Dissolved Oxygen
| Table 1: Minimum DO requirement | |
Body | Minimum dissolved oxygen (mg/L) |
| Trout | 6.5 |
| Smallmouth bass | 6.5 |
| Caddisfly larvae | 4.0 |
| Mayfly larvae | 4.0 |
| Bullhead | 2.5 |
| Large tent | 2.0 |
| Mosquito Larvae | 1.0 |
Gaseous oxygen dissolved in water is vital for the existence of most aquatic organisms. Oxygen is a key component
in cellular respiration for both aquatic and terrestrial life. The concentration of dissolved oxygen (DO) in an aquatic environment is an important indicator of ambient water quality.
Some organisms, such as salmon, mayflies, and trout, require high dissolved oxygen concentrations. Other organisms, such as catfish, mosquito larvae, and carp, can survive in environments with low dissolved oxygen concentrations. The diversity of organisms is much greater at high DO concentrations. Table 1 lists the minimum dissolved oxygen concentrations required to support various animals.
- Atmosphere diffusion
- Oxygenation by the movement of water over rocks or debris
- Oxygenation by wind or waves
- Photosynthesis of aquatic plants
DO sources
Oxygen gas is dissolved in water by a variety of processes, including diffusion between the atmosphere and water, oxygenation by the flow of water over rocks and other debris, the agitation of water by waves and wind, and photosynthesis by aquatic plants. Many factors affect the concentration of dissolved oxygen in an aquatic environment. These factors include temperature, stream flow, air pressure, aquatic plants, decaying organic matter, and human activity.
- Temperature
- Aquatic plant populations
- Decomposing organic material in water
- Flow of currents
- Atmospheric pressure and altitude
- Human Activity
Factors affecting DO levels
As a result of plant activity, DO levels can fluctuate during the day, rising throughout the morning and reaching a maximum in the afternoon. At night, photosynthesis ceases, but plants and animals continue to respire, causing DO levels to decrease. Since daily fluctuations are possible, DO tests should be performed at the same time each day. Large fluctuations in dissolved oxygen levels over short periods of time can result in algae blooms. Since the algae population is growing rapidly, dissolved oxygen levels increase. Soon, algae begin to grow.
die and are decomposed by aerobic bacteria, which use oxygen. As more algae die, the oxygen requirement for aerobic decomposition increases, which
This results in a sharp drop in oxygen levels. Following an algal bloom, oxygen levels can be so low that fish and other aquatic organisms suffocate and die.
| Table 2 | |
| DO level | DO Saturation Percentage |
| Supersaturation1 | 101% |
| Excellent | 90 – 100% |
| Appropriate | 80 – 89% |
| Acceptable | 60 – 79% |
| Poor | 60% |
Temperature is an important factor in oxygen's ability to dissolve, as oxygen, like all gases, has different solubilities at different temperatures. Colder waters have a higher oxygen capacity.
dissolved oxygen than warmer waters. Human activity, such as the removal of foliage along a stream or the release of warm water used in industrial processes, can cause water temperatures to rise across a given narrowing of the stream. This results in a reduced ability of the stream to dissolve oxygen.
Expected Levels
The unit mg/L2 is the amount of gaseous oxygen
dissolved in one liter of water. When relating DO measurements to minimum levels required by aquatic organisms, the unit mg/L is used. The procedure described in this document covers the use of a Dissolved Oxygen Sensor to measure DO concentration in mg/L. Dissolved oxygen concentration can range from 0 to 15 mg/L. Cold mountain streams will likely have DO concentrations of 7 to 15 mg/L, depending on water temperature and air pressure. At their lowest reaches, rivers and streams may exhibit DO concentrations between 2 and 11 mg/L.
When analyzing the water quality of a stream or river, it is convenient to use a unit other than mg/L. The term percent saturation is often used for water quality comparisons. The percent saturation is the dissolved oxygen reading in mg/L divided by the 100% dissolved oxygen value for the water (at the same temperature and air pressure). How percent saturation relates to water quality is shown in Table 2. In some cases, water may exceed 100% saturation and become supersaturated for short periods of time.
Summary of Methods
Dissolved oxygen can be measured directly on site or in water samples transported from the site. Measurements can be made on site either by placing the Dissolved Oxygen Sensor directly in the stream away from shore or by collecting a water sample with a container or jar and then making measurements with the Dissolved Oxygen Sensor back at shore. Water samples collected on site and transported back to the laboratory in sealed bottles should be stored in ice containers or in a refrigerator until measurements are taken. Transporting samples is not recommended because it reduces the accuracy of the test results.¨C9C
- Supersaturation can be harmful to aquatic organisms. It can lead to a disease called Gas Bubble Disease.
- The mg/L unit is numerically equal to the so-called parts per million or ppm.
Dissolved Oxygen¨C10C
¨C11C
How the Dissolved Oxygen Sensor Works
membrane¨C12C
membrane
platinum (cathode)
¨C13C
Ag/AgCl (anode)
KCl (aq)
Membrane cap
| ¨C14C |
The Dissolved Oxygen Sensor is a Clark-type polarographic electrode that measures the concentration of dissolved oxygen in water and aqueous solutions. A platinum cathode and a silver/silver chloride reference anode in a KCl electrolyte are separated from the sample by a gas-permeable plastic membrane.
