Lake stratification

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.