37
12. DISSOLVED OXYGEN THEORY
Dissolved Oxygen (DO) refers to the volume of oxygen that is contained
in water. There are two main sources of dissolved oxygen in water;
atmosphere and photosynthesis. Waves and tumbling water mix air into
the water where oxygen readily dissolves until saturation occurs. Oxygen
is also produced by aquatic plants and algae during photosynthesis.
The amount of dissolved oxygen that can be held by water depends on 3
factors:
1) TEMPERATURE:
DO increases with decreasing temperature.
(Colder water holds more oxygen.)
2) SALINITY:
DO increases with decreasing salinity.
(Freshwater holds more oxygen than saltwater.)
3) ATMOSPHERIC PRESSURE:
DO decreases with decreasing atmospheric pressure.
(The amount of DO absorbed in water decreases as altitude
increases.)
Solubility of oxygen in water contact with water saturated air at
standard atmospheric pressure
0
2
4
6
8
10
12
14
16
0
5
10
15
20
25
30
35
40
45
50
Temperature °C
So
lu
bi
lity
m
g/
L
Measurement Units
One unit of measure of DO in water is parts per million (ppm) which
is the number of oxygen molecules (O
2
) per million total molecules in
a sample. Calculating the % Saturation is another way to analyze DO
levels. % Saturation is the measured DO level divided by the greatest
amount of oxygen that the water could hold under various temperature
and atmospheric pressure conditions multiplied by 100.
