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.