Dissolved Oxygen (DO) tests for the presence of oxygen in water. This is important since oxygen is vital to the survival of aquatic plants and animals. The presence of oxygen is a good sign and the absence indicates pollution.

Most of the dissolved oxygen in the water comes from the atmosphere. Waves and tumbling water act to mix atmospheric oxygen with water. Algae and rooted aquatic plants also deliver oxygen to water through photosynthesis.

Levels rise from morning through the afternoon as a result of photosynthesis. At night plants and animals continue to respire and consume oxygen and the levels fall to a low point just before dawn.

Water tempertaure and the volume of water affect the levels of dissolved oxygen. Like other gasses, dissolved oxygen dissolves more easily in cooler water.

The build up of oragnic wastes is the main human factor that contributes to changes in dissolved oxygen levels.

Depletions in dissolved oxygen can cause major shifts in the diversity of aquatic life. Species that connot tolerate low levels of dessolved oxygen such as stoneflies, caddisflies, and beetle larvase, will be replaced by more tolerant organisms such as worms and fly larvae.

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