Dissolved oxygen (DO) refers to how much oxygen is dissolved in water. DO also refers to how much oxygen is available to aquatic organisms. For this reason, a river or lake’s dissolved oxygen levels say a lot about the quality of its water.
Because aquatic life (and humans) depend on dissolved oxygen in water, dissolved oxygen monitoring systems are essential to keeping a careful watch on water quality—especially in stratified water bodies, in troubled aquatic ecosystems, and near hydropower plants.
Dissolved Oxygen Definition
Dissolved oxygen represents the level of free oxygen that is non-compound, or not part of other compounds present in a liquid—almost always water. Limnologists, scientists who study lakes, assess dissolved oxygen levels as an essential water quality factor.
Dissolved oxygen is a critical water quality parameter due to its influence on aquatic life. Organisms living in a body of water are directly impacted by dissolved oxygen levels that are too low or high.
Free or non-compound oxygen (O2), is oxygen that occurs bound in molecular dyads, but is not bonded to any other elements. These free molecules of O2 in water, make up its dissolved oxygen levels.
Only the non-compound O2 molecules in the water add to these levels; although water contains oxygen (H2O), that oxygen is bonded in a compound, so it doesn’t add to dissolved oxygen levels. It’s similar to dissolving salt or some other compound in water; it doesn’t change the water’s H2O molecules, just adds other molecules to the solution.
Dissolved oxygen gets into water through natural processes, either as a byproduct of plant photosynthesis, or through the air. As aquatic plants such as seaweed, phytoplankton, and algae engage in photosynthesis, they create dissolved oxygen as a waste product.
Oxygen also diffuses across the surface of water bodies. This diffusion takes place at different speeds, depending on what helps the oxygen mix in. It slowly enters the water over time just by being in the atmosphere near the water, but it can also be mixed in more rapidly through natural or man-made aeration.
Natural aeration may be caused by groundwater discharge, waterfalls, rapids, other sources of running water, or in the case of larger water bodies, wind which creates waves. Examples of human-made aeration include anything from a massive dam to a smaller water wheel on a stream, or even an air pump in an aquarium.
Dissolved Oxygen in Water
Dissolved oxygen in water is essential to aquatic life. This is because animals living in water cannot access the oxygen that is compounded into water molecules.
This dissolved oxygen is a tiny amount—usually only around ten or fewer molecules of oxygen per million water molecules—yet it is critical to the health of aquatic ecosystems. It is also essential to the food web, as smaller organisms like phytoplankton rely on it and provide sustenance to larger creatures.
Dissolved oxygen in water is a core aspect of public health and water quality. DO is essential to aquatic life, of course, but humans also rely on economic resources such as recreational and commercial fisheries that rely on the right levels of DO in water.
Dissolved oxygen levels vary naturally in water depending on factors such as temperature and water flow. However, human activities such as release of nutrient-rich runoff from lawns, agricultural areas, and wastewater treatment facilities can also have a serious impact.
Why Monitor Dissolved Oxygen in Water
Why monitor dissolved oxygen in water? There are many reasons. Here are some of the most critical.
If dissolved oxygen levels get too low, fish mortality rates increase. More sensitive freshwater species such as trout can’t even reproduce when DO levels fall lower than 6 mg/L. As low as 2.0 mg/L, only freshwater species such as pike can survive.
In saltwater, fish that live near the coast have the highest dissolved oxygen requirements. These coastal species cannot live in a region when levels drop lower than 3.5 mg/L. DO levels of less than 2.0 mg/L are insufficient for invertebrates, and even benthic species are at risk in regions with dissolved oxygen levels below 1 mg/L.
Americans rely on aquatic life economically and culturally. And obviously, freshwater supplies are also essential sources of drinking water across the US.
Although there are natural processes that help regulate dissolved oxygen in water, there are many reasons to conduct dissolved oxygen monitoring. Preventing fishkills, winterkills, and gas bubble disease are among them. Mitigating hypoxia is another. Monitoring dissolved oxygen at hydropower facilities is an additional key area.
