




		Dissolved Oxygen : Background Information

What Is Dissolved Oxygen?

Stonefly nymphs have hair-like gills behind each leg

Dissolved oxygen is merely the oxygen molecules that have mixed in with water molecules. It gets there by diffusing from the air; when trapped by aeration or bubbling; and as a waste product from green plant photosynthesis.

Oxygen gas dissolves in water much like the carbon dioxide responsible for the fizz in a can of soda pop. Carbon dioxide (CO2), however, is 200 times more soluble in water than oxygen.

Oxygen is not very soluble and occurs dissolved in only trace amounts. The tiny amounts of dissolved oxygen are measured in the range of 1-14 milligrams per litre (mg/L). While one out of every five molecules in the atmosphere is oxygen, in water, only 1-14 molecules out of a million are oxygen. Said another way, the concentration of dissolved oxygen is 1-14 parts per million (ppm).

Note: The concentration units of mg/L and ppm are equivalent. (mg/L = ppm)

Aquatic organisms such as fish and macroinvertebrates rely on gills for breathing dissolved oxygen. Bacteria also use oxygen when they decompose dead organisms. So far, however, no one has devised a way for humans to extract oxygen from water during a dive.