Folks have been taking advantage of ice on lakes for a long time, whether it is to take a shorter route to a destination or to enjoy winter activities such as ice hockey or cross-country skiing. Unless a hole is drilled in it, very little is known about what happens under the ice.
Once a hole is drilled in the shallows, along with fish, insects can be seen in the weeds and on some bottoms. The eggs deposited the previous spring, summer and fall, are maturing and hatching at different depths and locations. A lot of the plant growth that happened in the previous open-water period is now dying and decomposing. This process introduces carbon dioxide into the water and is cumulative, and if it increases to a certain level, it can begin to affect a fish’s ability to survive as oxygen is depleted.
I must go back here and explain that contrary to popular belief, that while plants do transpire and create oxygen during the photosynthetic process, most of the oxygen in a lake comes from the air through direct contact with the surface of the open lake. Oxygen transfer is the greatest when the water surface is cooler in the spring and fall. It is imperative to the health of a lake to have a sufficient supply of oxygen in the water before the ice comes on.
If not, a number of shallower, weedy lakes may suffer a condition known as winter kill, where there is not enough oxygen in the water to sustain the fish. How that happens is explained in more detail below.
Lakes stratify as explained in the following information in italics taken from the International Institute for Sustainable Development website:
Thermal stratification occurs when the water in a lake forms distinct layers through heating from the sun. When the ice has melted in the spring, solar radiation warms the water at the surface of the lake much faster than in deeper waters. In fact, sunlight often only penetrates a few metres into the lake, directly warming just the top few metres. As the water warms, it becomes less dense and remains at the surface, floating in a layer above the cooler, denser water below.
The shallowest layer is that warm surface layer, called the epilimnion. The epilimnion is the layer of water that interacts with the wind and sunlight, so it becomes the warmest and contains the most dissolved oxygen. Though dissolved oxygen doesn’t play a direct role in lake stratification and turnover, it is important for all the aquatic organisms in a lake that require oxygen to survive.
The deepest layer is the cold, dense water at the lake bottom, called the hypolimnion. The hypolimnion often remains around 4°C throughout the year, rarely gets any direct warmth from the sun and is isolated from the air at the surface of the lake. The hypolimnion contains the lowest amount of dissolved oxygen and can often become anoxic (zero dissolved oxygen) while the lake is thermally stratified.
The middle layer is the transition zone of water between the warm epilimnion and cold hypolimnion, called the metalimnion. The metalimnion is a place where the shallowest of the cool waters in the hypolimnion gradually warm up until they mix into the epilimnion. The point of greatest temperature difference (and therefore density difference) is called the thermocline and occurs within the metalimnion.
Throughout the summer, wind and waves cause the warming water in the epilimnion to mix deeper and deeper, slowly incorporating hypolimnetic water through the metalimnion. The ability of a lake to mix through wind turbulence is determined by the “stability” of thermal stratification. Stratification becomes increasingly stable with heating from the sun. The larger the difference in temperature (and density) between the epilimnion and the hypolimnion, the more stable the thermal stratification.
Eventually, the epilimnion warms to the point where the difference in density between the epilimnion and hypolimnion (at the thermocline) is so large that wind and waves can no longer generate enough energy to incorporate hypolimnetic water.
As the summer turns to fall, the surface waters cool and sink, mixing the epilimnion down towards the hypolimnion and weakening the thermocline; as the temperatures and densities of the epilimnion and hypolimnion become more similar, the water currents and wind can once again mix water between the two layers.
Eventually, the epilimnion cools until the entire lake is at the same temperature (isothermal). This allows lake turnover to occur.
So, how does winterkill happen? If the ice goes on to a lake too early before adequate oxygen has been introduced into the water, or if enough carbon dioxide builds up under the ice, a winterkill can occur at one of two times; under the ice when there is insufficient oxygen to support fish, or at turnover in the spring when further mixing of whatever oxygen may have been remaining in the lake is distributed throughout the entire lake too thinly to be available to fish.