Monitoring Dissolved Oxygen to Prevent Fishkills
A fishkill refers to a large number of fish dying, either in a portion of a water body or the entire body of water. A fishkill can be water-wide, or species-specific. Low dissolved oxygen levels typically figure into fish kills, although they can occur for many reasons.
A body of water is most likely to be overproductive, or to use up its dissolved oxygen more quickly than it can be replenished, when there are too many organisms in the water. This frequently happens when a large algal bloom dies.
Fish kills happen more often in lakes that are eutrophic, that have high nutrient concentrations, especially nitrogen and phosphorus. These high nutrient levels fuel algal blooms, and at first this causes a jump in DO levels.
However, more organisms ultimately means more respiration. The algae need dissolved oxygen to respire, and when they die, they decompose. Bacteria make this happen, and they do this using up any available dissolved oxygen.
The result is an oxygen-depleted, or anoxic, water body that cannot support most aquatic organisms.
Monitoring dissolved oxygen to prevent winterkills
Another way for this kind of low-oxygen environment to occur is during wintertime, when snow and ice might cover a pond or lake. These kinds of conditions are called a winterkill when the respiration of aquatic plants, fish, and other organisms uses up the oxygen present too quickly and the organisms die off. In this situation, photosynthesis is the sole source of oxygen in the water, as atmospheric diffusion is cut off.
Should snow blanket the ice topping the water, photosynthesis will stop, and the organisms in the water will quickly use up the oxygen present. The situation is often also more serious for shallower bodies of water, which are typically home to more organisms.
Monitoring dissolved oxygen for supersaturation and gas bubble disease
Although low levels of dissolved oxygen in water can cause many serious issues, so can DO concentrations that are too high. Too much dissolved oxygen in water leads to supersaturated conditions and gas bubble disease.
The disease impacts both fish and invertebrates, although invertebrates are more tolerant of lower oxygen conditions. Should dissolved oxygen concentrations stay higher than 115 to 120 percent air saturation, you can expect to see significant mortality rates across species, and total death among some more sensitive species such as trout.
Supersaturated conditions are more likely to persist near hydropower dams and waterfalls, where the water is highly aerated. Excessive photosynthetic activity, such as that engaged in by algae blooms, can also cause supersaturation. This is often exacerbated by the fact that water becomes 100% saturated at lower dissolved oxygen concentrations when higher water temperatures are present.
Monitoring dissolved oxygen levels to prevent dead zones
A dead zone is so named because it cannot support aquatic life due to low levels of dissolved oxygen. Dead zones typically exist in oceans, along coasts and estuaries where there are large human populations, such as the Gulf of Mexico. Dead zones can also happen in larger rivers and lakes.
In general, the most common cause of these dead zones are large growths of phytoplankton and algae, usually caused by runoff. As the huge masses of algae and phytoplankton die, bottom-dwelling microbes decompose them, using up the local oxygen. These anoxic conditions are typically stratified, taking place only in the water body’s deeper layers.
There is an important difference between hypoxic regions that occur naturally, and anoxic dead zones. There are places where low dissolved oxygen levels occur naturally, and where benthic organisms in particular have adapted to survive. These conditions, fueled by water column stratification, occur most often in the deeper regions of the ocean and in very deep lake basins.
Monitoring dissolved oxygen by regulation
There are around 2,500 hydropower facilities in the United States. These facilities together generate approximately 7 percent of the nation’s electrical energy.
However, these dams serve as more than power sources. They can also function as local water supplies, and flood control points. Any of these functions demand careful monitoring for water quality, including dissolved oxygen.
Water that flows out of a hydropower facility is usually under-aerated, or much lower in dissolved oxygen than the water in the rest of the local ecosystem. The release of this under-aerated water downstream triggers a drop in dissolved oxygen downstream of the dam, threatening aquatic life.
