What is Dissolved Oxygen in Water?

Most aquatic organisms require dissolved oxygen, often abbreviated as DO, to survive, but the source of this oxygen is not the water molecule ( H₂O ).

DO is gaseous, molecular oxygen in the form of O₂ originating from the atmosphere or as a byproduct of photosynthesis. Once dissolved in water, it is available for use by living organisms and can play a significant role in many chemical processes in the aquatic environment. Besides being dissolved in water, this oxygen is no different from the oxygen we breathe.

What Are Sources of Dissolved Oxygen in Water?

The Earth's Atmosphere

Molecular oxygen can enter a water body from the planet's atmosphere in several ways. Suppose water has a lower oxygen concentration than the atmosphere above it. In that case, molecular oxygen will naturally diffuse from the air into the water until it is completely saturated with oxygen. Equilibrium conditions are met when the concentration of oxygen is the same in air and water.

Aeration of water occurs when water and air mix, resulting in increased levels of DO in water. This happens naturally at waterfalls and rapids or when windy conditions cause turbulence on a water body's surface.

Aquatic organisms need DO to survive, so that's why some water bodies have artificial aeration. Examples include a paddle wheel or a fountain in the middle of a pond, the use of an air stone in an aquarium, and pumping air into aeration basins at wastewater treatment plants to sustain microbes that break down contaminants.

Aerating water can be a considerable expense for wastewater facilities, but more municipalities are using DO sensors to optimize aeration, thus reducing their energy costs. Check out our white paper on How to Control Activated Sludge with Online Sensors to learn more.

Photosynthesis

Another major source of DO is photosynthesis. Aquatic plants and algae use photosynthesis to generate new cells and repair damaged cells. This process requires water, light energy, and carbon dioxide. A byproduct of photosynthesis is gaseous, molecular oxygen that can become dissolved in water. Not all plants are created equal, as some of them produce more oxygen than others.

Plants and algae produce oxygen during the day when photosynthesis occurs. They also consume it for respiration, which is the process by which plants convert glucose (i.e., the sugar produced during photosynthesis) and oxygen into usable cellular energy.¹ Plants and algae produce far more oxygen during the day than they consume. At night, plants and algae no longer produce oxygen, but they continue to consume it. Meanwhile, other organisms like fish consume oxygen at a steady rate around the clock.

So, in a healthy system, oxygen concentrations rise throughout the day and decline at night when respiratory activity consumes that oxygen.

What Environmental Variables Affect Dissolved Oxygen?

Dissolved oxygen concentrations in water are affected by temperature, barometric pressure, and salinity.

Temperature

The most significant variable is temperature, so it is essential to measure it in conjunction with dissolved oxygen.

The solubility of oxygen in water is inversely related to temperature – as temperature increases, DO decreases. Therefore, a water body in winter will have a higher DO concentration than in summer, assuming other variables are held constant. The same applies to nighttime – as a water body cools overnight, more oxygen can be dissolved. However, it is important to keep in mind the impact photosynthesis and respiration have on DO concentrations during the day and night – see Figure 5 and 6.

Salinity

Like temperature, the solubility of oxygen in water is inversely related to salinity – as salinity increases, DO decreases.

For instance, seawater can hold about 20% less oxygen under the same temperature and atmospheric pressure as freshwater. Therefore, it is critical to measure salinity – this is done with a conductivity sensor – when collecting DO data in estuaries, wetlands, coastal areas, aquaculture, or any other application where salinity can vary. See the Comparing Dissolved Oxygen Measurement Units section for more information on the impact of salinity on DO.

Most modern DO instruments, such as the YSI ProDSS, will provide real-time salinity-compensated DO measurements if a conductivity and DO sensor are connected. Otherwise, salinity will have to be entered into the meter for this compensation to occur.

Barometric Pressure

Unlike temperature and salinity, there is a direct relationship between barometric pressure and DO levels in water – as pressure decreases, DO decreases.

At lower elevations, the barometric pressure is high, so there is more pressure to push gaseous oxygen from the atmosphere into water. But at higher elevations, the barometric pressure is much, much lower.

