Dissolved oxygen, often abbreviated as DO, is the amount of free oxygen molecules (O2) dissolved in water. Oxygen enters the water through atmospheric diffusion, plant photosynthesis, or aeration (either natural or man-made). DO is typically represented in measurement units as percent of air saturation (%sat) and concentration in milligrams per liter (mg/L) or parts per million (ppm).
Water that is at 100% saturation is in a state of equilibrium with the atmosphere, meaning that there is no net movement of oxygen molecules between the water and air. However, since DO varies with temperature, pressure and salinity, the total amount of oxygen at saturation will vary as conditions change.
Temperature and salinity have an inverse relation to oxygen concentration. Water that is fresher and cooler can hold more oxygen molecules at saturation than saltier, warmer water. Salinity reduces dissolved oxygen solubility due to the attraction between salt ions and water molecules, limiting the availability of free molecules for gases like oxygen to bind to, and triggering the diffusion of remaining gases out of the water.
Pressure, on the other hand, increases the amount of oxygen molecules at saturation. This can be thought of as higher pressure forcing more oxygen from the air into the water.
Both temperature and salinity have a significant impact on dissolved oxygen concentration.
Why Measure Dissolved Oxygen?
Dissolved oxygen is fundamental to aquatic ecosystems. Fish, aquatic invertebrates, zooplankton, aerobic bacteria, and plants all depend on it for survival. Insufficient DO concentrations can cause reduced productivity and have other adverse effects on ecosystem health. In a worst-case scenario, rapid die-offs may occur due to hypoxic conditions.
Different organisms can survive at various oxygenation levels, but a minimum amount is required for every species that uses oxygen for respiration, as well as microbes which consume oxygen during decomposition. For this reason, DO concentrations greatly influence the distribution of aquatic life.
Most forms of aquatic life depend on dissolved oxygen for respiration or other processes.
For example, a lake with surface turbulence may experience relatively high atmospheric diffusion near the surface and support a broad range of species. However, if the same lake experiences seasonal stratification, limited photosynthesis, excessive organic matter decomposition, eutrophication, or thermal pollution, hypoxic zones may form, which can limit growth rates, cause migration to more oxygen-rich areas, or kill species that cannot escape.
Long-term monitoring can detect acute changes but may also be used to study long-term cycles. In some freshwater sources, concentrations commonly fluctuate from 12-14 mg/L during winter months when the water is colder to 6-8 mg/L in the warmer summer months. However, natural variations may occur.
Seasonal DO fluctuations occur as water temperature changes.
For example, high rates of photosynthetic activity during the relatively productive warm season can raise DO concentrations. Contrarily, light attenuation due to ice and snow cover during the winter may reduce photosynthetic activity and atmospheric diffusion, thereby decreasing oxygen concentrations despite the relatively high capacity for cold water to hold DO.
While insufficient oxygen levels can harm aquatic life, high levels beyond the point of saturation also pose a potential threat. Supersaturated waters may cause gas bubble disease in fish, which, over an extended period of time, can be fatal. Such conditions occur most frequently at the outfall of dams or waterfalls but can also be caused by algal blooms, which produce an excess of photosynthetic activity. Sites where this is identified as a potential risk can benefit from measuring total dissolved gas (TDG) and other dissolved gases in addition to DO.
How Is Dissolved Oxygen Measured?
Dissolved oxygen sensor technology has evolved extensively over the years. Various electrochemical designs have been largely displaced by optical technology used in modern sensor designs.
Based on the principle of dynamic luminescence quenching, optical sensors have a light source, typically an LED, which emits blue light towards an oxygen-sensitive fluorescent dye on a semi-permeable membrane that allows interaction with gases in the water. The light excites the material, and a photodetector measures the returned luminescence.
The presence of oxygen molecules inversely affects the luminescence lifetime returned to the photodetector. It is this “quenching” by oxygen molecules that is used to calculate the DO concentration. Some sensor designs may incorporate a red light that does not interact with oxygen to act as a reference.
Optical DO sensors emit light to a membrane that interacts with oxygen in the water and measure the resulting luminescence.
The International Organization for Standardization’s ISO 17289 applies to optical DO sensors. This standard provides guidelines on sensor design, including accuracy, response time, and calibration procedures. These standards ensure the reliability and comparability of measurements across different environments, including those with varying temperatures and salinity levels, which can affect DO solubility and sensor response.
How to Select a Dissolved Oxygen Sensor?
When selecting a dissolved oxygen (DO) sensor for use in natural waters (lakes, rivers, streams, coastal, and ocean), the key considerations are sensor type, measurement range, accuracy, and maintenance needs.
