Turnover a Lake

April 2013 Prof. Scarlet Leslie

In honor of Earth Day this month, I decided to revisit one of my most treasured science topics - Water! Inspired by the Mysteries of the Lake class offered by Prof. Rorey Padfoot that I took last term, let's look at the importance of lake turnover.

Freshwater lakes are unique, mainly self-sustained ecosystems. Dissolved oxygen is essential for all the creatures that live in the lake. There are two sources of dissolved oxygen. Plankton and other organisms can perform photosynthesis, provided that there is sunlight. Oxygen can also diffuse in from the atmosphere. The larger the lake, the more surface area available for the diffusion of oxygen.

This all sounds so wonderful, but the truth is that lakes, especially the deeper ones, tend to form layers. This process is called stratification. There are usually three distinct layers. The top layer is called the epilimnion. It is the most well mixed. Below that is the thermocline, where the temperature drops rapidly according to the depth of the water. Water is most dense at 4 degrees Celsius and will settle to the bottom of the lake to form the hypolimnion.

In the summer, the lighter, warmer water stays near the surface of the lake. The denser, cooler water collects at the bottom of the lake. Wind might be able to mix the top few layers, but it generally cannot reach the hypolimnion of a deep lake in the summer. By the end of the season, there may not be any oxygen in the deepest layer.

The cool autumn weather brings the extremely important fall turnover. When all of the water in the lake is at the same temperature, the winds are able to mix the entire lake. This brings the water that was in the hypolimnion all summer up to the surface and replenish the dissolved oxygen supply for the winter.

Water is the only substance that has a solid form that can floats on its liquid form. When the ice freezes over a lake in the winter, it forms a thin cap that protects the lakes inhabitants from the chilling winter wind and tough weather. Below the ice, all of the water stays around 4 degrees Celsius. If the ice cap is thin enough, it is still possible to have photosynthesis. However, long extended winters may result in "fish kills" when there is insufficient dissolved oxygen for all the organisms.

Then comes spring! The warmer weather melts the ice until it reaches the same density of the water below it. This allows for spring turnover, where the wind is able to mix all of the contents of the lake once again... at least until summer comes along!

There you have it, a year in the life of a lake. Don't stop by your favorite lake and see what layers you can turnover!