If more oxygen is consumed than is produced, dissolved oxygen levels decline and some sensitive animals may disappear. DO levels fluctuate daily and seasonally. They also vary with water temperature - cold water holds more oxygen than warm water. The most critical time for many aquatic animals are early mornings on hot summer days, when river flows are low, water temperatures are high, and plants have not been producing oxygen since sunset. The amount of oxygen that water can hold varies with temperature.
Cold water can hold more oxygen than warm water.
Immerse thermometer four to six inches below the water surface for at least thirty seconds to allow it to equilibrate. Read the thermometer immediately after removing it from the water. The water sample must be collected in such a way that you can stopper the bottle while it is still submerged. That means that you must be able to reach into the water with both arms. For DO samples, an extension pole is not feasible.