Thermal stratification is the process in which deep water bodies, such as lakes, develop distinct layers of water based on temperature differences, which directly affects water density. Water density is not linear with temperature; maximum density occurs at approximately 4 degrees Celsius (39.2 degrees Fahrenheit). Since colder water is denser than warmer water, less dense water floats atop denser water, creating a stable vertical separation. This phenomenon is pronounced in deep lakes during warmer months and influences the distribution of dissolved materials, including oxygen and nutrients.
The Three Distinct Layers
During the peak of stratification, typically in summer, a lake separates into three identifiable layers that remain unmixed due to density differences. The uppermost layer is the Epilimnion, the warmest layer exposed directly to solar radiation and wind mixing. This surface layer is uniform in temperature and is well-oxygenated because of atmospheric diffusion and photosynthetic activity.
Beneath the warm surface lies the Hypolimnion, the deepest and coldest layer of water, which remains isolated from surface influences. This bottom layer maintains a temperature near 4 degrees Celsius and is characterized by its stability and lack of circulation. The hypolimnion remains sealed off from the epilimnion throughout the summer due to extreme temperature and density differences.
Separating these two main zones is the Metalimnion, a transitional layer defined by a rapid change in water temperature with depth. The most intense part of this transition is the Thermocline, where the temperature gradient is steepest. This narrow zone acts as a physical barrier, preventing the wind-driven mixing of the epilimnion from reaching the cold, deep waters of the hypolimnion.
Seasonal Cycles and Lake Turnover
The formation and destruction of thermal stratification follows a predictable seasonal cycle in temperate lakes, a process known as lake turnover. Stratification begins in the spring as surface waters warm and stabilize, but this stability breaks down in the fall as air temperatures drop and days shorten. The surface water of the epilimnion cools, becoming denser until its temperature and density approach that of the hypolimnion.
This cooling process leads to the Fall Turnover, where density differences disappear, and the entire water column reaches a uniform temperature, known as an isothermal condition. Wind action easily mixes the water from top to bottom, resulting in complete circulation. This mixing redistributes dissolved oxygen to the deeper waters and brings accumulated nutrients from the bottom sediments up toward the surface.
As winter progresses, a temporary Inverse Stratification can form beneath a layer of ice. Water beneath the ice is near 0 degrees Celsius, making it slightly less dense than the 4-degree water that settles below. When the ice melts in the spring, the surface water warms and sinks until the entire lake reaches a uniform 4 degrees Celsius. This allows the wind to mix the water column in a Spring Turnover. The cycle then repeats as the sun warms the surface, re-establishing summer stratification.
Ecological Consequences in Aquatic Environments
Thermal stratification profoundly affects the aquatic ecosystem, particularly within the isolated hypolimnion. During summer, the deep hypolimnion becomes a zone of oxygen depletion because it is cut off from the atmosphere and sunlight. The decay of sinking organic material consumes dissolved oxygen, often resulting in extremely low levels or zero oxygen, a state known as anoxia.
This lack of oxygen makes the deep layer uninhabitable for many aquatic organisms, including most fish species, which are forced to seek oxygenated water. Consequently, cold-water fish, such as trout, are often restricted to a narrow band of cold, oxygenated water just above the thermocline. The stable stratification also traps nutrients like phosphorus and manganese in the deep, oxygen-starved water, preventing them from cycling back to the surface where they could fuel primary production.
When turnover occurs in the fall, the sudden mixing of the entire water column has a temporary impact. The nutrient-rich, oxygen-poor water from the bottom is brought to the surface, which can sometimes trigger large algal blooms. This event also briefly disrupts water chemistry and can release gases like hydrogen sulfide. However, the overall effect of turnover is beneficial as it recharges the lake with oxygen and resets the distribution of essential elements.