Water circulation is a natural process that is vital to the life of the pond. Water at the surface of a pond transfers oxygen from the surface (which interacts with the air and aquatic plant life) down to the bottom. Water contracts and becomes more dense and heavier as it gets colder and sinks to the bottom of the pond, pushing the water it has displaced to the surface, where it too can cool. When the water at both the surface and the bottom of the pond are close to the same temperature, water circulates more easily between them.
When the density difference between the top and bottom become more extreme they separate into distinct layers with a middle layer, known as the thermocline, that acts as a barrier to any mixing of the deeper waters. Starting in the spring and over the course of the summer, surface waters absorb a lot of the sun’s energy and can heat extensively, causing them to become quite buoyant. There is too much difference in temperature between the surface water and the bottom to allow for complete mixing of all the water in the pond. Toward the end of summer, the deep water becomes quite depleted of oxygen because no mixing has taken place.
As the days get shorter and cooler, and energy is transported out of the pond, mixing becomes easier. At about 50°, the cooler water (with a higher oxygen content) at the surface begins to sink into and through the thermocline, forcing warmer and less dense water to the surface, eventually erasing the temperature stratification built up over the summer. At some point, the majority of the water in the pond reaches an approximately uniform temperature. Now, storms and sustained high winds can begin to perform the task of overturning and mixing all of the water in the pond — referred to as fall turnover. The deep water contains an abundance of decaying matter and sulfurous gases; when it reaches the surface, it produces a telltale odor that indicates the process has begun. Eventually the turnover mixes fresh oxygen into the entire pond mass, replenishing the deep waters with the life-giving stuff and cleansing the sulfurous fumes from the water, allowing fish to return to the depths where they will spend the winter months.
As winter approaches, the water that has now reached 39° sinks to the bottom, allowing colder and less dense, buoyant water to remain at the surface to freeze.
Below 39°F, water expands and becomes less dense, allowing it to float above the warmer water. The water that cools below that temperature, to 32°, freezes and stays on the top, effectively capping the pond and stops further energy loss from the pond. Everything beneath the surface of the ice never gets any colder than 39°.
The ice thickens because it is not a good insulator; water in contact with the underside of the ice cools further and freezes, adding to the surface layer.
A stratification similar to that of the summer months will occur in the water column during the winter months, but not to as great an extent. With the advent of spring, the warming/melting of the ice layer at the surface and the much smaller temperature differences in the water column, winds and storms are able to create a spring turnover with little difficulty. As the waters continue to warm, stratification begins again and the endless cycle continues.