Dissolved Oxygen in Water: Why It Matters

Dissolved oxygen (DO) is the oxygen gas mixed into water, measured in milligrams per liter (mg/L), which in fresh water is essentially parts per million. It is what fish, aquatic insects and water-cleaning bacteria breathe. Most fish are stressed when DO falls below 5 mg/L, and below 3 mg/L water generally can't support them at all. Fresh water can hold no more than about 14.6 mg/L near freezing and only about 7.6 mg/L at 30 °C, so temperature is the biggest single control on how much oxygen you'll find. Here's where it comes from, what moves it, and how to read the numbers.

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Dissolved oxygen at a glance

The most oxygen water can hold when it is in balance with the air is called its saturation value, and it falls steadily as water warms:

Read top to bottom and the pattern is clear: warming water from 0 to 30 °C strips out nearly half of the oxygen it can hold. That inverse relationship is why the U.S. Geological Survey (USGS) reports that dissolved oxygen in surface water tends to be high in winter and early spring and lower in summer and fall.

Saturation values are for fresh water at sea-level pressure, rounded from standard oxygen-solubility tables. Real water is often above or below them, as the next sections explain.

Where dissolved oxygen comes from, and where it goes

Water trades oxygen constantly. The DO you measure is the balance between what is being added and what is being used up.

Where it comes from:

• The atmosphere. Oxygen dissolves in at the surface. Turbulence from riffles, waterfalls and wind-driven waves keeps mixing fresh air in, which is why a fast mountain stream carries more oxygen than a still pond.
• Photosynthesis. Aquatic plants and algae release oxygen in daylight. In productive water this can push DO above 100% saturation on a sunny afternoon.
• Groundwater. Discharge adds a meaningful amount only where groundwater supplies a large share of a stream's flow.

Where it goes:

• Respiration. Fish, insects, plants and algae all consume oxygen, and plants and algae keep doing it after dark.
• Decomposition. Bacteria use oxygen to break down dead plants, leaves, manure and sewage. The oxygen consumed this way is called biochemical oxygen demand (BOD), and it rises with the amount of organic matter in the water.
• Warming. As water heats up, it can hold less, and the surplus escapes to the air.

Because photosynthesis stops at night while respiration doesn't, DO in many ponds and slow rivers follows a daily cycle: highest in late afternoon and lowest just before dawn. A pond that looks healthy at noon can be dangerously low at 5 a.m.

What changes how much oxygen water holds

Four physical factors set the ceiling; biology then pulls the real number up or down.

Temperature. Solubility falls as water warms, as the table above shows. Heat also speeds up respiration and decay, so warm water both holds less oxygen and loses it faster.

Pressure and altitude. The amount of oxygen that dissolves is proportional to the pressure of the air above the water. Thinner mountain air means less dissolved oxygen: roughly 3–4% less for every 1,000 ft of elevation, by our estimate. At about 5,000 ft, 20 °C water tops out near 7.5 mg/L instead of the 9.1 mg/L you'd see at sea level.

Salinity. Dissolved salts crowd out oxygen. Seawater holds roughly 20% less than fresh water at the same temperature.

Turbulence and depth. Mixing pulls water toward saturation, while depth cuts water off from both air and sunlight. In a stratified summer lake, the warm surface can be too hot for cold-water fish while the deep layer runs short of oxygen.

Why dissolved oxygen matters

Aquatic life

Every aquatic animal needs dissolved oxygen to breathe. As levels drop, pollution-sensitive insects such as some mayflies, stoneflies and caddisflies disappear first and are replaced by tolerant worms and fly larvae. Fish move away, weaken or die. Many summer fish kills are oxygen problems: hot, calm weather, warm water and heavy decay all arrive together.

Nutrients and dead zones

Excess nitrogen and phosphorus fuel algal blooms. When the blooms die, bacteria decompose them and use up the oxygen in the process. Scaled up, that is how the seasonal dead zone in the northern Gulf forms. USGS defines hypoxic water as anything below 2 mg/L.

