What is dissolved oxygen?

What is dissolved oxygen(DO)?

Dissolved oxygen (DO) is a measure of how much oxygen is dissolved in water or other liquids. It is an important parameter in assessing water quality because it has an impact on the organisms living in the water body. The amount of dissolved oxygen in a stream or lake can tell us a lot about its water quality. High or low DO levels can harm aquatic organisms and affect water quality.

In scientific terms, dissolved oxygen is the level of free, non-combined oxygen present in water or other liquids. Uncomplexed oxygen or free oxygen (O2) is oxygen that is not bound to any other element. Dissolved oxygen is the presence of these free oxygen molecules in the water. The bonded oxygen molecules (H2O) in water are present in compounds and are not counted in the dissolved oxygen level. As you can imagine, free oxygen molecules dissolve in water in a manner very similar to how salt or sugar dissolves when stirred.

Dissolved oxygen units are usually expressed in parts per million (ppm) or milligrams per liter (mg/L) concentrations. Concentrations can also be expressed as percent saturation, where saturation is the maximum amount of oxygen that can theoretically be dissolved in water at a given pressure and temperature.

Where does dissolved oxygen come from?

Dissolved oxygen in water mainly comes from the atmosphere and photosynthesis. Oxygen from the air enters the water naturally through diffusion, while wind, waves, flowing water and artificial aeration can increase the rate of oxygen transfer. Aquatic plants and algae also release oxygen into the water during photosynthesis when sufficient light is available. In rivers, lakes and reservoirs, the actual DO level depends on the balance between these oxygen inputs and oxygen consumption by fish, microorganisms, plant respiration and the decomposition of organic matter.

Dissolved oxygen in water

Dissolved oxygen(DO) is essential for freshwater organisms such as fish, aquatic insects, zooplankton, and aerobic microorganisms. However, oxygen requirements vary considerably among species and life stages. Cold-water fish such as trout and salmon generally require higher dissolved oxygen levels, while some warm-water species are more tolerant of lower DO conditions. As dissolved oxygen decreases, sensitive species may experience reduced feeding, growth, reproduction, and activity before more tolerant organisms are affected.

Marine organisms, including fish, shrimp, crabs, shellfish, and many bottom-dwelling invertebrates, also depend on dissolved oxygen for respiration. Oxygen availability in seawater is influenced not only by biological activity and temperature but also by salinity, since oxygen becomes less soluble as salinity increases. In coastal waters and estuaries, low DO can force mobile organisms to leave affected areas, while less mobile species may experience physiological stress or mortality.

Dissolved oxygen can fall rapidly when large amounts of organic matter enter the water. Algae, plant material, wastewater, and other organic matter are decomposed by bacteria and other microorganisms, which consume oxygen during the process. When oxygen consumption exceeds oxygen replenishment from the atmosphere, photosynthesis, or water mixing, hypoxic conditions can develop. If oxygen is almost completely depleted, the water becomes anoxic. Prolonged hypoxia or anoxia can stress or kill aquatic organisms and, in severe cases, contribute to the formation of so-called “dead zones.”

Temperature and dissolved oxygen

Dissolved oxygen concentration in surface water is affected by temperature and has a seasonal and daily cycle. Cold water can hold more dissolved oxygen than warm water. In winter and early spring, when water temperatures are cooler, dissolved oxygen concentrations are higher. Dissolved oxygen concentrations tend to be lower in summer and fall, when water temperatures are warmer.

All forms of aquatic life use dissolved oxygen in surface water; therefore, this component is often measured to assess the “health” of lakes and streams. Oxygen enters streams from atmospheric and groundwater emissions. However, the contribution of oxygen in groundwater discharge is significant, but only in areas where groundwater is an important component of streamflow, such as in areas of glacial deposition. Photosynthesis is the primary process affecting the dissolved oxygen/temperature relationship; in turn, water clarity, light intensity and duration affect the rate of photosynthesis.

Temperature（C)	CsDissolved oxygen（mg/L)	Temperature（C)	CsDissolved oxygen（mg/L)
0	14.64	18	9.46
1	14.22	19	9.27
2	13.82	20	9.08
3	13.44	21	8.90
4	13.09	22	8.73
5	12.74	23	8.57
6	12.42	24	8.41
7	12.11	25	8.25
8	11.81	26	8.11
9	11.53	27	7.96
10	11.26	28	7.82
11	11.01	29	7.69
12	10.77	30	7.56
13	10.53	31	7.43
14	10.30	32	7.30
15	10.08	33	7.18
16	9.86	34	7.07
17	9.66	35	6.95

Dissolved oxygen measurement

Dissolved oxygen is considered an important indicator of water quality because it is a direct indicator of the ability of aquatic resources to support aquatic life. Dissolved oxygen levels are measured using a calibrated water quality probe meter, usually in combination with temperature and pH measurements. While each organism has its own dissolved oxygen tolerance range, in general, DO levels below 3 milligrams per liter (mg/L) are of concern, and water below 1 mg/L is considered hypoxic and usually lifeless.

Dissolved oxygen can be measured using electrochemical or optical sensors. Optical DO sensors are often preferred for long-term online monitoring because they require less routine maintenance and do not consume oxygen during measurement, while electrochemical sensors remain widely used in portable and general water-quality measurements.

Dissolved oxygen reading interpretation (mg/L)

0-2 mg/L: not enough oxygen to sustain life
2-4 mg/L: Only a few fish and insects can survive
4-7 mg/L: Acceptable for warm water fish
7-11 mg/L: ideal for most stream fish, including cold-water fish

For percent saturation.

Below 60%: poor; water too hot or bacteria depleting dissolved oxygen
60-79%: acceptable for most aquatic organisms
80-125%: very suitable for most aquatic organisms
112% or more: too high and may be dangerous to fish

Dissolved oxygen in different water applications

In aquaculture, DO affects fish and shrimp health, so online sensors are often used for continuous monitoring and aeration control.

In wastewater treatment, DO supports aerobic microorganisms and helps control aeration efficiency.

In rivers and lakes, DO indicates ecological conditions and is commonly checked with portable or online meters.

In industrial water, DO is monitored for corrosion, oxidation and process control.

More articles on dissolved oxygen:

Why is water quality important?
Main water quality indicators
What is salinity?
What is pH in water test?