Dissolved oxygen in water - how to measure it and what it means

Methods guide · June 12, 2026

Ask anyone who looks after a pond, fishes, or studies a river what worries them most in a hot, windless week - and you will hear about oxygen. It is one of those measurements with a real stake behind it: literally everything that lives in the water depends on the amount of dissolved oxygen. Fish, insect larvae, the microorganisms that break down matter - each breathes the same oxygen dissolved in the water column.

People who measure it regularly - fish farmers, observant anglers, students, ecologists, river keepers - learn to read the water like a barometer. They know that in the early morning, after a warm night, oxygen is at its lowest, and that this is exactly when oxygen crashes (fish kills) happen. Turning that intuition into a concrete number, recorded day after day, gives you something priceless: an early warning and hard evidence of how the water changes over time. It is a measurement that truly saves lives - in the water body.

What dissolved oxygen tells us

Oxygen enters the water from two sources: from the atmosphere (mixing, wind, waves) and from the photosynthesis of plants and algae. It is consumed, on the other hand, by the respiration of organisms and the decomposition of organic matter. The result is the interplay of these forces - which is why oxygen responds so sensitively to the state of the water body.

Two things are worth remembering:

• Temperature: cold water holds more oxygen, warm water less. In summer, oxygenation drops from physics alone.
• The daily rhythm: during the day photosynthesis oxygenates the water (in the afternoon there can even be supersaturation), at night respiration consumes it - it is lowest in the early morning. A reading at different times gives two different numbers.

How much oxygen the water can hold at all

The concentration at full saturation (100%) depends almost entirely on temperature. Values for fresh water at 1 atm:

Calculated with the Benson-Krause equation - the same one used by our oxygen saturation calculator, so the numbers agree on both sides. In practice: water at 25 °C holds about 35% less oxygen than the same water at 5 °C - before anything starts consuming it. Altitude and salinity lower these values further (the calculator applies both corrections).

How it is measured - two routes

An oxygen probe (electrochemical or optical) - the fastest, for fieldwork. You submerge the sensor and get a reading instantly, usually in mg/l and/or % saturation. It needs calibration (most often in water-vapour-saturated air) and a clean membrane/sensor.

The Winkler method - classic chemical titration, treated as the accuracy benchmark. The sample is fixed with reagents in the field, and the determination is finished by titration. More labour-intensive, but independent of electronic calibration and excellent as a probe check.

In both cases, record the temperature along with oxygen - without it the result is hard to interpret.

What the result means

These bands are an editorial convention, not a quotation from a standard. We built them as a common denominator of water quality criteria for aquatic life - the reference point is the US EPA criteria for dissolved oxygen (in cold-water, salmonid waters the criteria are markedly stricter than in lowland ones). Requirements differ by species and life stage - fry and juveniles tolerate less than adults, and a value that persists is far worse than a brief dip. Legally binding classification thresholds are set by national regulations - always check the version in force in your country.

% saturation, in turn, tells you how far the water is from equilibrium with the atmosphere: ~100% is equilibrium, values well above (supersaturation) occur during the day with blooms, and below - at night and during intense decomposition. The two numbers say different things and you need both: 7 mg/l is 77% saturation at 20 °C but only 55% at 5 °C - the same figure in mg/l once means moderately oxygenated water, once clearly depleted (in cold water most of the available oxygen has been used up).

Measure deliberately - the most common pitfalls

• Time of day. Because of the daily rhythm, measuring at a fixed hour is crucial. An early-morning comparison will tell you the most.
• Depth. In fertile, stratified lakes the oxygen near the bottom can drop to zero even though there is plenty near the surface. If you can - measure at several depths.
• Probe calibration. An uncalibrated sensor is a nice-looking but wrong number.
• Surface only. A reading from the top alone can be misleading - life goes on throughout the whole water column.

How often

In season and during heatwaves it is worth checking more often (even daily in the morning during critical periods), otherwise - in a steady weekly rhythm. Here regularity really pays off: a series from the same hour reveals a trend that a single measurement will not.

Sources

• Oxygen solubility as a function of temperature (the table above): Benson B.B., Krause D. (1984), The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere, “Limnology and Oceanography” 29(3): 620-632, doi:10.4319/lo.1984.29.3.0620. This equation was adopted by Standard Methods (APHA) and the USGS; our calculator uses it too.
• Determination methods: iodometric titration (Winkler) - ISO 5813:1983; electrochemical probe method - ISO 5814:2012; optical sensor method (fluorescence quenching) - ISO 17289:2014. All three methods have a standard of their own. ISO 17289 explicitly covers field measurement and continuous monitoring, and points to the optical method for coloured and turbid waters, where Winkler titration struggles.
• Criteria for aquatic life: US EPA - National Recommended Water Quality Criteria.

Read oxygen together with the temperature profile - only together do they show whether the deep layer is still oxygenated.

📊 See it live: Global Rivers & Lakes Watch - a public dashboard from three stations in three countries: the Maumee River (Ohio, USA), the river Main in Frankfurt and the LéXPLORE research platform on Lake Geneva. Water temperature, dissolved oxygen, pH, conductivity, turbidity and chlorophyll-a are fetched automatically (USGS Water Services, WSV Pegelonline, Eawag Datalakes), with correlation tiles and a station map. No login.

How to make it easier

Oxygen is a measurement that says the most as a series and at a fixed time - which is exactly what is hard to keep up with on loose notes, especially when you measure at dawn, in a hurry, right by the water.

LimnoLog takes that part off your hands:

• recording right on your phone by the water - even with no signal (it syncs when you are back in range);
• the measurement goes to a station and a session, so “the same hour, the same place” takes care of itself;
• the readings turn into a chart - daily and seasonal swings are visible at a glance, and drops stand out before they become dangerous;
• you can keep several depths in one station, and share the results via a link or export them.

And, as usual - with the same system you record, alongside oxygen, temperature, pH, transparency or any other indicator. One set of data instead of scattered spreadsheets.

🧮 Tool: Oxygen saturation calculator - from temperature and concentration you can work out the percent saturation (with altitude and salinity correction).

Early users get free access for now and keep their features for good. If you watch over your water’s oxygenation - take a look and try it.