Have you ever skated on a frozen lake? Or even seen a fully frozen lake for the first time? You might be wondering, “Hey, what happens to all the fish?” Well, here’s the thing, lakes don’t completely freeze.

If you’re from a cold climate, this probably isn’t a surprise (or it might be if you’ve never pondered it), but that frozen lake isn’t ice all the way to the bottom.

Usually, a top layer will form on the body of water. It may be so weak that it cracks at the first sign of weight or strong enough to hold an impromptu ice hockey tournament. That’s why you should always check ice’s thickness, and don’t walk on it unless absolutely sure it’s safe.

But what determines the thickness of the ice? And why exactly don’t lakes completely freeze? Let’s look into it.

How Does a Lake Freeze?

A lake is a body of water completely surrounded by land. Most contain freshwater, although saltwater lakes do exist. They come in various sizes and depths and can be found all over the world.

When the outside temperature consistently stays above 32 degrees Fahrenheit, most lakes are thermally stratified. This means that warmer water, which has a lower density, sits on the surface while colder water, which has a higher density, sits below. As the outside temperature decreases, the warm water on the surface begins to cool down and this density difference changes.

Eventually, the exposed water on the surface of the lake cools to 39.2 degrees Fahrenheit, which is the temperature at which water is the densest. This new density causes the water to sink to the bottom of the lake while warmer water is pushed to the top. The cycle continues until the temperature outside is cold enough that it causes the top layer of water to drop below 39.2 degrees Fahrenheit and eventually freeze below 32 degrees.

Because ice is less dense than water at 39.2 degrees, the layer of ice floats on top of the warmer water below. The colder the temperature outside, the thicker the layer of ice will be. However, ice is a good insulator, and depending on the depth of the lake, it is difficult for the outside temperature to get cold enough to make the lake freeze all the way through. The layer of ice on top protects the liquid below from the cold air and keeps it at a temperature above freezing.

It would take a very shallow lake or an extremely cold winter (think: outer space) for large lakes to completely freeze.

Why It’s Important That Lakes Don’t Completely Freeze

Freshwater habitats like lakes provide homes for over 100,000 species of animals and plants. If lakes froze completely in the winter, many of those animals and plants would be cut off from oxygen and food and subsequently die. It would be disastrous for these ecosystems to start from scratch every year.

The natural process of water circulation during the winter also redistributes nutrients and oxygen throughout the lake. Without this occurrence, the deepest waters wouldn’t have enough oxygen to support life.

Fish and other life forms that live in lakes also contain unique adaptation qualities that allow them to survive under the ice. Their metabolisms slow down and they will instinctively reduce their energy expenditures for those months. Some other animals hibernate and some plants survive against all odds. Whether we know the reason or not, essentially all living creatures are equipped to survive this fascinating phenomenon.

The next time you’re out and about in the cold and see what appears to be a frozen lake, don’t be so quick to assume it’s safe and approach with caution.