A dissolved oxygen monitoring system confirms that dissolved oxygen levels up- and downstream of the dam are sufficiently similar. Dams are also river obstructions, impacting water quality and flow, and local animal behavior. This is why dissolved oxygen monitoring is integral to the proper, minimally impactful functioning of hydropower facilities.
Furthermore, dissolved oxygen monitoring is required for hydropower facilities by the Federal Energy Regulatory Commission (FERC) and other federal and state agencies. These facilities must report on dissolved oxygen levels according to procedure and within set limits to minimize the impact of the dam on the local ecosystem.
The FERC guidelines also provide mitigation measures managers can follow when dissolved oxygen levels fall too low. However, this kind of mitigating action is only effective in protecting the local ecosystem if managers can immediately recognize a problem.
Real-time dissolved oxygen monitoring systems provide data that help decision makers evaluate whether they are meeting water quality criteria. These systems generate data that reveal water quality relationships and trends, such as links between nutrient runoff and hypoxia, and the timing of hypoxic events. And they confirm for policymakers whether or not mitigating treatments and infrastructure improvements are working as planned.
Dissolved Oxygen and Aquatic Life
Just like land organisms, aquatic organisms need oxygen to respire. This means dissolved oxygen is essential to many forms of aquatic life, including bacteria, invertebrates, fish, and plants.
Crustaceans and fish respire with oxygen using their gills. And while phytoplankton and plant life typically use light to respire via photosynthesis, under no light conditions, they require dissolved oxygen.
How much dissolved oxygen an organism needs varies. Creatures that dwell near the bottom of a water body such as clams, crabs, and worms require less dissolved oxygen, between 1 to 6 mg/L. Fish that live in shallower water need more, between 4 to 15 mg/L dissolved oxygen.
Even fungi, bacteria, and other microbes need dissolved oxygen to decompose organic matter at depths. In fact, microbial decomposition fuels the recycling of nutrients in water.
As for those microbes, they adapt best to DO fluctuations. If bacteria run out of oxygen, they can engage in denitrification, the process of decomposing organic matter using nitrate. Should they run out of nitrogen, they can move on again, using sulfate. Eventually, if too much organic matter accumulates, and bacteria cannot decompose it all, the lake’s sediment simply changes, becoming more enriched.
However, too much decaying organic material (DOM) can accumulate as algae and other organisms die. When this happens, oxygen gets used up too rapidly at lower water levels in a stratified water body—one with infrequent or no turnover. This may enrich the sediment, but it has other negative consequences.
Monitoring Dissolved Oxygen in Water
Dissolved oxygen can be measured in multiple ways. The relationship between mg/L and % air saturation is important to understand, too.
Thanks to groundwater discharge, aeration from rapids, and relatively large surface area, streams and rivers usually stay close to 100% air saturation or slightly above it. This means that fluctuations in dissolved oxygen levels are usually due to changes in water temperature. Counterintuitively, groundwater-fed streams can hold more oxygen although groundwater DO levels are typically low. This is because the colder groundwater causes mixing as it comes into the stream or river.
Oceanic dissolved oxygen levels change with pressure, salinity, temperature, and other factors. This means annual dissolved oxygen concentrations near the surface are lower near the equator where salinity rises, around 4 mg/L, and higher near the poles, around 9 mg/L. Dissolved oxygen levels also drop at depth.
In some places, DO levels are directly controlled by regulations. For example, in states with their own Water Quality Standard Acts, minimum dissolved oxygen levels differ. However, in reality these variations are fairly minor, and standards are usually within 6 to 7 mg/L for cold-water fisheries and around 5 to 6 mg/L for warm-water fish.
This is because ideally, dissolved oxygen should be near 100% air saturation. This means freshwater tanks mimic optimal environmental conditions best around 8 mg/L of dissolved oxygen. For marine tanks, that level depends on the salinity, but should stay between 6 to 7 mg/L of dissolved oxygen.
