By Miriam Turnbull
It doesn’t really feel like winter is here until a thick enough layer of ice has formed on the pond in my family’s backyard to clear off the ice for some pond hockey, broomball, or ice skating. Many of us Minnesotans take for granted the experience of being able to walk out onto lakes, put up an ice house and fish, or ski, snowshoe and even drive across some of them. This ice and what happens underneath are truly amazing.
If heat rises, why does ice form on the top of lakes, ponds and other bodies of water? Water has a higher density at 39 degrees Fahrenheit than at any other temperature. As the water in a lake or pond gets colder and approaches this magic temperature, it gets denser and sinks to the bottom. But once it reaches and passes that threshold, it starts moving back upwards. The crystal structure of ice moves water molecules further apart from each other, making the water less dense and allowing ice to float.
This amazing property of water makes it possible for many forms of life to exist under the ice.
As any ice fisherman will tell you, many resident fish continue to swim around under lake ice, even venturing into the colder waters above the bottom. Many turtles and frogs that winter in lakes and ponds burrow into mud at the bottom where temperatures are a smidge warmer. Their body temperature drops as the water temperature drops, meaning they need less oxygen and energy to sustain their lives. This hibernation-like process is known as brumation.
This winter, take advantage of your ability to remain active in a way that frogs and turtles can’t. Enjoy the beauty of winter ice on lakes, ponds, creeks and other bodies of water. Just make sure to check ice thickness and safety before walking out onto it. Minnesota DNR recommends that new, clear ice be at least four inches thick before venturing out on foot, but a number of other factors also affect ice safety, and ice thickness may not be consistent from one end of a lake or pond to another.
Miriam Turnbull is an interpretive naturalist at Springbrook Nature Center, 100 85th Ave. NW in Fridley. Director Mike Maher welcomes comments at [email protected] or 763-572-3589. Go to springbrooknaturecenter.org for more information.