Diffusion is caused by partial differential pressure of oxygen in the air and in the water.
As the oxygen content of the air is considered to be constant, the rate of diffusion is determined by oxygen saturation of the water layer interfacing with the air. In the case of stagnant water the uppermost layer of water becomes saturated quickly and the convection current of oxygen into the water slows down. The mass transfer rate which determines the rate of diffusion (g/O2/m2/hour) varies in a wide range. It can be seen from a collection of the data of several authors that the mass transfer rate has varied between 0.03-5.0 g/m2/hour depending on the circumstances (Table 4).
In fish ponds the rate of diffusion is dependent on the mixing of water layers caused mainly by wind action. The oxygen intake from the atmosphere by diffusion is 1.5 g/m2/day in small ponds and 4.8 g/m2/day in big ponds, where the wind action is stronger.
Depending on oxygen saturation of the water, diffusion can show a reversed direction. In case of over-saturation of the water oxygen diffuses to the atmosphere. Diffusion is assisted by the mixing of water caused by wind action in this case.
It is well known that wind action has a good effect on the water quality of fish ponds through increased diffusion. In the case of construction of new fish ponds the prevailing wind direction has to be taken into account in order to utilize natural diffusion.
In intensive fish ponds where the oxygen budget of the pond water can be regulated artificially, diffusion by wind action has less importance.