Water Density
To understand lake stratification, we first must address the relationship between water density and temperature. Water is unique in that it is more dense as a liquid than a solid; therefore, ice floats. If ice sank, our lakes would behave much differently in the winter!
Water is most dense at 4 degrees Celsius (39 F), and as water warms or cools it gets less dense. This has implications for a lake’s structure because the denser water is heavier and will be at the bottom of a lake while the less dense water is lighter and will generally be at the top of the lake.
Summer
In the summer in Minnesota, the sun heats the top layer of a lake, the epilimnion, which causes it to become less dense. The bottom layer of the lake, the hypolimnion, does not receive sunlight and therefore remains cold. Since the epilimnion is less dense, it floats on top of the hypolimnion and the two do not mix. The thermocline is the dividing area between the top and bottom layers.
Since the epilimnion is the only part of the lake that sunlight can penetrate, it is where plants and algae grow. Around the shoreline of a lake, the area where sunlight penetrates and vascular plants grow is called the littoral zone. In the middle of the lake, the epilimnion is home to algae and zooplankton.
When algae and zooplankton die, they sink to the bottom of the lake. Invertebrates and microbes living in the benthos recycle and decompose this dead material. This recycling process uses up oxygen. Since the lake does not mix during the summer, the hypolimnion is completely cut off from the epilimnion and does not receive a fresh supply of oxygen. Therefore, they hypolimnion can become anoxic during the summer in a mesotrophic or eutrophic lake.
Fall
In the fall in Minnesota, the sunlight is not as strong and the nights become cooler. This change in season allows the epilimnion to cool off. As the water in the epilimnion cools, the density difference between the epilimnion and hypolimnion is not as great. Wind can then mix the layers. In addition, when the epilimnion cools it becomes more dense and sinks to the hypolimnion, mixing the layers. This mixing allows oxygen and nutrients to be distributed across the whole water column.
Winter
In the winter in Minnesota, the lakes are covered with ice. Under the ice, the water cannot mix because it is not exposed to wind. Most of the hypolimnion remains 4 degrees Celsius (39 F). There is a thin layer of water under the ice that is colder than 4C and therefore less dense. This thin layer of water floats on top of the hypolimnion throughout the winter, but this stratification is not quite as stable as in the summer because the density difference is much smaller. This phenomenon is called inverse stratification because cooler water is sitting on top of warmer water.
As in the summer, the hypolimnion is cut off from oxygen, so as decomposition takes place in the benthos, oxygen gets used up. When the hypolimnion becomes anoxic in the winter it is called winter kill because fish and other living organisms that need oxygen die. In addition, when the bottom of the lake is anoxic, chemical processes at the sediment/water interface cause phosphorus to be released from the sediments. When the ice melts in the spring and the lake mixes again, this increased phosphorus fuels algae growth.
Spring
In the spring in Minnesota, the ice melts off the lake, the wind picks up and the lake mixes again. This is called spring turnover. Oxygen and nutrients get distributed throughout the water column as the water mixes. Then, as the weather becomes warmer, the surface water warms again and sets up summer stratification.
Most lakes in Minnesota are considered dimictic, meaning they mix twice a year – spring and fall. Shallow lakes behave differently and can mix more often.