Most growers are familiar with the need to have some form of aeration in their nutrient solution - whether they be in a recirculation or a media based system. In NFT systems, this is often accomplished with the use of an air pump or by allowing the nutrient to fall back into the reservoir, thus introducing oxygen. However, the effect of temperature of the solution on the dissolved oxygen levels and on root respiration rates also needs to be taken into account. As the temperature of your nutrient solution increases, the ability of that solution to 'hold' dissolved oxygen decreases. For example, the oxygen content of a fully aerated solution at 10°C (50° F) is about 13ppm, but as the solution warms up to 20° C (68° F) the ability of the liquid to 'hold' oxygen drops to 9 - 10ppm, by the time the solution has reached 30° C (86° F), then it's only 7ppm.
While this may not seem like a huge drop in the amount of dissolved oxygen, we have to remember that as the temperature of the root system warms, the rate of respiration of the root tissue also increases and more oxygen is required by the plant. For example, the respiration rate of the roots will double for each 10°C rise in temperature up to 30°C (86° F). So the situation can develop where the solution temperature increases from 20° - 30° C (68° - 86° F) during the day, with a mature crop and a large root system, then the requirement for oxygen will double while the oxygen carrying capacity of the solution will drop by over 25%. This means that the dissolved oxygen in solution will be much more rapidly depleted and the plants can suffer from oxygen starvation for a period of time.
http://simplyhydro.com/nutrient_temp.htm
