Lake stratification

Lakes can have layers of water.

Warm weather makes these layers. The top part is warm. The middle part is in between. The bottom part is very cold.

This happens because of how water moves. Cold water is heavy. It sinks to the bottom. Warm water stays on top.

Wind can mix the water. This helps move food around. It also moves the air we breathe. This helps the fish.

Some lakes stay in layers for a long time. Other lakes mix all year. It is a busy world underwater!

Lakes often form separate layers of water. This is called stratification. It happens during warm weather.

Water changes its weight based on its heat. Cold water is denser, which means it is heavier. Because it is heavy, it sinks to the bottom. Warm water is less dense. This means it stays on top.

Most lakes have three main parts. The top warm layer is the epilimnion. The middle layer is the metalimnion. This layer can change depth during the day. The cold bottom layer is the hypolimnion.

Wind helps move the water. This movement is called turbulence. It can mix the layers together. Mixing is important for life. It moves oxygen and nutrients through the water. Without mixing, the bottom might run out of oxygen. This can hurt animals like shellfish.

Some lakes mix all year long. These are called polymictic lakes. Other lakes only mix in the spring and fall. These are called dimictic lakes. Changes in weather can change how lakes mix.

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.

Thermal stratification is the process where lakes form distinct layers of water based on temperature. This phenomenon occurs because the density of water changes depending on its temperature. Cold water is denser and heavier than warm water. In a stratified lake, this density difference causes the water to separate into layers that do not mix easily.

This separation is vital for understanding how nutrients, oxygen, and heat move through freshwater ecosystems.

A typical stratified lake consists of three specific sections. The top layer is called the epilimnion, which is the warmest part of the water. Below the epilimnion is the metalimnion, often referred to as the thermocline. This middle layer acts as a transition zone where temperature changes most rapidly, and its depth can shift throughout the day. The third layer is the hypolimnion, which is the coldest section and extends down to the lake floor. Heat moves incredibly slowly between these layers; for example, it can take about one month for heat to diffuse just one vertical meter.

Lakes follow different mixing regimes, which are the yearly patterns of stratification. Dimictic lakes are common in temperate regions and undergo a process called turnover during the spring and fall. During this turnover, the water mixes, recirculating oxygen and nutrients like phosphorus. If a lake stays stratified for very long periods, it is classified as meromictic. In contrast, shallow lakes often do not form these distinct layers because wind or cooling causes them to mix constantly. These are known as polymictic lakes. The specific regime of a lake depends on its morphometry, which is its physical shape and structure, and environmental conditions.

In some environments, stratification takes on unique forms. In northern areas, seasonally ice-covered dimictic lakes may be described as cryostratified or cryomictic. Cryostratified lakes show inverse stratification near the ice surface and have average temperatures near 4 °C. Cryomictic lakes have no under-ice thermocline and have average winter temperatures closer to 0 °C. In large, shallow lakes like Lake Taihu in China, the water is highly dynamic. Even though the depth is limited, turbidity, or water cloudiness, allows the lake to stratify and de-stratify quickly as it absorbs solar radiation.

Stratification can lead to significant environmental risks. In meromictic lakes, such as Lake Nyos and Lake Monoun in Cameroon or Lake Kivu in Rwanda, large amounts of carbon dioxide (CO2) can accumulate. If a limnic eruption occurs, this gas can be released suddenly, displacing the oxygen that humans and animals need to breathe. Additionally, strong stratification can lead to anoxic conditions, where oxygen levels near the bottom become extremely low. This can be harmful to benthic organisms, which are creatures that live on the bottom, and can even wipe out entire populations of shellfish.

Human activities, known as anthropogenic influences, are currently altering these natural patterns. Rising global temperatures act like a physical shift in geographic location, making tropical zones particularly sensitive. This warming can change a dimictic lake into a monomictic lake, or a monomictic lake into a meromictic one. Local land use also plays a role. Urban expansion increases runoff, and salt from roads can create a saline layer at the bottom. This salty layer interferes with vertical mixing, preventing oxygen from reaching the sediments.

These changes have a ripple effect through the entire food web. Shifts in stratification alter the availability of nutrients and oxygen, which changes the composition of phytoplankton and zooplankton communities. For example, in deep lakes, temperature changes affect the deep chlorophyll layer taxa. These biological shifts can eventually impact fish recruitment, such as the population of walleye. When the timing of prey and predator populations becomes out of sync due to changing weather and stratification, it can take many years for these ecosystems to return to their normal state.