In addition to altitude, barometric pressure can change due to a change in weather. A quick pressure drop can indicate a storm is on the way. Most modern DO instruments have a built-in barometric pressure sensor that will automatically compensate DO readings for barometric pressure changes.

See the Comparing Dissolved Oxygen Measurement Units section to see the impact of barometric pressure on DO readings.

What Units are Used When Measuring Dissolved Oxygen?

DO is expressed in many different units, but most often in mg/L or % saturation (DO%). The unit mg/L is straightforward, as it is the milligrams of gaseous oxygen dissolved in a liter of water.

The best place to start when explaining % saturation is with the atmosphere – approximately 21% of the earth's atmosphere is oxygen. Another consideration is the barometric pressure at sea level, which is equal to 760 millimeters of mercury. The part of the overall pressure caused by oxygen – termed partial pressure – is equal to 160 mmHg (21% * 760 mmHg = 160 mmHg).

If a DO sensor is calibrated at sea level, it should calibrate to a percent saturation of 100%, assuming the water and air are in equilibrium. But what if the barometric pressure is less than 760 mmHg? What will the sensor calibrate to?

Let's say the barometric pressure determined by a meter is 750 mmHg. To determine what the sensor will calibrate to, divide 750 mmHg by 760 mmHg; this equals 98.68% (750 mmHg / 760 mmHg = 98.68%). At this pressure, saturation cannot be greater than 98.68% as long as water and air are in equilibrium. Therefore, the sensor will calibrate to 98.68%.

Some may wish to report Local DO where the calibration value is 100% regardless of the barometric pressure at the time of calibration. The 100% calibration value reflects that the calibration environment is at 100% oxygen pressure for that specific location. Several YSI instruments are capable of reporting Local DO.

Comparing Dissolved Oxygen Measurement Units

You can think of dissolved oxygen percent (DO%) as the unit being determined directly by any instrument that uses an Electrochemical Sensor or Optical Sensor. The only variable that impacts DO% is barometric pressure, as can be seen in Table 1 below.

In contrast, DO mg/L is calculated by the instrument from DO%, temperature, and salinity. Table 2 below demonstrates the impact of varying temperatures and salinities.

What is Dissolved Oxygen Supersaturation?

Dissolved oxygen percent values in the natural environment can reach over 100%, but how is this possible?

Photosynthesis can be a significant driver of supersaturation, as this process produces pure oxygen. Sometimes it can even account for DO% values up to 500%!

Another cause is rapid temperature changes. While the equilibration of water with the air above it is seldom rapid, the temperature of a water body can change rapidly. So, let's say the temperature of a stagnant lake quickly increases by 5 degrees once the sun starts shining. DO levels in water should decrease as temperature increases. However, if the equilibration between air and water is not as rapid as the temperature change, the lake will technically be supersaturated with DO until an equilibrium state is once again established.

Another cause of supersaturation is turbulent conditions or anything else that can cause mixing of the air and water (e.g., air stones, whitewater rapids).

To learn more about supersaturation, check out our technical note on Environmental Dissolved Oxygen: Values Greater Than 100% Air Saturation.

Why Measure Dissolved Oxygen?

DO is one of the most commonly measured water quality parameters, but the reason for measuring it varies based on the environment.

Why Measure Dissolved Oxygen in Surface Water and Aquaculture?

Dissolved oxygen is a direct indicator of a water body's ability to support aquatic life – aquatic organisms need DO to survive!

The level of DO required varies by species. In general, most fish species will grow and thrive within a range of 5-12 mg/L. However, if levels drop below 4 mg/L, they may stop feeding and become stressed, possibly leading to large fish kills. Hypoxia occurs when the concentration of dissolved oxygen decreases to a level that can no longer support living aquatic organisms.

Check out our blog post on Dissolved Oxygen Management and Related Costs in Pond Aquaculture to learn more about the importance of measuring DO in fish farming and other forms of aquaculture. We also created a Hypoxia Infographic that helps explain how hypoxia occurs in the environment.

