Fall Turnover

Ok so the fabulous colors of autumn have flared and faded, but that is not the end of extraordinary changes going on around us. Besides the scores of birds embarking on long distance journeys of 1,000’s of miles or the mammals preparing to hunker down to sleep off the more severe aspects of winter, we have a relatively quiet but essential shift going on in our lakes and ponds.

Let me start by saying that water is amazing. Without its unusual characteristics, life would not exist as we know it. Certainly not here in the north country. Let’s talk temperature…. Water is its most dense at 39° F. So what, you might think. But wait a minute, that means that water when it is frozen at 32° F is less dense. So ice floats. To understand why that is so important let me explain the dynamics of what is happening in the depths of the lake.

Think back to those steamy days of summer by the lakeside. As one would expect the surface temperatures of the water are warmer than the cooler temperatures found a few feet lower. The sun’s rays warm up the surface temperatures of the water. This warmer water at the surface being less dense than the cooler water below, sets up a temperature gradient. As the season moves along, there is relatively little mixing between the warm water at the surface and cool water below. As a result there is no mixing of oxygen, nutrients or aquatic waste products during this time so the surface waters can be depleted.

But just wait, a big shift is on the way. As summer fades into autumn, the cool nighttime temperatures allow the surface water to also cool. And since cooler water now on the surface is more dense than the warmer water just below, it begins to sink. It only sinks to where it’s temperature is the same as the surrounding water. Gradual at first, but the surface water temperature continues to cool and mix with the deeper water. Eventually the entire lake or pond is completely mixed with resources of nutrients, dissolved gasses distributed evenly throughout the body of water. This mixing process continues until the water reaches its maximum density at the magic temperature of 39°F.

What is so miraculous about that you might ask? Well if water was more dense at 32°F and if ice did not float, then the lake would freeze from the bottom up, preventing many aquatic species from surviving winter. With water at 39°F throughout the lake, as nighttime temperatures dip even colder, water at the top will freeze essentially placing a lid over the lake insulating it from freezing all the way through.

Check back for more of what is going on in our lakes and ponds as winter approaches. In the meantime checkout any local pond or lake for signs of fall turnover.