Chemistry parameter
Dissolved oxygen
The oxygen dissolved in water and available to aquatic life. Dissolved oxygen is the primary indicator of the health of a river, and its collapse is the mechanism behind most fish kills.
Key facts
- Unit
- mg/l
- Metric
- Dissolved oxygen
What it measures
Molecular oxygen in solution, either as concentration or as percentage of the maximum the water could hold at that temperature and salinity.
How it is measured
By optical or membrane electrode, or by Winkler titration. Must be measured in situ or with great care, because a sample exchanges oxygen with the air immediately.
How to read a value
| Value (mg/l) | What it indicates |
|---|---|
| Above 8 | Well oxygenated. Supports salmonids and other sensitive species. |
| 5 – 8 | Adequate for most coarse fish. |
| 2 – 5 | Stressful. Sensitive species are lost. |
| Below 2 | Hypoxic. Fish kills occur; the water body is functionally dead for most animal life. |
Bands are conventional interpretation aids, not regulatory limits. Where a legal standard exists it is stated on the relevant standards page.
What causes it
- Solubility falls as temperature rises — warm water simply cannot hold as much oxygen
- Decomposition of organic matter consumes it, which is why sewage and slurry inputs are so damaging
- Algal blooms produce oxygen by day and consume it by night, driving severe pre-dawn minima
- Turbulence and weirs re-aerate water
Why it matters
- The combination of a hot summer, low flow and an organic discharge is the classic fish kill mechanism, and each factor amplifies the others
- Biochemical oxygen demand measures the oxygen a discharge will consume, and is the basis of most effluent consent conditions
Related intelligence
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- National Institute of Standards and Technology — NIST Chemistry WebBook — thermophysical properties of water. Open · US Government work — public domain