An imbalance of DO occurs when there is a harmful algal bloom (HAB). During the early and peak growth phases of a HAB, DO can increase significantly in the vicinity of the bloom due to photosynthetic activity during the day. More oxygen is generated than can be consumed by either the algae or the other organisms, day or night – this can lead to supersaturation.

As the bloom fades and dies, the algae become food for bacteria and other things that consume oxygen. This can cause DO levels to drop drastically, resulting in hypoxia. Check out our blog post, HABs | Everything You Need to Know, to learn more!

Large fish kills can also result from thermal pollution around power plants and industrial manufacturers. While the effluent from these plants is typically clean, it is often much, much warmer than the surface water it enters. As temperature increases, the level of DO in the water decreases. Therefore, a sudden influx of warm water can result in large fish kills.

Thermal pollution and HABs aren't the only events that endanger aquatic organisms. Road salt is commonly applied to icy roads in winter. This salt runs off the road and into surface water bodies, increasing salinity. As salinity increases, DO levels decrease. So, even though oxygen is more soluble in cold water, high salinity can result in large fish kills in winter due to suffocation.

Why Measure Dissolved Oxygen in Groundwater?

Many assume DO is absent below the water table, but this is an incorrect assumption. Before water percolates downward from the surface, water is in contact with the atmosphere, and oxygen becomes dissolved. DO can exist at great depths in an aquifer as long as there is little or no oxidizable material.²

Dissolved oxygen can be a helpful parameter to measure when conducting groundwater investigations. DO can help determine when stable conditions have been reached during purging and can be used to evaluate well construction.

Measuring DO can also help ensure proper groundwater sampling procedures are being followed when collecting samples for the analysis of metal and volatile organic compounds. Any artificial aeration can impact laboratory analyses for these compounds.³

DO plays a significant role in chemical reactions that occur in the subsurface. It regulates the valence state of trace metals and constrains the metabolism of dissolved organic compounds (e.g., oil) by microbes.⁴

Microbes can degrade oil that has leaked into an aquifer. Like other organisms, microbes need to respire (i.e., breathe). Respiration requires an electron acceptor, and since oxygen is the most preferred one, DO is quickly depleted where there is contamination present. Therefore, DO can only be found outside a plume of contaminated groundwater.⁵

Other electron acceptors are used once dissolved oxygen has been depleted. After oxygen, nitrate will be used up, so nitrate can only be found relatively far away from the plume, just like DO. The electron acceptor used last is carbon dioxide (CO₂). The process of using CO₂ is called methanogenesis; this will be occurring closest to the source of contamination.⁵

Other environments can become anoxic due to microbial activity, such as the open water contaminated by the Deepwater Horizon oil spill in 2010.

Why Measure Dissolved Oxygen in Wastewater?

Microbes consume waste and transform it into harmless end products in the treatment process at wastewater treatment plants. DO plays a critical role in this process, as these microbes rely on it to break down wastewater contaminants, such as organics or ammonia. In the activated sludge process (ASP) – the most common plant configuration – air is pumped into aeration tanks filled with microbes suspended in water.

Our blog post Wastewater or Water Resource Recovery? | Getting the Waste Out of Wastewater discusses aeration technology in more detail.

Effluent, which is the treated water leaving the plant, must contain a limited amount of nutrients to ensure eutrophication does not occur in the environment. Biological nutrient removal (BNR) processes can be used to ensure compliance with nutrient effluent limits, but these processes require controlled conditions within the treatment plant.

BNR is characterized by the presence of unaerated anaerobic and anoxic zones upstream and downstream of aeration zones. Mixed liquor recycle and sludge return streams are arranged to make the best use of the organic content in the activated sludge system.

Check out our webinar on the Biological Nutrient Removal of Nitrogen to learn more about this treatment strategy.

How to Measure Dissolved Oxygen in Water

How is dissolved oxygen measured? There are a few different methods to measure dissolved oxygen in water and the following section provides an overview. 