¨C15C
membrane cap¨C16C
The platinum electrode has a fixed voltage applied. Oxygen is reduced as it diffuses through the membrane to the cathode:
½ O2 + H2O + 2e- 2 OH-
The oxidation that takes place at the reference electrode (anode) is:
Ag + Cl- AgCl + e-
Accordingly, a current flows that is proportional to the rate of oxygen diffusion, and therefore to the concentration of dissolved oxygen in the sample. This current is converted to a proportional voltage, which is amplified and read by one of the Vernier interfaces.
¨C17C
Dissolved Oxygen Sensor Storage
Follow these steps when storing the electrode:
- Long-term storage (more than 24 hours): Remove the membrane cap and wash the inside and outside of the membrane with distilled water. Shake the membrane cap dry. Also wash and dry the exposed anode and cathode interior elements (dry with laboratory paper towels). Carefully reinstall the membrane cap onto the electrode body for storage. Do not screw it on too tightly.
- Short-term storage (less than 24 hours): Store the Dissolved Oxygen Sensor with the membrane end submerged in approximately 1 inch of distilled water.
Automatic Temperature Compensation
The Vernier Dissolved Oxygen Sensor has automatic temperature compensation because it uses a thermistor built into the sensor body. The temperature output from this sensor is used to automatically compensate for changes in membrane permeability due to changes in temperature. If the sensor did not have temperature compensation, you would notice a change in the dissolved oxygen reading as the temperature changed, even though the actual concentration of dissolved oxygen in the solution did not change.
Here are two examples of how automatic temperature compensation works:
- If you calibrate the Dissolved Oxygen Sensor in the laboratory at 25° C and 760 mm Hg barometric pressure (assuming negligible salinity), the value you enter for the saturated oxygen calibration point should be 8.36 mg/L (see Table 3). If you were to take a reading on distilled water that is saturated with oxygen by vigorous, rapid stirring, you would get a reading of 8.36 mg/L. If the water sample were then cooled to 10° C without further agitation, the water would no longer be saturated (cold water can hold more dissolved oxygen than warm water). Therefore, the temperature-compensated Dissolved Oxygen Sensor reading should be 8.36 mg/L.¨C18C
- If the solution is cooled, however, to 10° C and is constantly stirred so that it remains saturated by the additional oxygen solution, the temperature compensation of the sensor should produce a reading of 11.35 mg/L which is the value shown in Table 3. Note: Temperature compensation It does not mean that the reading of a saturated solution will be the same at two different temperatures, since the two solutions have different
oxygen concentrations Saturated dissolved oxygen vs. temperature
dissolved and the sensor readings should reflect this difference.
¨C19C
Sampling in Oceanic Saltwater or Estuaries
(at salinity levels greater than 1000 mg/L)
The dissolved oxygen concentration for air-saturated water at various salinity levels, DO(salt), can be calculated using the formula:
DO(salt) = DO – (k•S)
- DO(salt) is the concentration of dissolved oxygen (in mg/L) in saltwater solutions.
- DO is the dissolved oxygen concentration for air-saturated distilled water as determined from Table 3.
- S is the salinity value (in ppt). Salinity values can be determined using the Vernier Chloride Ion Selective Electrode or the Conductivity Sensor as described in Activity 40. Dissolved Oxygen¨C20C
- k is a constant. The value of k varies according to temperature and can be determined from Table 5.
| Table 5: Salinity correction constant values | |||||||
| Temp. (°C) | Constant, k | Temp. (°C) | Constant, k | Temp. (°C) | Constant, k | Temp. (°C) | Constant, k |
| 1 | 0.08796 | 8 | 0.06916 | 15 | 0.05602 | 22 | 0.04754 |
| 2 | 0.08485 | 9 | 0.06697 | 16 | 0.05456 | 23 | 0.04662 |
| 3 | 0.08184 | 10 | 0.06478 | 17 | 0.05328 | 24 | 0.04580 |
| 4 | 0.07911 | 11 | 0.06286 | 18 | 0.05201 | 25 | 0.04498 |
| 5 | 0.07646 | 12 | 0.06104 | 19 | 0.05073 | 26 | 0.04425 |
| 6 | 0.07391 | 13 | 0.05931 | 20 | 0.04964 | 27 | 0.04361 |
| 7 | 0.07135 | 14 | 0.05757 | 21 | 0.04854 | 28 | 0.04296 |
Example: Determine the calibration value of saturated oxygen DO at the temperature of 23°C and the pressure of 750 mm Hg, when the Dissolved Oxygen Sensor is used in seawater with a salinity value of 35.0 ppt.
First you have to find the dissolved oxygen value in Table 3 (DO = 8.55 mg/L). Then you have to find the value of k in Table 5 at 23° C (k = 0.04662). These values, as well as the salinity value, must then be substituted into the previous equation:
DO(sal) = DO – (k•S) = 8.55 – (0.04662 • 35.0) = 8.55 – 1.63 = 6.92 mg/L
Use the value 8.46 mg/L when performing the DO saturation calibration point (air saturated water), as described in Step 6. The Dissolved Oxygen Sensor will now be calibrated to give the correct value for DO readings in saltwater samples with a salinity of 35.0 ppt.
Important: For most dissolved oxygen assays, No It is necessary to compensate for salinity; for example, if the salinity value is 0.5 ppt, using 25° C and 760 mm Hg, the calculation for DO(s) should be:
DO(sal) = DO – (k•S) = 8.36 – (0.04498 • 0.5) = 8.36 – 0.023 = 8.34 mg/L
At salinity levels less than 1.0 ppt, neglecting this correction means an error of less than 0.2%.
