Water is an oxygen poor environment. It contains only 5%
of the oxygen that the same volume of air does:
- 1 litre of oxygen weighs 1,428mg
- 1 litre of air contains 285mg oxygen
- 1 litre of freshwater contains 14.6mg oxygen at 0°C
Oxygen is depleted by:
- plants at night
- livestock
- biological filtration
Oxygen is replenished by:
- plants during the day
- diffusion at air-water interfaces
Oxygen and saturation
If a beaker of sterile freshwater is left to stand at 25°C
then the normal maximum amount of oxygen that it can dissolve
is 8.2mg/litre. At this point the water sample is said to
be SATURATED. If it contained only 4.1mg/l then it would be
50% saturated. A level of dissolved oxygen of 6mg/l as recommended
in the OATA "Water Quality Criteria" is equivalent to 73%
saturation.
Solubility of oxygen
As the table below demonstrates, that as the water temperature
rises the amount of oxygen it may dissolve before becoming
saturated diminishes. Seawater dissolves less oxygen than
freshwater before it becomes saturated. Altitude and atmospheric
pressure play a small part in determining oxygen solubility.
For practical purposes both may be ignored.
| Temp
°C |
mg/l oxygen freshwater |
OATA
criteria in terms of % saturation |
mg/l oxygen saltwater |
OATA
criteria in terms of % saturation |
0
5
10
15
20
25
30 |
14.6
12.8
11.3
10.1
9.1
8.2
7.5 |
41
47
53
59
66
73
80 |
11.7
10.4
9.3
8.5
7.8
7.1
6.5 |
47
52
58
65
71
77
85 |
Oxygen and the weather
The weather may combine all of the
factors to create problems (some of them may arise also in
fish houses).
- Sunlight
Sunlight increases water temperature and hence decreases
oxygen solubility.
- Freezing
Ice seals the surface preventing the entry of oxygen and
the escape of toxic gases.
- Still periods
When there is little wind such as before thunder storms,
the rate of diffusion of oxygen is diminished by the reduction
of the pond surface area (ripples in a light wind may increase
the surface area of a pond by two or three times).