Colorimetric Method

Colorimeters, also known as filter photometers, are instruments that measure color intensity. When using these instruments, chemical reagents are mixed with the sample. If the target parameter is present, the solution will have a color, and its intensity will be proportional to the concentration of the parameter being tested.

Light is passed through a test tube containing the sample solution and then through a colored filter onto a photodetector. Filters are chosen so that light of a specific wavelength is selected. When the solution is colorless, all of the light passes through. With colored samples, light is absorbed, and that which passes through the sample is proportionately reduced.

There are two different colorimetric methods of determining DO – Indigo Carmine and Rhodazine D. Indigo carmine reacts with DO to form a blue complex. In contrast, Rhodazine D reacts with DO to form a bright pink complex.

Winkler Titration

Reagents are also used when determining DO concentrations via a Winkler titration. In this method, reagents form an acid compound that's titrated with a neutralizing compound. Also, like the colorimetric method, a color change results, and the DO concentration is determined by observing the point at which this color change occurs.⁶

Many standard operating procedures (SOPs) still call for a Winkler titration, especially at wastewater treatment labs that are determining biological oxygen demand (BOD). Winklers need to be done in triplicate, with the results being averaged.

Electrochemical Sensors

Unlike the measurement of DO by performing a Winkler titration or using a colorimeter, electrochemical sensors, also known as membrane-covered DO sensors, don't require reagents. These sensors provide fast measurements and have a wide range, but water must continuously move across the membrane as oxygen is consumed during the measurement.

There are two types of electrochemical sensors – polarographic and galvanic. In 1956, Dr. Leland Clark invented the polarographic electrode while working with YSI Scientists. The galvanic electrode was developed later on, but it measures DO the same way as the polarographic sensor. Either sensor type can be used with YSI instruments such as the ProQuatro and Pro20.

Electrochemical DO sensors consist of an anode and a cathode confined in electrolyte solution by an oxygen-permeable membrane. Oxygen molecules dissolved in the sample diffuse through the membrane before being reduced (i.e., consumed) at the cathode. This reaction produces an electrical signal that travels from the cathode to the anode, ultimately reaching the instrument/meter.

The amount of oxygen diffusing through the membrane is proportional to the partial pressure and concentration of oxygen outside the membrane. As the oxygen concentration varies, so does the oxygen diffusing through the membrane, and this causes the probe current to change proportionally.

Polarographic

Polarographic sensors have a silver anode and a gold cathode. These materials require the probe to warm up, or polarize, before use – this takes about 10 minutes. Polarographic sensors have a longer lifespan than galvanic sensors because it is not always on (i.e., not always polarized).

Galvanic

Galvanic sensors have a zinc anode and a silver cathode. These materials allow the sensor to be continuously polarized even when the meter is off, so no warm-up period is required. There is a drawback to always being on – these sensors have a shorter life than polarographic sensors.

Optical Sensors

Optical and electrochemical sensors have some similarities. For starters, these sensors measure the pressure of oxygen dissolved in the sample. 'Raw' readings are expressed as DO%, and the only variable that affects DO% is barometric pressure. The higher the barometric pressure, the more oxygen will be pushed into the water. It is important to note that DO mg/L is calculated from DO%, temperature, and salinity.

Like electrochemical sensors, no reagents are required when using optical sensors. Both sensor types are also placed directly in the sample when taking a measurement.

There are several key structures of an optical DO sensor. The sensor cap of an optical DO sensor contains a diffusion layer across which DO is constantly moving. Unlike electrochemical sensors, oxygen is not consumed during the measurement, so water does not need to flow continuously across the sensor cap.

There are also different LEDs, one of which (the blue light in most of our YSI sensors) causes another layer of the sensor cap – the dye layer – to luminesce (i.e., glow).

As oxygen moves across the diffusion layer, it affects the luminescence of the dye layer. The amount of oxygen passing through the sensing layer is inversely proportional to the lifetime of the luminescence in the sensing layer. The lifetime of the luminescence is measured by the sensor and compared against the reference (the red light in our example), allowing for DO to be determined.

How to Select the Right Dissolved Oxygen Sensor

There are several options for measuring dissolved oxygen in water, and it can be challenging for those new to measuring DO to select the right method for them.

