How is a lake or river that freezes over to the organisms in the water?

This is what i found online:
When the surface of a lake or a river freezes, the ice serves as a barrier between the water and the cold air above. The temperature of the water below the ice is kept at a stable level, and it allows the organisms in the water to survive at a sustainable temperature. It also prevents further freezing of the water body and allows the organisms to stay in motion.