Typical Levels of Dissolved Oxygen in Water
Various natural processes affect dissolved oxygen concentrations all of the time: aeration and diffusion, decomposition, respiration, and photosynthesis, for example. Water naturally tends to achieve equilibrium at near 100 percent air saturation. However, pressure changes, salinity, and temperature also cause dissolved oxygen levels to fluctuate.
Dissolved oxygen levels therefore range from greater than 20 mg/L to less than 1 mg/L, depending on how pressure changes, salinity, and temperature interact. Achieving stable dissolved oxygen concentrations is further complicated in freshwater systems such as ponds, streams, rivers, and lakes. This is because these are all affected by location, water depth, and seasonal variations.
Even a single body of freshwater might experience significant fluctuations in dissolved oxygen levels. Seasonal variations alone can mean dissolved oxygen levels that drop by half during the summertime, when the water is busy with life and autumn turnover hasn’t yet happened—or in wintertime, when ice covers a pond. Even daily fluctuations in dissolved oxygen levels due to photosynthetic production can vary notably.
Oceanic dissolved oxygen concentrations are usually lower than freshwater DO levels, because saltwater holds less oxygen. Surface water in the ocean changes with pressure, salinity, temperature, and other factors like mixing.
Dissolved Oxygen Units
You can report dissolved oxygen concentrations in water in different ways. Most often, DO levels are reported as a percent of air saturation or in milligrams per liter (mg/L).
However, some studies report dissolved oxygen in micromoles (umol) or parts per million (ppm). 1 ppm is equal to 1 mg/L. As mentioned above, the relationship between % air saturation and mg/L varies as salinity, pressure, and temperature fluctuate.
It is critical to monitor for dissolved oxygen throughout the water column. Stratification happens when factors such as temperature and the presence of dissolved substances such as oxygen and salt separate a water body into layers. Lake stratification is a common study area for scientists, although stratification also takes place in some estuaries, certain rivers with deep enough pooling in segments, and in the ocean.
Dissolved Oxygen Levels and Hydroelectric Facilities
Hydropower plants can have a significant impact on local environments. Among the most critical ways such a facility changes the environment around it is to lower levels of dissolved oxygen downstream.
To think about the impact of a hydropower facility in the right way, it’s important to distinguish the ways that natural processes like organism respiration, underwater photosynthesis, and wind effects cause dissolved oxygen levels to vary. This is because hydropower dams can and usually do cause more rapid and profound changes.
Reservoirs are typically deep enough for stratification to be an issue. This means that as dam managers release water from the reservoir’s bottom, the dissolved oxygen levels in that layer drop. Algae also use up oxygen in the impoundment as they respire, which can further lower dissolved oxygen levels. Therefore, dissolved oxygen concentrations should be monitored continuously to prevent harm to the environment, and to mitigate any existing risk.
FERC Dissolved Oxygen Monitoring
The Federal Energy Regulatory Commission (FERC) and other agencies have established dissolved oxygen limits for use by hydropower facility operations. These metrics exist to minimize the impact dams have on local aquatic habitats.
NexSens dissolved oxygen monitoring systems make near real-time monitoring of dissolved oxygen levels possible. These DO monitoring systems include flexible sensors that measure conditions, and data logging and communications capabilities.
Dissolved Oxygen Monitoring and FERC Licensing
Almost all operating hydroelectric facilities at the non-federal level must report to the Federal Energy Regulatory Commission (FERC). FERC issues operating licenses for these facilities, but the detailed rules and requirements that control licensees are set forth by environmental regulations controlled by lawmakers and promulgated by state and federal agencies.
For example, a FERC license places responsibilities on its holder based on the Clean Water Act (CWA). This means that a state level agency such as a Department of Natural Resources can require a CWA Section 401(a) Water Quality Certification under the Clean Water Act.
In other words, the state’s DNR can, under the law, oversee both recommended and required monitoring efforts mitigation plans designed to achieve compliance with water quality standards. Furthermore, any such requirements are typically part of the FERC hydropower license.
