As temperatures begin to plummet your pond life reacts accordingly. Life in ponds has evolved to survive through winter but that is not to say that a pond owner can’t help it through the season especially if it is an extremely long or harsh winter, or both! However you also want to avoid creating other problems such as Hyper-Cooling when assisting your pond through winter..
Some pond life such as frogs and turtles retreat down into the sludge at the bottom of your pond and hibernate until everything warms up. Fish also prefer to stay at the bottom of the pond in the warmer pockets of water. What happens to the koi and other pond fish is that they go into a state of torpor.
Torpor is not quite full hibernation, because it is shorter than hibernation, but otherwise it is a very similar state. The fish have reduced body temperature, slowed metabolism, slow reaction times, reduced breathing rate and primary body functions. Fish in torpor save energy that would otherwise be needed for higher levels of activity such as feeding. Because of this it is a good idea to keep things as calm and stable as possible around the pond. Use a quiet way of keeping a hole in the ice of your pond, such as an air pump. This stability can be interrupted however if you have super or hyper-cooling in your pond.
In winter when temperatures drop your pond will naturally have cooler water at the surface and warmer water deeper in the pond. In a pond or lake without sufficient aeration your pond may have a thermocline. This is where the water at the top is very cold and the warmer water sinks to the bottom. With thermocline there is very little mixing of the layers.
You may inadvertently cause super or hyper-cooling when you are trying to aerate your pond. This is when you move the extremely cold water from the surface of your pond down to the deeper water. When this happens you could dramatically reduce the temperature of your water. Then your fish would have nowhere warm to be and this can result in fish deaths. When using your air pump make sure it is introducing warm air into the pond. Keep your air pump either indoors or in an insulated box. Don’t also have the diffuser too deep in your pond or have it running at less than full strength.
An air pump, used correctly, is invaluable in winter keeping your pond healthy through the colder months but like anything use it wisely for the best results.