Dissolved Oxygen
Why are we concerned?
Aquatic plants and animals depend on dissolved oxygen (D.O.) for survival. Lack of dissolved oxygen can cause aquatic animals (e.g. fish, macroinvertebrates) to quickly leave the area or face death.
D.O. concentrations are influenced by many factors including water temperature, the rate of photosynthesis, the degree of light penetration (turbidity and water depth), the degree of water turbulence or wave action, and the amount of oxygen used by respiration and decay of organic matter.
You can assess the range of dissolved oxygen levels that aquatic plants and animals at your stream site must tolerate by monitoring twice in one day – early in the morning, just before sunrise, and later in the afternoon when plants have been exposed to the most direct sunlight for an extended period.
The background science
Oxygen is a clear, colorless, odorless, and tasteless gas. Small but important amounts of it are dissolved in water. It is supplied by diffusion of atmospheric oxygen (air) into the water and by production of oxygen from photosynthesis by aquatic plants. Wind, waves, and tumbling water in fast-moving streams increase the rate of diffusion.
Measuring dissolved oxygen is probably the most significant water quality test to determine the suitability of a stream for fish and many other aquatic organisms. However, these measures only provide a snapshot of the oxygen levels at that particular time. Levels can fluctuate widely throughout the day and year. Fish and other organisms have to live and breathe in that water all year long. A short time without oxygen can be fatal.
Dissolved oxygen (D.O.) is reported as milligrams of oxygen per liter of water (mg/L) which can be called parts per million by weight (ppm). Different aquatic organisms have different oxygen needs.
Organisms |
Minimum Dissolved Oxygen |
|---|---|
|
Trout during spawning |
7 mg/L |
|
Trout out of spawning season |
6 mg/L |
|
Stone fly and similar macroinvertebrates |
6 mg/L |
|
Warm water fish such as bass and bluegills |
5 mg/L |
|
Forage fish |
3 mg/L |
|
Carp |
1 mg/L |
|
Bloodworms |
1 mg/L |
The oxygen demand of aquatic plants and cold-blooded animals also varies with water temperature. A trout uses five times more oxygen while resting at 80° F (26.7° C) than at 40° F (4.4° C).
Daily Cycle of Dissolved Oxygen
There are many factors that affect the amount of dissolved oxygen in the water. A major one is photosynthesis. Aquatic plants produce oxygen by photosynthesis during daylight hours but they also use oxygen for respiration at night. High daytime levels of D.O. are often countered with low nighttime levels as shown in the sample diel cycle for dissolved oxygen in the image. This is due to respiration of living organisms, including fish, bacteria, fungi and protozoans, as well as the cessation of photosynthesis. Wide daily fluctuations of D.O. stress fish and other aquatic animals.
Factors that change dissolved oxygen levels
In addition to dissolved oxygen levels changing on a daily cycle, there are many other reasons why dissolved oxygen might be higher or lower in a stream.
Factors that could INCREASE the amount of dissolved oxygen in water |
Factors that could DECREASE the amount of dissolved oxygen in water |
|---|---|
|
Cold water |
Warm water |
|
Clear water |
High levels of turbidity ( such as from erosion); stained or colored water |
|
Photosynthesis of plants and algae living in the water |
Respiration of animals and plants living in the water |
|
Presence of excessive amounts of plants (during daytime) |
Presence of excessive amounts of plants (overnight) |
|
High atmospheric pressure |
Low atmosphere pressure |
|
Turbulence and / or wave action |
|
|
|
Organic materials such as sewage, manure, or fertilizers; the chemical reactions of the decaying process |
Recording dissolved oxygen differs from other tests in that it requires two distinct calculations. We are interested in both the absolute amount of D.O. (mg/L or ppm) and how close the value is to the equilibrium value for that temperature and air pressure, which is the percentage of saturation. Values between 91% and 110% of saturation are excellent, between 71% and 90% are good, and between 51% and 70% are fair according to the Field Manual for Water Quality Monitoring (13th Edition). Supersaturated (over 100%) values may sound good but they can also indicate problems, such as excessive plant growth.
More information is available in the fact sheet Dissolved Oxygen: Aquatic Life Depends on It.
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