The Lake Where You Live

Something is happening beneath your lake’s ice that at this time of year it’s becoming significant.

One can think of a lake in winter as a giant scuba tank that in fall was fully charged with oxygen, which then gradually gets depleted. When frozen, the lake is largely a sealed container, although fed to some degree by groundwater springs and perhaps by a creek. So whatever oxygen is there was the ice sets in will have to suffice to carry the fish and other creatures through until spring.

And here another interesting property of water comes into play. In summer most lakes are thermally stratified, a layer of warm, less dense water floating on colder, denser water below. In fall the surface cools, the lake temperature becomes essentially uniform, and wind and wave action stirs the water, which takes in oxygen from the air. 

It’s as if the lake inhales a deep cleansing breath. This process continues through October and November, and as the water chills, the amount of oxygen it can hold increases. In fact, by the time the water reaches the freezing point, it carries nearly twice as much oxygen as it did in summer when its temperature was about 80 degrees F.

So when the ice seals off the surface, the lake holds more oxygen than at any other time of year. But now that oxygen slowly gets consumed. With the aquatic plants and algae died back, there’s very little photosynthesis to add new oxygen. The chemical reactions that decompose the dead plant matter and algae continue to function. 

And the water creatures, from fish, to turtles and frogs, to crayfish and mussels, to the insects in their different life stages, still need oxygen to breathe. So, little by little, the oxygen gets consumed. Fortunately, the cold-blooded creatures’ activity slows down greatly. So does their metabolism, and thus their need for oxygen. And decomposition and other biochemical processes slow down as well. 

So, it turns out that entering winter, the lake held is maximum quantity of oxygen, at a time when the living creatures needed the least amount of it. And therein lies the miracle of life persisting through months of cold and isolation.

In most lakes, the creatures make it through just fine, in healthy condition. The occasional exceptions are very shallow lakes, where the scuba tank’s capacity is relatively low. There, if the ice sets in early (such as mid-November) and stays late (as into early May), the lake’s life can experience a winter kill.

It’s not that the ice forms clear to the lake bottom. The problem is that the water runs out of oxygen, and the fish suffocate. Fortunately, even in shallow lakes, such events are uncommon, happening only in the most severe winters. Some lake associations look to limit the risk by installing aerators that pump in oxygen through the winter.

In most years, though, the ice leaves, the lake wakes up, and life goes on. Fish and other creatures continue to grow, reproduce, and metamorphose. And the lake is ready for yet another annual cycle of life.

Ted Rulseh, a writer, author and advocate for lake protection, lives on Birch Lake in Oneida County. Visit him and his blog at https://thelakeguy.net.