Dissolved Oxygen in a Kratky System

After running the Kratky hydroponic systems for three months, from December through March, it became clear that this would not work without through summer in the tropics.  As the temperatures started to rise above 32 degrees C, the lettuce started to wilt during the day, a sure sign that something was wrong.  The water level was still high in the beds, so that wasn’t the problem.  However, the water was not looking good – it was coated with a slime and the roots all looked rotten.
After doing some more research I decided to try adding some oxygen to the water.  First, I washed all the lettuce plant roots.  Many were rotten and just washed right off.  I then placed airstones in each bed and ran a line to a central air pump for aquariums.  Within two days the lettuce started looking better and I could see the tips of the roots were now white, although the temperature had not changed drastically.  The plants that had started to bolt remained at the same height and started forming new leaves at the top.
This taught me a very important lesson:  The dissolved oxygen level in water is extremely important, and it is highly dependent on temperature.

Here are some interesting insights:

1. The higher the temperature of the water, the more it expands leaving less place for O2 molecules.  
2. Water and air exchange O2 molecules through the water surface, and this is the reason air pumps help since they agitate the water constantly, creating a space for this exchange to take place
3. A slimy surface on your water will guarantee that very little or no O2 will enter your water

So for those of you attempting Kratky in hot climates and seeing problems, try adding oxygen and see whether that helps.