Written by Ted J. Rulseh

When you look out on your favorite lake, what do you see? Beautiful blue water? A place for a refreshing dip on a summer day? A surface on which to paddle a canoe or kayak? Favored spots to catch fish for sport or dinner? Your lake is all this, but also much more. A lake is a fascinating living system, full of mysteries and things to discover, if you look closely. Here are ten things you may not know about the world beneath the waves.

1. It all starts with the sun. That’s right. The walleye you fry up for supper owes its existence, first and foremost, to the sun. It’s sunlight that enables plants and algae in the lake to manufacture food through photosynthesis. The food these primary producers make forms the base of the lake’s food chain.
2. Your lake’s water is a thin soup. The water is the broth; the meat and vegetables consist of tiny organisms called plankton. The vegetables are the cells of algae that float freely in the water; they’re called phytoplankton. The meat is made up of small creatures, called zooplankton, that swim through the water, feeding as they go. They feed on the algae and in turn become food for fish in the very early stages of their lives.
3. Your lake has ‘fleas.’ Tiny creatures called Daphnia, crustaceans from the same family as crayfish and shrimp, float in the thin soup. They’re often called water fleas because their herky-jerky swimming patterns remind observers of the jumping of fleas (those you hope never infest your dog). Daphnia are an essential food source for baby fish (called fry), water insects and the immature forms of frogs and toads. You don’t need a microscope to see them – they’re about a millimeter long. So if you scooped up a jar of lake water and looked through it, you’d probably see a Daphnia or two kicking about.
4. Your lake has layers. The water is not a pool with a uniform temperature, at least not in the warm months of the year. As spring turns to summer, the lake separates into layers. Cold water lies at the bottom. Warmer water, being less dense, floats on top. The zone where warm water meets cold is called the thermocline. You can experience the thermocline by swimming out into fairly deep water, then doing a feet-first surface dive. When your feet reach a depth of about 12 to 15 feet, you will feel a sudden change from warm to cool. You’ve penetrated the thermocline.
5. There’s only “one water.” There are lakes, rivers, and the vast resource known as groundwater. These are not really separate entities. They are all part of the same system. The top of the groundwater is called the water table. In an important sense, a lake is a depression in the land that intersects and exposes the water table.
6. Your lake has a “skin.” You’ve seen the rounded shape of water droplets on a lakeside leaf. What gives that droplet its shape is something called surface tension – it’s as if the water had a very thin, invisible skin. That’s why the insects called water striders can skim across your lake’s surface on their long, spindly legs: The surface tension keeps them from sinking.
7. Water has a unique behavior. Most liquids, as they cool, become progressively denser. Water is different. It becomes denser until it reaches 39 degrees F. Below that temperature it becomes less dense, until finally it becomes ice, only about 90 percent as dense as water (this is why ice floats). That’s important, because imagine what would happen if ice were denser than water and would sink. Through the winter, ice forming on the lake’s surface would drop to the bottom, and eventually the entire bowl of the lake would be frozen solid. It might never thaw; almost everything in it would be dead.
8. Making ice is hard work. Your lake can take a long time to freeze, even with a number of cold and wintry days and nights. Because of a property of water called the heat of fusion, it is eighty times harder to freeze a given volume of water than to lower its temperature by one Celsius degree. Put another way, a drop of water has to give as much energy to freeze as it would give up to lower its temperature by 80 Celsius degrees.
9. Your lake breathes. Dissolved oxygen in your lake is the single most important component of water quality, because without adequate oxygen, next to nothing could live. Your lake breathes by taking in oxygen from the air (with help from the stirring action of waves) and as a product of photosynthesis, the process by which algae and plants use sunlight to create food. The air we breathe contains about 20 percent oxygen. By comparison, the amount in lake water is tiny. A healthy lake contains 6 to 8 parts per million of oxygen – or 0.0006 to 0.0008 percent. Yet, that’s enough to enable fish to breathe, because of the miraculous structures they have called gills.
10. Your lake is aging. All lakes go through a long, slow process of getting older and filling in. Every year, silt enters the lake with runoff from rainfall. Water plants grow and die, and their remains sink to the bottom. Gradually the lake takes on more nutrients, and more plants grow and die. It’s called eutrophication. We can all help slow this process down by keeping nutrients out of the lake. We do this by forgoing the use of fertilizers on our lakefront lots, keeping our septic systems maintained and in good repair, and limiting runoff into the water by keeping land near the water’s edge in a natural condition.

The closer you look at your lake, the more you’ll discover, and the more you will treasure and want to protect that natural wonder.

Ted J. Rulseh writes the newspaper column, “The Lake Where You Live.” An advocate for lake improvement and protection, he lives in the lake-rich region of northern Wisconsin. This article is adapted and excerpted from his book, A Lakeside Companion. It is printed by permission of the University of Wisconsin Press. © 2018 by the Board of Regents of the University of Wisconsin System. All rights reserved.