Now that we understand some basics of dissolved oxygen, we can get back to the temperature problems in your pond. Temperature problems are a seasonal issue. As the sun exposure to the pond increases through spring and into summer, the water warms. As the summer progresses, the water at the surface continues to warm faster than the water below because not as much sunlight penetrates the lower portions of the water column. The area of the water column that receives sunlight is called the Euphotic Zone. The layer of warm water is known as an epilimnion. Also, warm water is less dense or lighter than cooler water, so warm water stays at the surface and colder water sinks to the bottom.
As the summer continues, this temperature difference expands. The surface water is very warm and the water below much cooler. The cold water layer is known as the hypolimnion. As stated earlier, the cooler the water the more oxygen it can hold. However, if the cool water has no exposure to the air or oxygen, it cannot hold the oxygen, no matter how cold it is. Therefore, the cooler water near the bottom does not have a continuous oxygen supply, and over time can turn anoxic or have all of its oxygen used up. When this occurs, organisms such as fish and bacteria need to move into areas of higher oxygen or end up dying. This limits the area your fish have to live and also greatly limits the amount of decomposition of organic matter at the pond bottom.
The transition area between the warm water and the cold water is called a Thermocline. The thermocline is very important to your pond health and can act as a barrier between aerobic and anaerobic areas, and prevent water mixing. For the organisms in your pond that require oxygen, the thermocline can mean life of death. Basically, the lower down the water column the thermocline is located, the better off they are.