Dissolved oxygen (DO, pronounced dee-oh)

Dissolved oxygen (DO, pronounced dee-oh) is oxygen that is dissolved in water. It gets there by diffusion from the surrounding air; aeration of water that has tumbled over falls and rapids; and as a waste product of photosynthesis. An over simplified formula is given below:
Photosynthesis (in the presence of light and chlorophyll):

Carbon dioxide	+	Water	à	Oxygen	+	Carbon-rich foods
CO2		H2O		O2		C6H12O6

Fish and aquatic animals cannot split oxygen from water (H2O) or other oxygen-containing compounds. Only green plants and some bacteria can do that through photosynthesis and similar processes. Virtually all the oxygen we breathe is manufactured by green plants. A total of three-fourths of the earth’s oxygen supply is produced by phytoplankton in the oceans.
If water is too warm, there may not be enough oxygen in it. When there are too many bacteria or aquatic animal in the area, they may overpopulate, using DO in great amounts.
Oxygen levels also can be reduced through overfertilization of water plants by run-off from farm fields containing phosphates and nitrates (the ingredients in fertilizers). Under these conditions, the numbers and size of water plants increase a great deal. Then, if the weather becomes cloudy for several days, respiring plants will use much of the available DO. When these plants die, they become food for bacteria, which in turn multiply and use large amounts of oxygen.
How much DO an aquatic organism needs depends upon its species, its physical state, water temperature, pollutants present, and more. Consequently, it’s impossible to accurately predict minimum DO levels for specific fish and aquatic animals. For example, at 5 ^oC (41 ^oF), trout use about 50-60 milligrams (mg) of oxygen per hour; at 25 ^oC (77 ^oF), they may need five or six times that amount. Fish are cold-blooded animals,so they use more oxygen at higher temperatures when their metabolic rate increases.
Numerous scientific studies suggest that 4-5 parts per million (ppm) of DO is the minimum amount that will support a large, diverse fish population. The DO level in good fishing waters generally averages about 9.0 parts per million (ppm).
When DO levels drop below about 3.0 parts per million, even the rough fish die. The table in this section shows some representative comparisons.

Table 4. Effect of dissolved oxygen level on fish
Fish Species	Lowest DO level at which fish survive for:
	24 hours (summer)	48 hours (winter)
Northern Pike	6.0 mg/L	3.1
Black Bass	5.5	4.7
Common Sunfish	4.2	1.4
Yellow Perch	4.2	4.7
Black Bullhead	3.3	1.1

How Dissolved Oxygen Affects Water Supplies
A high DO level in a community water supply is good because it makes drinking water taste better. However, high DO levels speed up corrosion in water pipes. For this reason, industries use water with the least possible amount of dissolved oxygen. Water used in very low pressure boilers have no more than 2.0 ppm of DO, but most boiler plant operators try to keep oxygen levels to 0.007 ppm or less!