During the spring and fall months, we see many changes take place as nature transitions from winter to summer and back to winter again. Birds migrate, flowers bloom, and leaves change color. But, there are also many changes that take place we never see, such as those now occurring in our lakes and ponds. And, although those changes may be invisible to us, they are vitally important to the health of those bodies of water.
During the summer, the heat of the sun warms the upper layer of water in a pond or lake, making water less dense. Where sunlight is less or doesn’t reach at all, the water is cooler and denser, so it remains at the bottom. The area between the two is known as the thermocline. Sometimes, if you go swimming in a lake or a pond, you can notice that boundary pretty clearly. These two layers rarely mix in the summer because the hot sun keeps that top layer warmer and less dense, basically floating on top of the cooler, denser layer underneath. This means oxygen can sometimes become depleted in the bottom layer, where there is decomposition occurring, but no photosynthesis.
But, during the fall, that top layer becomes cooler due to less sunlight and cooler air temperatures. As a result, that top layer becomes a little, more dense and begins to sink. Since there’s not as much of a density difference now, wind is able to easily mix the two layers. This mixing process, called turnover, allows nutrients and oxygen to mix throughout the lake.
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Winter, though, poses a unique situation. Water does become denser the cooler it gets, but only until it reaches 39 degrees, then it becomes less dense again as it approaches freezing. So, water is actually less dense as a solid (ice) than it is as a liquid, which is why ice floats. When ice forms in the winter, it blocks what little sunshine there may be, causing the water to be cold throughout a lake or a pond; so there isn’t that much of a temperature or density difference. But, the coolest water does tend to be at the top, just under the ice, because it’s closer to freezing and not very dense. The “warmer” water tends to be at the bottom because it’s closer to that 39 degrees where water is densest.
Come spring, as the ice melts and there’s more sunlight, the top layer of water will warm up again, making it closer to the temperature at the bottom. Since the temperature is relatively uniform throughout the lake or pond, so is the density, allowing the wind to once again mix up the water, including the oxygen and nutrients.
So, right now, our ponds and lakes are “turning over” and the oxygen and nutrients are mixing throughout the lake. Without this mixing each spring and fall, our lakes would be much less productive, with many “dead zones.” Like so many other things in nature, there’s a lot going on that we can’t see. But, right now, there are big changes occurring in our lakes and ponds, changes that will insure that they will remain productive throughout the year.