How Frogs, Turtles and Insects survive winter

This week the temperature plunged and our lakes froze over pretty quickly.  We humans retreated into the shelter of our heated homes and warm coats.  The only animals you see outside now are warm-blooded animals such as rabbits, deer and squirrels.  So what happens to the cold-blooded animals like aquatic insects, frogs and turtles when the lakes freeze?

First of all, remember that water is most dense at 39 degrees Fahrenheit.  In the winter, lake water is insulated by the ice and remains about 39 degrees at the bottom of the lake.  This temperature is still cold, but it stays relatively stable and is a much better habitat than the winter air.

In the winter, frogs and turtles enter a form of hibernation.  They survive by slowing their metabolism to where their heart beats so slowly that you can barely detect it.  Turtles spend the winter dug into the mud at the bottom of a pond or stream.  But how do they breathe?  Snapping turtles can take up some oxygen from the water across the skin lining the throat.  Others, like painted turtles, have altered their metabolism to survive without oxygen.  In cold water, painted turtles can stay submerged for as long as 3 months with zero blood oxygen.

Aquatic frogs don't usually bury themselves in the mud like turtles do.  They can't get enough oxygen that way.  Frogs spend a good portion of the winter just lying on top of the mud or only partially buried. They may even slowly swim around from time to time.  They need to stay in high-oxygen areas so they can absorb oxygen into their bodies through their skin.

Aquatic insects spend the winter a bit different than frogs and turtles.  Aquatic insects overwinter at the bottom of lakes and streams in egg or larvae form.  You know how a butterfly starts out as a caterpillar and then forms a cocoon and emerges finally as a butterfly?  Aquatic insects such as mayflies, dragonflies and midges do the same thing.  You can think of their larvae form as equivalent to a butterfly's caterpillar form.  Aquatic insect larvae live at the bottom of lakes and streams and then emerge in the spring as flying insects.

When living at the bottom of a lake in the winter, the 39º water is pretty tolerable.  Those insects that spend the winter as larvae and nymphs are called "freeze tolerant".  This is an amazing adaptation.  Usually when body tissue freezes, the ice crystals puncture blood vessels and organs, and makes survival pretty difficult.  Freeze tolerant insects use the sugar alcohol, glycerol, for antifreeze in their organs and blood vessels to prevent freezing, and then actually allow ice crystals to form outside their organs.  In this way, they can survive short periods of freezing by not damaging their organs. 

As you can imagine with Minnesota winters, many of the aquatic insects living here are freeze tolerant.  Insects such as stoneflies, mayflies, caddisflies, and dragonflies are common overwintering aquatic insects.

You can be glad that these aquatic animals and insects survive the winter – otherwise they wouldn't be around!  This winter, I will talk more about what goes on under the ice in lakes.  If you have any particular questions or topics you're wondering about, please contact me and I will address them.

Until next week, enjoy the lakes!

Moriya Rufer is the Lakes Monitoring Program Coordinator for RMB Environmental Laboratories in Detroit Lakes, 218-846-1465, lakes.rmbel@eot.com.