in seawater is significantly lower than that in freshwater at every temperature [1]. This is particularly relevant for aquaculture, where salinity and dissolved oxygen levels must be closely monitored.
As air pressure increases, so does the solubility of oxygen in water. At high altitudes, where air pressure is lower, water therefore contains less dissolved oxygen than at sea level [1].
What DO levels are considered normal?
The DO content varies considerably depending on the type of water body and the application. The following values serve as a general guide [1][2][3]:
| Application / Water bodies | Typical DO range |
|---|---|
| Freshwater (25 °C, saturation) | approx. 8.3 mg/L |
| Healthy waterways | 7 to 12 mg/L |
| Aquaculture (optimal) | 6 to 9 mg/L |
| Critical lower limit for fish | less than 3 mg/L |
| Anaerobic conditions | less than 0.5 mg/L |
| Seawater (25 °C, saturation) | approx. 6.7 mg/L |
A DO level below 3 mg/L is considered critical for most fish species. Levels below 1 mg/L are referred to as hypoxic conditions, which can lead to the death of aquatic organisms [3].
How is dissolved oxygen measured?
There are three basic methods for measuring dissolved oxygen [1]:
Wet chemical method (Winkler titration)
The Winkler titration is the most accurate method for determining dissolved oxygen and is considered the reference method. A water sample is collected in a special bottle that prevents the sample from coming into contact with air. Chemical reagents are then added, and titration is performed until a color reaction indicates that oxygen consumption has ceased [1].
The drawback: This method requires careful sampling, specialized laboratory equipment, and rapid transport to the laboratory. It is therefore not suitable for field measurements or continuous monitoring.
Membrane sensors (electrochemical)
Conventional membrane sensors operate on electrochemical principles. A probe with a membrane is immersed in water and measures Dissolved Oxygen through a chemical reaction on the membrane surface.
These sensors are suitable for field measurements, but they have some drawbacks: The membrane and the calibration solution must be replaced regularly. In addition, polarographic sensors require a warm-up period and are sensitive to water flow velocity [1].
Optical sensors
Optical DO sensors operate on a completely different principle. Blue light from the probe excites a fluorescent material, whose fluorescence is quenched by oxygen molecules in the water. The more oxygen present, the more strongly the fluorescence is suppressed. This signal is converted by the device into a DO value [1].
Optical sensors have clear advantages over diaphragm sensors:
- No membrane or filling solution required
- No need to warm up before taking the measurement
- not sensitive to flow velocity
- fast and stable readings
- significantly lower maintenance requirements
Apera’s DO meters feature state-of-the-art optical sensors that are low-maintenance, rugged, and suitable for use in virtually any environment.
Overview of all Apera DO meters
Calibration of dissolved oxygen meters
Both membrane sensors and optical sensors can be calibrated using air as a source of oxygen. Since the oxygen concentration in the atmosphere is a constant, known value (20.9%), moisture-saturated air serves as a stable calibration point [1].
A cap containing air saturated with water is often used for calibration. Alternatively, water saturated with air or a standard solution with known Dissolved Oxygen, determined using the Winkler method, can be used [1].
Why is DO measurement important?
Aquaculture and fish farming: Fish and other aquatic organisms are directly dependent on dissolved oxygen. If dissolved oxygen levels are too low, this leads to stress, reduced growth, and, in the worst case, mass mortality. Continuous monitoring is essential in professional facilities [2].
Environmental monitoring: In natural water bodies, dissolved oxygen (DO) levels are an important indicator of pollution. Organic waste and nutrient inputs promote the growth of bacteria, which consume oxygen as they break down organic matter, thereby lowering DO levels [3].
Water treatment: In drinking water treatment and wastewater treatment, dissolved oxygen (DO) levels are a key control parameter for biological degradation processes.
Laboratory: Reproducible DO measurements are essential for many scientific experiments, such as those in biochemistry or environmental science [1].
Conclusion
Dissolved oxygen is one of the most important parameters in water analysis. Its concentration is significantly influenced by temperature, salinity, and pressure, and has a direct impact on aquatic life and many industrial processes. Modern optical sensors now enable fast, low-maintenance, and reliable DO measurement directly on-site, without the limitations of older membrane sensors.
Frequently Asked Questions About Dissolved Oxygen
What does DO mean in water analysis?
DO stands for dissolved oxygen. It refers to the amount of molecular oxygen (O2) dissolved in a liquid and is expressed in mg/L or as a percentage of saturation.
Why does warm water contain less oxygen?
As the temperature rises, the solubility of gases in water decreases. At 0 °C, the saturation concentration in freshwater is approximately 14.6 mg/L; at 30 °C, it is only about 7.6 mg/L.
What is the optimal DO level for fish?
For most fish species, dissolved oxygen levels between 6 and 9 mg/l are considered optimal. Levels below 3 mg/l are critical and can lead to stress and increased mortality.
Why does seawater contain less oxygen than freshwater?
Dissolved salts reduce the solubility of gases in water. As a result, seawater has a lower oxygen saturation than freshwater at the same temperature.
What is the difference between a membrane sensor and an optical sensor?
Membrane sensors operate on an electrochemical principle and require regular membrane replacement as well as a warm-up period. Optical sensors measure using fluorescence quenching, require no membrane or filling solution, and are faster and require less maintenance.
How is a DO meter calibrated?
Calibration is usually performed using air saturated with water vapor, since the oxygen concentration in the atmosphere is constant and known to be 20.9%. Alternatively, water saturated with air or standard solutions can be used.
How does oxygen get into the water?
Mainly through diffusion from the atmosphere and through photosynthesis by aquatic plants and algae. Turbulence, wind, and waves significantly accelerate the uptake of oxygen from the air.
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.