The FERC license itself sets forth duties that are specific to the facility, such as operational conditions and metrics. These can include minimum rules for flow, mitigation, metrics for substances like dissolved oxygen, and criteria for judging the health of aquatic habitats and life. A typical FERC hydropower license is issued for 30 to 50 years.
These licenses are specific, setting forth requirements that can vary substantially—and not just from state to state. FERC requirements might and often do vary from site to site based on both potential for environmental degradation and the dam structure itself.
All periods of non-compliance that are uncontrolled must be reported under the license, including both compliance with its standards and deviation from its articles. When periods of non-compliance come up, they must always be reported, but deviations from the license’s terms will have different consequences for the licensee, depending on the facts.
When natural events such as bad weather cause the period of non-compliance, the FERC licensee must still report the problem and correct them. However, in cases like these, penalties are typically not incurred. On the other hand, there may be penalties for intentional or systematic non-compliance.
Dissolved Oxygen Monitoring for Impoundments and Reservoirs
Hydroelectric facilities with deep impoundments or reservoirs typically face more licensing regulations. This is because local aquatic habitats and landscapes are so dramatically changed by the creation of any deep impoundment.
Dislocation, erosion, and flooding are all risks inherent to impoundment style facilities, but these facilities can also hurt water quality both up- and downstream. This is because like other still waters, reservoirs easily experience stratification. As demands on oxygen rise and aeration isn’t possible, the bottom layer of water is at risk of becoming hypoxic. Alongside a deep intake point, the release of oxygen-depleted water downstream can affect the dissolved oxygen concentrations throughout the river.
This is why FERC licenses often require dissolved oxygen monitoring at several points—most often up- and downstream as well as inside the impoundment. How often DO monitoring takes place at each point varies by state and locality, but continuous or near-continuous dissolved oxygen monitoring is the gold standard for deep hydro intake dams.
Real-time data is invaluable to quick detection of problems and timely mitigation efforts at hydropower facilities. However, even less frequent sampling intervals such as scheduled weekly events enhance the team’s ability to re-establish compliance with FERC after a rapid respond to low dissolved oxygen conditions.
Dissolved Oxygen Monitoring Technology
The classic means for testing dissolved oxygen levels involves the Winkler titration method. However, any field scientist can tell you how difficult and time-consuming this is compared to using dissolved oxygen sensors.
DO sensors are more accurate and provide real-time results—and they can even be deployed for quality assurance and spot sampling. There are galvanic, rapid-pulsing, optical, and polarographic dissolved oxygen sensors on the market, but optical and submerged rapid-pulsing technologies are most frequently used in hydropower facilities. This is because it’s quicker and easier to launch more successful control measures with real-time water quality data.
Typical Dissolved Oxygen Monitoring System
FERC requires many hydropower facilities to monitor dissolved oxygen levels, usually with three monitoring stations. One provides background data collected upstream. One offers risk analysis data from within the impoundment. The last is situated at a point impacted by the dam release, downstream.
Dissolved oxygen monitoring data is most effective in real-time. An integrated telemetry system built into the DO monitoring system is the most efficient way to achieve this goal. As sensors are placed at multiple depths, a data logger can support them all, taking and logging data at intervals the user can define. The system then uses secure telemetry to transmit the data in real-time for online access from any smartphone or computer.
FERC license requirements are specific, but they still allow for customized, integrated telemetry stations and data logging. These systems can house as many sensors as an application requires, all powered by a recharging solar panel and central battery system.
A dissolved oxygen monitoring system for a hydroelectric power facility usually includes an integrated data logging system which includes the data logger itself, along with the power supple and telemetry module. The system might be mounted to infrastructure such as a bridge, pole, wall, or pier. Specific satellite monitoring stations can either be attached to infrastructure or deployed with buoys.
Deployment pipes constructed of metal or PVC typically protect the dissolved oxygen sensors underwater. This protects them not from the water itself, but from damage related to debris and fouling. It also enhances ease of use, making the sensors easier to remove, calibrate, and deploy.
