First, an understanding of water
Water is an amazing substance. Its physical and chemical properties allow life to exist on earth. Our 4 billion years of evolution of life on the planet required water adjust the structure and function of every organism on earth.
Now back to the water in your pond
Many changes take place when your pond water cools (or heats) and, depending on the structure of your water basin, different things may happen when the temperature changes. Don’t worry, however; all of these changes are all predictable.
Now the important pond stuff
1. Water is densest at 40 degrees Fahrenheit or 4 degrees Centigrade.
2. As water cools the molecules in water move less and collide less frequently. Therefore, the water per unit of measure is slightly heavier as the temperature goes down until it hits 40 degrees F.
3. As the water moves toward it’s densest temperature and gets heavier it sinks to the bottom of the pond or lake. This is called the fall overturn. There is also one in the spring as the water warms. The curious thing about water is that as it cools below 40 degrees F, the chemical properties of water cause the molecules to start going into alignment and become lighter on a per unit basis. Ice cubes float because the crystals take up space and become lighter than the fluid. The thing to remember is that each molecule moves independently depending on its temperature. That is how water sinks or rises.
4. The spring and fall overturn means that for a short time the water throughout the water column is at 40 degrees F. In the fall that will cool at the surface and gradually sink to the bottom. It will continue until the water at the bottom is 40 degrees F.
5. As the air temperature drops and the average air temperature goes below 40 degrees F, the water becomes lighter. When the temperature hits 32 degrees F, it freezes at the surface.
6. The surface ice then becomes an insulator to some extent. As the temperature gets colder and colder, the ice gets thicker and protects the water from turning over and freezing at the bottom. Water at the bottom is always warmer than the water at the top.
But what happens after the ice over?
1. The ice has both benefits and detriments. The good is the insulating properties that keep the water from freezing to the bottom. The bad comes from a lack of oxygen dissolving in the water. This results from plants absorbing oxygen and fish respiring in addition fish poop decomposing with no aeration. This is called BOD (biological oxygen demand).
2. Aeration in the winter can be important, but how you do it can mean the difference between life and death for your fish. Too often people leave their waterfall or aeration on during the winter. During most years, the relatively mild Pennsylvania winters allows us to get away with this. In colder climates, however, this can be a disaster. Turning water over in a very harsh cold snap (when air temperatures average below 32 degrees) can result in a super-cooling effect causing the water temperature to drop all the way to the bottom. This happens because the stratification in the water column is destroyed by the warm water from below being cooled as it is brought to the top by aeration and water turnover. You will have no warning when this happens. It will just freeze solid to the bottom and your fish will die. The study of the biological and physical properties of water in basins is called Limnology.
The best way to manage your pond in the winter
1. A surface heater will help if the cold spell is prolonged for weeks at a time. The heater will open a space around it and if a small airstone is placed within a few inches of the bottom of the heater you will get some aeration.
2. Turning your waterfall or substantial aeration when the air temperature is above 40 degrees is the best. Just don’t stir up the bottom if you have not cleaned out your pond during the fall. The “muck” from the bottom can decompose and use up the oxygen very quickly when the debris is stirred into the water column.
3. Do not feed your fish once your water temperature is below 55 to 60 degrees F. Once your temperature comes down to 70 degrees F in early fall start feeding fall food with wheat germ.
4. The best way to help keep from having a winterkill from oxygen depletion is to clean your pond out in early fall, but make sure to aerate very well when you do this so you do not get an oxygen depletion from the stirred up organics.