Water is densest at the temperature of 39.2 degrees. When it becomes warmer or colder it becomes less dense. This density is the difference that causes most ponds or lakes to stratify into 3 temperature layers. The deepest and coldest, densest layer is called the "Hypolimnion"; and the shallowest and warmest, lighter layer is called the "Epilimnion." Between these two layers is a layer called the "Thermocline" where temperatures change rapidly.
The epilimnion which is warmer and lighter water, floats on top of the cooler, heavier water in the dephs. And because this water is lighter it is easily circulated by the wind. The layer called thermocline may have some water circulation, but the water below it circulates very little.
When a pond stratifies in early summer the stagnation process begins, as decaying organic matter on the pond bottom, along with the living organisms in the water consume dissolved oxygen. With no circulation to restore it and no aquatic plants to produce it, the oxygen levels in the hypolimnion layer begin declining. Just how fast they decline depends on the water fertility.
The temperature layers remain constant through the summer, as long as the weather stays warm enough to keep the surface water several degrees warmer than the water that is in the deepest layer. But when the fall cool weather comes and the shallowest water reaches the same temperature as the depths, the fall turnover begins. Since all the water in the pond is now the same temperature and density, the wind can now circulate the entire water mass. As the surface of the water continues to cool down, the water becomes denser then it is in the deepest part, so it sinks, which helps the mixing process.
The mixing of the layers from the surface to the bottom continues through the fall. and in the northern areas the surface water freezes, but the water on the bottom stays several degrees above the freezing temperature. If water did not have this unique property of being densest at a temperature just above freezing, ponds and lakes, would freeze up completely to the bottom.
Note: In the fall the pond is still stratified into three different temperature layers. Fish stay in the upper two layers, because the oxygen levels are two low in the hypolimnion layer. In the mid-fall the upper layer cools down, and surface water starts to sink, thus starting the turnover process that scatters fish to all depths. In late fall the surface water is cooler than the water on the bottom, as practically all of the pond fish are in the hypolimnion layer, the deepest part of the pond.
If you have a small or medium, or large pond in your backyard, this information should help you to understand how fish live throughout the different seasons and how to prevent fish losses from overgrowth of aquatic weeds or from lack of the proper aquatic plants. even if you feed your fish daily. Having some aquatic plant-life in your pond can insure that oxygen will be produced naturally in your pond.
I mentioned fishing in one of my above posts, because several friends of mine raise trout and bass, along with their koi, in backyard ponds. Though they may fish once in a while from their pond, their children have given the fish pet names. I think "Charley" and "Sam", are the two biggest koi in their pond. I have handfed the koi, but leave the bass to those who don't mind getting nipped by a hungry bass after a frog.
Just in case your wondering what I mean't by warm-water fish and cold-water fish, here are some examples: Flathead Catfish, Channel Catfish, Bullhead, White Bass, Bluegill, Largemouth Bass, Striped Bass, and Crappies are considered warm-water fish. (epilmnion layer warmest)
Bottom depth cold-water fish: Brown Trout, Cutthroat Trout, Chinook Salmon, Rainbow Trout, Coho Salmon, Brook Trout, and Lake Trout (hypolimnion layer coldest)
Fish Habitat
This is very important for keeping your pond fish healthy and happy. You want to provide your fish with the right kind of cover and proper range of water temperature, adequate dissolved oxygen and a reliable supply of food. If you skip any one of these important elements long-term survival of your pet pond fish would be almost impossible.
