Why do some lakes freeze?
This process continues until all the water in the lake has reached a temperature of 4° C, when the density of the water is at its highest. This could be because air cools and warms faster than water. If water were most dense as a solid, lakes would freeze from the bottom up and eventually become a solid. It can take some time for the water to freeze.
This happens because the water at the edge of the lake is shallower and therefore cools faster.
Why doesn't the whole lake freeze over?
When the anoxic zone creeps up in the water column, the fish cling to the underside of the ice as oxygen is depleted until they suffocate. However, because water is a special molecule, it expands rather than contracts when it freezes, meaning that the ice at the surface of the lake is less dense than the water below. However, St Clair Lake had only a thin layer of ice on Thursday morning, so small shallow lakes may have already started to freeze. Also, a lot of energy has to be removed from the water to freeze, and the air is usually much colder than the ground at the bottom of the lake.
If the plants, animals and other organisms that live there do not have some adaptation that protects their tissues from freezing, they would die.
What prevents a lake from freezing?
Many fish and aquatic plants can survive a cold winter because the layer of ice that forms on the surface of the lake insulates the water below and prevents the lake from freezing over. Anyone who has ever left a full water bottle in the freezer and then had it break when the water froze and expanded knows this. So now we understand why ice floats on water, but how does it work on a lake? Imagine it is the beginning of winter and the temperature has just dropped below freezing. As soon as the fresh water freezes, the ice floats and insulates the rest of the water underneath so that it doesn't cool down any further.
This is a serious challenge for the fish, because if the lake stays frozen for too long, the oxygen level can get so low that the fish die.