Freshwater is densest at 4 C. When the water freezes, the ice forms a layer on the water surface, which acts as an insulator. In northern Maine the ice can exceed 3′ in thickness in a cold winter, but it takes increasing cold for the ice to continue to thicken.
All of the aquatic animals except for mammals and birds are cold-blooded or ectothermic. As the temperature drops their biological processes slow down. This includes respiration, which uses oxygen. The amount of dissolved gasses water can hold increases with decreasing temperature, so the coldest water also holds the highest dissolved oxygen levels, which is, in turn, used more slowly. Many animals slow down enough so they go into a state of torpor which decreases their metabolic needs even more.
Other biological processes continue under the ice. Algae under the ice still photosynthesize slowly. Other cold-loving organisms are also active. Obviously trout, yellow perch and other fish are active and feeding, otherwise ice fishing wouldn’t catch anything.
The unique properties of water allow animals to exist in freshwater year round. If water froze like other substances, where the solids settle to the bottom, lakes and ponds would freeze from the bottom up and many would freeze solid.