Both optical and rapid-pulsing dissolved oxygen sensors can be deployed at multiple depths in the water column remotely. This enables facility managers to measure DO levels both up- and downstream, and identify stratification problems more readily.
Dissolved oxygen monitoring stations
How many FERC-mandated monitoring stations a hydroelectric facility needs and where they should be located depends mostly on the structure of the facility and the license requirements—most frequently, three stations are mandated. Each station can be mounted to infrastructure or buoy-based, and should include as many sensors as needed and a data logging telemetry system.
Required monitoring stations can be located downstream, upstream, and/or inside the impoundment. To ensure accurate data, dissolved oxygen monitoring stations should be installed at fixed points.
Downstream stations obviously provide information about immediate impact, and can be located just at the outlet point, following a weir, in the tailrace, or downstream from the impoundment and facility. Background data is essential to assessing impact, and this kind of information comes from upstream monitoring stations. Impoundment monitoring stations offer stratification data from multiple depths, as well as dissolved oxygen levels and fluctuations near the intake point.
For measuring more comprehensive data and a complete water quality profile throughout the water-column, it may be more effective to deploy a buoy-based monitoring system. Buoy-based systems can be set up almost anywhere, providing a stable platform that can support multiple sensors throughout the water column.
Like dissolved oxygen monitoring systems attached to infrastructure, data buoys hold a solar-powered battery pack, a data logger, and whatever telemetry options are best for their location. Data buoys come in various sizes, but usually 150 to 450 lb. net buoyancy is sufficient for monitoring dissolved oxygen in rivers and reservoirs.
Preparing to monitor dissolved oxygen near a hydroelectric facility
Before deployment, it is critical to calibrate and test all equipment. Calibration and testing is just as important throughout the life of the equipment, and knowing how it all works before deployment is essential to making maintenance successful.
You can mount dissolved oxygen monitoring systems almost anywhere a structure exists, either with or without a protective stainless steel enclosure. Frequently used deployment locations include bridges, dam walls, docks, lock structures, piers, poles, railroad trestles. The sensors work in the water suspended from the station inside a perforated pipe made of metal or PVC. This guards sensors against damage from debris in the water and ensures more accurate results and easier removal for recalibration.
Buoy-based systems must be moored to the bottom to ensure consistent results. Two-point mooring is recommended for monitoring dissolved oxygen in a reservoir. This is because two-point mooring clears the water column below the buoy for sensors, and provides increased stability against turbulence or currents.
FERC licensure may not specifically require maintenance at particular intervals, but such maintenance is inevitable and necessary for all dissolved oxygen monitoring equipment. Without upkeep, there is no way to promote longer instrument life or prevent sensor drift.
Dissolved oxygen sensor maintenance includes replacing any missing or damaged O-rings to prevent water ingress, and cleaning the sensor itself to prevent fouling. Other servicing of dissolved oxygen probes can be needed, and other sensors will demand attention if the station monitors for other parameters. Although site conditions vary, maintenance intervals should be frequent, and DO sensors should checked and calibrated once or twice a month, or as the manufacturer recommends.
Dissolved oxygen sensors can be calibrated with a single point, although two-point calibrations verify accuracy across the range of measurement. Conduct a single-point calibration at 100% air saturation, adding a second calibration with a zero oxygen solution to make your process a two-point calibration.
When monitoring a hydropower dam for dissolved oxygen levels, it is useful to have a spare dissolved oxygen probe, sensor, or sonde on hand. This cuts downtime when sensors occasionally fail, which can impact FERC reporting requirements.
It’s just as important to ensure monitoring equipment remains calibrated, and to periodically verify that the data from the dissolved oxygen sensors are accurate. Use a separate, portable DO instrument such as a dissolved oxygen meter for spot sampling to confirm your system is still providing reliable data.
Optical dissolved oxygen sensors are often best for this kind of spot checking, as they are more accurate and work independent of flow conditions. On the other hand, if you have already deployed a sonde, it is wise to spot check with a similar instrument.