Why it matters
Aquatic organisms use dissolved oxygen for aerobic respiration. Persistently low concentrations can create stressful conditions, although the meaning of a reading depends on temperature, flow, time, and the organism being considered.
What changes it
Cool, turbulent water often holds more oxygen than warm, slow water. Photosynthesis can add oxygen during daylight, while decomposition and respiration consume it.
How to interpret it
Compare dissolved oxygen with the date, temperature, station, recent trend, and any applicable reference value. A single reading should be treated as evidence to investigate, not proof of a cause.