Remember that warm day last week when you went swimming in your lake, and you felt that the water was warmer near the surface and colder at the bottom?  Do you know what this phenomenon is called and why it happens? 

Lakes that are large enough, like Lake Metonga and Lucerne, and deep enough are stratified or layered.  Stratification of a lake is simply the layering of different water temperatures within the lake. The three layers that make up the lake are the epilimnion,the metalimnion (also referred to as the thermocline), and the hypolimnion. 

The top layer of water is referred to as the epilimnion. This layer is the warmest in the summer because this water is on the surface of the lake and is heated by the sun throughout the day. The sun’s rays only penetrate so deep within the water, which is why the water on the bottom of the lake does not get as warm as the water on the top of the lake.  

The middle layer of water is referred to as the thermocline, or the metalimnion. This layer of the lake has water that is moderate in temperature. It isn’t the warmest spot, but it’s not the coldest spot either. This water receives some sunlight, but not enough to warm up the water molecules so that they can become a part of the epilimnion.  

The bottom layer of the lake is known as the hypolimnion. This layer consists of the coolest, densest water since the sunlight typically does not reach the bottom of the lake.  

Here in Northern Wisconsin, these three layers do still exist in the winter, but they are underneath the layer of ice that is formed on top of the water.  These layers change throughout the year during turnover. Turnover is caused by the density of the water particles changing and it allows for the colder water to move to either the top or bottom of the lake, depending on the season. The density of water changes as the temperature of the water changes. For example, cooler water is more dense than warmer water, meaning that in the summertime, the cooler water sinks down to the bottom of the lake. 

However, once the water reaches a temperature of 0 degrees Celsius (32 degrees Fahrenheit) it freezes. Water in its solid form (ice) is less dense than water in its liquid form, which is why the coldest water is found on the top of the lake in the winter as a sheet of ice. 


Conservation Corner is a weekly article produced by the Forest County Land & Water Conservation Department. For more information contact Steve Kircher, County Conservationist-Land Information/GIS Director at 715-478-1387 or by e-mail at .