Can you imagine living under the ice? Are you a fish, a crayfish, an adult beetle, a beetle larva, a dragonfly nymph, or a mayfly naiad? The list is long and varied. They don’t have to imagine living under the ice. They do it.
Water is liquid below the ice, full of life, even if it is slowed by the cold. The residents are not gasping for air or slipping into hypothermia (or if they are, it is an adaptation for coping with winter). This is their world, cut off from the sky and the land, committed to the sealed pond. Months go by without seeing the sun in full. That is just fine with everyone involved.
Cold water holds more oxygen than warm water. Cold water slows the metabolic rate. In clean water, with little decomposition that depletes oxygen, life is well supplied. The animals are not asphyxiated. They could be said to thrive.
The water will not mix with air again until spring melts the ice. Myriad organisms are adapted for just such isolation. Life cycles continue as programmed, and will do whatever it is that each species does come the thaw.
Until spring, ice shields the living things below from the frozen, dry air above. Out of our sight, in a home that cannot be our home, a web of life carries on. Hard to imagine? That’s ok.