Quick answer: Dissolved oxygen (DO) is the oxygen gas present in water and available to aquatic organisms and roots. It is commonly reported in milligrams per liter and may also be expressed as percent saturation.
Why DO matters
Aquatic animals, plant roots, and aerobic microorganisms depend on oxygen. DO can change with temperature, circulation, aeration, photosynthesis, respiration, organic matter, stocking, and biological demand.
Temperature relationship
Cooler water can generally hold more dissolved oxygen than warmer water. This means a DO value should be interpreted with water temperature and the needs of the actual crop, fish, invertebrate, or process.
Daily and spatial variation
Photosynthesis can raise DO during illuminated periods, while respiration consumes oxygen. Deep, stagnant, crowded, or heavily loaded areas can differ from the surface. Measure representative locations and look at time-of-day trends instead of relying on one spot reading.
How to measure DO
- Follow the sensor's preparation and calibration requirements.
- Record DO, unit, temperature, location, depth, time, aeration, and recent events.
- Avoid bubbles on the sensing surface unless the instrument method specifically accounts for them.
- Repeat unexpected results and check probe condition before major corrective action.
Response to low DO
Low-oxygen response depends on the application. Check aeration, circulation, temperature, biological load, decaying material, pump operation, and sensor accuracy. Use species- or crop-specific action thresholds rather than a universal value.
Sources
- U.S. Geological Survey: Dissolved Oxygen and Water
- U.S. Geological Survey: Diurnal Water-Quality Variation
Reviewed August 1, 2026. Educational content; application-specific targets should be confirmed for the actual system, crop, or aquatic species.