12
1 Introducing Sea Water
1.2.5 Dissolved Gases
As well as salts, gases from the atmosphere are dissolved in sea water. There
are two gases, oxygen and carbon dioxide, which are of special interest due
to their involvement in biological and geochemical processes occurring in the
ocean. Oxygen enters water by diffusion at the atmosphere-ocean interface or
at the surface of air bubbles; it is also released into the ocean by the process
of photosynthesis (Harvey, 1985):
(1.6)
Oxygen is also utilized by organic respiration and decomposition in the breakdown of carbohydrates.
As temperature rises, the gas-absorbing capacity of water decreases rapidly
and reaches zero at 100°C. Under normal atmospheric pressure and a temperature of 20°C, water contains about 2% (by volume) dissolved air. The
dissolved air contains between 33% to 35% oxygen, O2, depending on temperature. At any given temperature, the highest possible amount of dissolved
oxygen is called 'oxygen saturation'.
- - .. - ----§
0
.s I=>Q.l
"0
...
~
~
1000
2000
3000
o
2
3
4
5
6
Oxygen concentration (ml/ I)
Fig. 1.5: Typical oxygen profiles: a Pacific Ocean, b Indian Ocean and c Atlantic
Ocean (adapted from Horne, 1969)