The Lake Where You Live

They say a lake, though filled with an inanimate substance, is a living system. And in many ways that’s true. For example, your lake breathes. And right now it is about to take a great big, deep, cleansing breath.

And it turns out that breath — that top-to-bottom infusion of oxygen — comes at the ideal time of year. All summer long, lakes unless very shallow are stratified by temperature, a warmer and less-dense surface layer floating on top of colder, denser water below.

As summer goes on that deeper layer receives dead algae and other organic matter that sinks from the surface water. As that material decomposes, the oxygen in the deep becomes depleted and not very hospitable to life. 

Then along comes autumn. The air cools, the surface water gives up its heat. Wind and wave action then stirs the water until the entire lake’s volume is at a nearly uniform temperature, thoroughly mixed, no more warm and cool layers. This is called the fall turnover.

Now for the really great thing that mixing action does: It re-supplies the lake with oxygen, essentially from surface to bottom. And that’s important, because the fish, turtles, frogs, crayfish, mussels, mayfly and midge larvae, dragonfly and damselfly nymphs and other creatures need a supply of oxygen to get them through the winter, when the ice cover makes the lake mostly a closed system, sealed off from the air above.

Even more important, the mixing action continues through the cold and windy days of late October, all of November, and into early December before the ice takes hold. And cold water holds significantly more oxygen than warm water.

In fact, water at the freezing point can hold roughly twice as much oxygen as water at a mid-summer temperature of around 80 degrees. This means the lake hold more oxygen in the cold weather and under the ice than at any other time of year.

Meanwhile, because the metabolism of the cold-blooded water creatures slows down, they consume less oxygen while going about their slow, gelid, cold-water business. In other words, during the months when the lake is frozen, the fish and other critters have the most oxygen in the water during a time when they need it the least.

This is one reason why all but the shallowest lakes are largely immune from the problem of winter kill, which occurs not because the ice freezes all the way to the bottom but because the water runs out of oxygen.

In the deeper lakes, fish and crayfish take in the oxygen through their gills, turtles and frogs (in or near a state of hibernation) breathe through their skin, absorbing the minimal oxygen they require.

So call it nature’s wisdom, or call it a happy accident of chemistry, biology and physics. The fall turnover gives your lake that deep, cleaning breath just when it is the most necessary. And that can only be called perfect timing. 

Ted Rulseh is a writer, author and lake advocate who lives on Birch Lake in Oneida County. His new book, “Ripple Effects,” has been released by UW Press. You can learn about it by visiting my website at https://thelakeguy.net.