Wastewater treatment

Treatment plants blow air into their basins on purpose, because the bacteria that break down waste need oxygen to do it. Untreated waste with a high BOD can pull a receiving stream's oxygen down for miles.

Your home water

Dissolved oxygen in drinking water is harmless, and there is no federal drinking water standard for it. It still shows up in practical ways:

• Cloudy tap water. Milky water that clears from the bottom of the glass upward is dissolved air (oxygen and nitrogen) coming out of solution as pressure drops or water warms. It is not a contaminant.
• Corrosion. Oxygen is a major driver of corrosion in steel and iron piping, which is why boiler systems deliberately remove it.
• Iron and sulfur in wells. Deep groundwater is often low in oxygen, which lets iron and manganese dissolve and hydrogen sulfide form. When that water reaches air, oxygen converts the dissolved iron into rust-colored particles that stain fixtures. If that sounds like your water, the iron filter sizing calculator is a good next step.

DO is also one of the standard inputs to a water quality index, which you can explore with the water quality index calculator.

What is a healthy dissolved oxygen level?

Tolerance varies by species, and eggs and young fish generally need more oxygen than adults. Still, a few rule-of-thumb ranges cover most freshwater situations:

The 5 and 3 mg/L cutoffs follow EPA guidance. Hypoxia is defined slightly differently depending on the agency: USGS uses below 2 mg/L, while EPA's aquatic survey indicator uses below 1 mg/L, hence the range in the last row.

How dissolved oxygen is measured

Three methods cover almost every measurement:

• Winkler titration. The classic laboratory method. Oxygen in a sealed sample is chemically fixed and then titrated. It is the reference that other methods are checked against.
• Electrochemical (membrane) probes. Oxygen diffuses across a membrane and produces a small current proportional to its concentration. These probes need water flowing past the membrane to read correctly.
• Optical (luminescent) sensors. A dye's glow fades in the presence of oxygen, and the sensor reads the change. They use up no oxygen, need no flow and generally take less maintenance.

Whichever you use, the meter needs an accurate temperature reading. Calibrate before each use, read the water in place when you can (a sample sitting in a bucket trades oxygen with the air), and measure at more than one time of day, since early morning and late afternoon bracket the daily swing.

Work out percent saturation

Percent saturation strips temperature out of the picture, which makes it the quickest way to judge whether a reading is high or low for the conditions:

To go the other way, multiply the saturation value by the percentage a meter reports:

Near 100% means the water is in balance with the air. Well below 100% points to respiration or decay using oxygen, while above 100% (supersaturation) usually means photosynthesis or aeration is adding it faster than it escapes. Fish respond to concentration, though, so check mg/L too: a pond at 28 °C reading 4.5 mg/L is only about 58% saturated (4.5 ÷ 7.8), already in the stressful range. Our dissolved oxygen calculator does the lookup for you.

Common real-world examples

Maximum DO for four typical situations:

How to raise dissolved oxygen

You raise DO by adding oxygen faster than it is used up, or by lowering the demand:

• Aerate and agitate. Fountains, air stones, spillways and waterfalls increase surface contact and mixing. In a deep pond, bottom aeration reaches the oxygen-starved layer.
• Keep it cool. Shade, depth and cold inflow all raise the ceiling and slow oxygen use.
• Cut the oxygen demand. Reduce nutrient runoff, overfeeding, overstocking and piles of decaying leaves or algae. Aeration can't keep up with a heavy organic load for long.
• Keep water moving. Stagnant water is the worst case; circulation pumps help.

The bottom line

Dissolved oxygen is the small amount of oxygen gas in water that every aquatic animal depends on. Fresh water holds at most about 14.6 mg/L near freezing and about 7.6 mg/L at 30 °C, and biology then pulls the real number up or down. For fish, 5 mg/L is the line between comfortable and stressed, and 3 mg/L is the line below which they can't be supported. Track both mg/L and percent saturation, and remember the low point usually comes just before dawn on a hot day. For an exact figure at a specific temperature, use the calculator below.