Air under ice

Have you ever wondered how air gets under ice on a pond?

Sometimes the movement of an animal, such as a frog or turtle hibernating in debris on the pond bottom, will free trapped gases and bubbles will rise.

These bubbles will become trapped against the ice, unable to escape through the ice cover.

When this happens, there often will be a cluster of small bubbles. If enough small bubbles rise at once, they may bump into one another as they reach the ice and form one larger area of trapped gas.

Muskrats, otters, mink and beavers do not hibernate. They can swim under the ice cover. Muskrat and beaver dens have burrows leading from their dry living quarters.

Some of their burrows will lead to aboveground exits. But there usually is at least one escape burrow leading to an exit under water. When ice covers the pond, it is easy to see long trails of small bubbles left by these animals as they exhale air while swimming. The more bubble trails you see trapped under the ice, the more muskrats or beavers you might expect live there.

The picture I show here looked, at first glance, like an otter sticking its nose up and breaking the thin ice. I soon realized that what looked like whiskers were really cracks in the thin ice. What looked like an otter's nose was really a cypress knee.

The air under the ice was caused in this case by overall lowering of the pond water.

Most ponds slowly lose water naturally over time. The temperature dropped below freezing and a thin layer of ice formed on the surface of the pond. As the water level slowly receded, the cypress knee held up the ice frozen to it.

The thin ice was unable to support itself as it became separated from the water below and cracks formed around the knee and allowed air to get in.