uestion: This fall the lake in our development had a foul odor. What causes this? (Asked by a concerned, anonymous lake dweller, of Greenwood, S.C.)

Reply: Most likely, it is just nature taking its course, which occurs in lakes and large ponds. First, let’s look at a little lake water characteristics and dynamics.

In the summer, when swimming and standing neck deep in a lake, you have undoubtedly noticed that the water at your feet is cooler than the surface, and even more so at greater depths.

This is because the sun warms the surface, and warm water (being less dense) “floats” on the cooler, denser water beneath.

Now, fast forward to fall and cold weather. As the surface water cools, it becomes denser and sinks. Over time and with increased cooling, a descending “column” of water occurs with the displaced water ascending, setting up a thermal cycle. This cycle may reach the bottom of the lake, depending on its depth and the temperature of the weather.

During the summer, organic material (such as leaves, grass, weeds and animal waste) settles to the lake bottom, creating thick layers of decaying “muck.” Bacterial action produces muck decay, but with lake bottoms being depleted of oxygen, there is no common aerobic (requiring oxygen) bacteria.

Therefore, anaerobic (without oxygen) bacteria take over, which produce gasses such as methane and hydrogen sulfide (which smells like rotten eggs) during the decaying process.

When the thermal water cycle extends deep enough to disturb the muck settled at the bottom, the lake “turns over.”

That is, the bottom anaerobic layer rises to the top, bringing the foul-smelling odors with it. This smell is sometimes mistaken for a sewage leak.

Depending on the lake and changing weather conditions, these “turnovers” can occur at other times of the year, such as in spring.

Curious Science Bonus: I pointed out that because the density of lake water increases with temperature, it becomes heavier and sinks. But what would happen if the water cooling and density increase occurred down to zero degrees Celsius (freezing)?

With freezing point at the lowest level, the lake would start to freeze at the bottom. But we all know that lakes freeze at the top, so what is wrong here?

Nothing. Just another little-known quirk of nature. Water has a strange characteristic of a maximum density of four degrees Celsius.

This means that the density of water increases down to four degrees Celsius, but decreases from four degrees Celsius to zero degrees Celsius, and water cooled at the lake’s surface in this range does not sink. So, when cooled to zero degrees Celsius and below, the lake water freezes from the top down, which is better for aquatic life (and ice skating).

C.P.S. (Curious Postscript): “The best way to get odors out of the kitchen: eat out!” — Phyllis Diller