Optical DO sensors are ideal for long-term monitoring due to their low maintenance requirements, long-term stability, and resistance to fouling. They do not require warm-up time, there is no need for stirring, and they are not affected by gases such as hydrogen sulfide.
Measurement range and accuracy are important considerations. The sensor should be capable of measuring the full range of DO concentrations expected in the target environment. For most natural waters, a sensor that can measure from 0 to at least 20 mg/L should be sufficient. Accurate measurements are paramount, especially in environments where DO levels are critical to ecological health or regulatory compliance.
Optical DO sensors provide lower maintenance and better resistance to biofouling than traditional oxygen-consuming type sensors, such as Clark electrodes. This is particularly important for long-term deployments.
Since the optics are internal to the sensor, biofouling does not physically interfere with the measurement as it does with sensors emitting light into the water. However, the sensor must be kept clean to allow oxygen to maintain equilibrium inside and outside the membrane. Some optical DO sensors include built-in anti-fouling features, such as a wiper mechanism, or are designed to minimize surfaces where fouling organisms can adhere. In salt and brackish environments, biofouling resistance is even more crucial.
In addition to these considerations, factors like depth rating and temperature/salinity compensation (since DO levels are temperature and salinity-dependent) should also be considered.
What to Consider When Preparing a Dissolved Oxygen Sensor?
Optical dissolved oxygen sensors can often maintain their calibration for extended periods of time (several months) with minimal drift. The calibration of DO sensors involves a point at air saturation, and sometimes an additional zero-oxygen reference point.
The first calibration point should be done in water-saturated air. For water-saturated air, place the sensor in a humid environment that is also exposed to air, for example, a partially sealed plastic bag with a moist paper towel inside.
Note that the saturation level will vary with barometric pressure. Air saturation of 100% is expected at sea level, and it declines at higher elevations and lower barometric pressure.
A zero calibration is often beneficial at least once prior to a long-term deployment to verify the responsiveness and accuracy of the sensor. Place the sensor into a zero-oxygen standard such as sodium sulfite (Na2SO3). Allow the sensor time to stabilize before accepting the calibration.
Since DO measurements are temperature-sensitive, avoid areas with fluctuating temperatures during calibration. Make sure the sensor’s membrane or optical surface is clean and free from obstructions or air bubbles. After calibration, it’s good practice to rinse the sensor and place it in a controlled water sample, such as tap water, to confirm the sensor’s performance.
During deployment, perform quality control (QC) checks regularly (depending on site conditions and sensor type) to ensure that the sensor is performing consistently and accurately.
How to Deploy a Dissolved Oxygen Measurement System?
Dissolved oxygen sensors can be deployed from either shoreside or open water deployments. Some of the factors to consider when planning for deployment are measurement depth(s), type of water body, availability of suitable mounting structures, and regulatory requirements.
Shore-based DO sensors may be securely deployed in pipes that allow flow through to the sensor.
One common application for DO measurement is at hydropower dams. Authorities such as the Federal Energy Regulatory Commission (FERC) in the US often require monitoring of DO because hydropower processes can affect concentrations. In these cases, shoreside deployments are often preferential because the DO sensors can be securely mounted in deployment pipes fixed to the power plant structure. This simplifies maintenance and allows for safe access.
Buoy platforms support open-water deployments and DO profiling applications.
In other cases, such as limnology research, buoy-based systems are often a better alternative. Buoys allow sensors to be placed at any location in a water body, and they can support single-sensor measurements or profiling strings with sensors placed at multiple depths in the water column. Profiling systems offer better data resolution and can provide insight into natural processes occurring in water bodies, such as seasonal stratification or diurnal cycles.
Buoy systems typically contain a data logging unit secured within the buoy hull and powered by a solar-charged battery for continuous operation. If a single, near-surface measurement is acceptable, the buoy frame or a perforated deployment pipe provides a secure location to mount the turbidity sensor. For deeper measurement, the sensor can be mounted onto a line suspended from the buoy.
The buoy is secured in place with a single-point or multi-point mooring system depending on the sensor depth and environmental conditions. The mooring system must allow the buoy to move with any waves and water level fluctuations that the site may experience.
Conclusion
Dissolved Oxygen is one of the most commonly measured and studied water quality parameters because it is critical to aquatic ecosystems. Most aquatic life forms depend on it for respiration or other functions.
Factors influencing DO concentration in natural waters include biological processes, surface-atmosphere interaction (aeration), temperature, salinity, and local barometric pressure. Successful long-term monitoring depends on system design, sensor calibration, and regular cleaning and recalibration.
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