Lake stratification

Lakes often form separate layers of water during warm weather. This process is called thermal stratification. It happens because water changes its density based on its temperature. Cold water is denser, so it is heavier and sinks. Warm water is less dense, so it stays near the surface. This creates distinct sections of water that do not mix easily.

Most stratified lakes have three main parts. The top warm layer is called the epilimnion. Below that is the metalimnion, which is also called the thermocline. This middle layer can change its depth during the day. The bottom layer is the cold hypolimnion, which reaches the lake floor. Heat moves very slowly between these layers. It can take about a month for heat to move just one meter.

Lakes follow different mixing regimes throughout the year. Dimictic lakes are a common type in temperate regions. These lakes turn over during the spring and the fall. This mixing moves oxygen and nutrients through the water. Shallow lakes might mix all year long. These are called polymictic lakes. If stratification lasts for a very long time, the lake is meromictic.

Some lakes have very special or dangerous patterns. In Africa, three meromictic lakes can hold large amounts of carbon dioxide. These are Lake Nyos and Lake Monoun in Cameroon, and Lake Kivu in Rwanda. If a limnic eruption happens, the gas can leave the lake quickly. This can push away the oxygen that people and animals need. Other lakes, like the large Lake Taihu in China, are shallow and dynamic.

Human activities can change how lakes behave. Changes in land use and rising air temperatures affect stratification. For example, urban expansion can cause more runoff to enter a lake. Salt from roads can also create a heavy layer at the bottom. This layer can stop surface water from mixing with the bottom. This can change the types of fish and tiny plants that live there.