Colorimeters are not typically used when the only parameter being measured is dissolved oxygen, as they are not convenient – it takes time to mix the reagent and solution! Additionally, there are some pretty tight limitations on the measurement range.

Winkler titrations are time-consuming and challenging to perform. Suppose you have to perform a Winkler titration because your standard operating procedure (SOP) follows ISO 5813 or ASTM D888. In that case, we recommend using an automated titrator – check out some titration options from YSI – rather than doing the titrations by hand. For customers who require measuring DO in situ or have a high throughput of samples, we recommend using an electrochemical or optical sensor for DO measurement if you have a choice of method.

Electrochemical and optical sensors are by far the most commonly used tools when measuring DO. Unlike other water quality sensors (e.g., nitrate) that are often designed for a specific application, DO sensors can be used in a wide variety of applications – surface water, aquaculture, groundwater, wastewater, and more!

So which DO sensor is right for you? Table 3 has some considerations.

Want to learn more about the measurement of DO, differences between DO sensors, and best practices? Download our DO Handbook!

How to Select the Right Dissolved Oxygen Instrument

While electrochemical and optical DO sensors are suitable for many applications, the instruments they're used with are often designed with specific applications in mind. Examples include:

The MultiLab 4010-3W is the ideal instrument to use in a lab (e.g., wastewater lab) that's measuring pH, DO/BOD, ammonia, or another combination of parameters. This is a lab instrument – it's not meant to be used outside! While the sensor technology is the same as is used on field instruments, the sensor bodies are designed for use in a controlled environment (e.g., some pH sensors have a refillable glass body).

The ProDSS is a portable system with a handheld, single cable, and a bulkhead where the sensors are installed. This is a true field instrument – rugged, waterproof case (IP-67 rated); metal, military-spec (MS) cable connectors; and titanium sensors. This instrument is meant to be used for spot sampling, meaning it is not meant for unattended monitoring.

For assistance selecting the right portable dissolved oxygen system, check out our blog post on 5 Considerations When Selecting a Portable Dissolved Oxygen Meter or our video on How to Select a Handheld Dissolved Oxygen Meter. See available dissolved oxygen meters. 

Video: How to Select a Handheld Dissolved Oxygen Meter

The EXO sonde is similar to the ProDSS, but it features more sensors (e.g., the NitraLED) and is designed for continuous, unattended monitoring in many types of environments. It has onboard batteries, data logging, and an onboard wiper, all of which allow for months-long deployments in harsh environments. This is the most advanced outdoor water quality monitoring platform we offer. Not convinced? Check out our blog post on Bayou Sorrell | An Unexpected Bonus with EXO Sondes.

Aquaculture monitors like the 5200A, 5400, and 5500D are also meant for continuous monitoring, but these systems require a power source and are typically stationary. These can be connected to the AquaViewer II App that allows the user to configure tanks and map them on the app, monitor water quality conditions, receive alarms if there's a problem, and more.

The IQ SensorNet system is ideal for wastewater monitoring and control. Operators use systems like these to view their process in real-time and react accordingly. A variety of controllers, modules, and sensors (e.g., the FDO 700 optical sensor) allow facilities to monitor only what they need to – up to 20 probes can be connected! Sensors are built into rugged, corrosion-resistant probes.

The table below provides a list of YSI's platforms, typical use for the platform, and the type of DO sensor(s) available.

Still not sure which dissolved oxygen sensor or instrument is right for your needs? Ask our experts or schedule a free virtual consultation today!

Sources

1. UC Davis, Photosythesis & Respiration
2. Texas Water Development Board, A Field Manual for Groundwater Sampling
3. Ohio EPA, Ground Water Sampling
4. Groundwater Monitoring & Remediation, Monitoring Dissolved Oxygen in Ground Water Some Basic Considerations
5. Parsons, Principles and Practices of Enhanced Anaerobic Bioremediation of Chlorinated Solvents
6. SERC, The Winkler Method - Measuring Dissolved Oxygen