If you notice the piece of poly-pipe sticking out of the snow, thats connected to an inverted foam cooler & has a length of air line running to a garden shed that houses a small air pump that supplies enough air for gas exchange over the winter.
The air stone goes down only 6" or so, not enough to disturb the fish. My freeze line in my area goes down to 15"-18" so no matter how much snow & ice there is above, it's about 40-42 deg. below.
I saw this hint in the pond column of Aquarium Fish Mag. & it's been working fine for 5 winters now.......Rich