The surface of a pool of water freezes first. Then why do you usually find large bubbles of air under the ice?
In a small pool of water, such as water frozen in a bowl or flower pot, the air bubbles are the result of gases that were dissolved in the water. As the water nears the freezing point, the gases come out of solution. Once the surface of the water freezes, any gas that is released from the water underneath will be trapped as bubbles. You can see the same thing with the tiny bubbles that are trapped in the center of ice cubes.
In larger pools, such as ponds and lakes, living things can contribute gases to the bubbles through processes such as photosynthesis, digestion, and decomposition.