Lake turnover is a cyclical process that occurs in many deeper lakes, fundamentally altering the physical structure of the water column. This phenomenon involves the complete, top-to-bottom mixing of the entire volume of water in the basin. It effectively resets the lake’s internal environment, transforming it from a layered system into a homogeneous body of water. This deep circulation is driven by physical forces that temporarily overcome the resistance to mixing that exists for much of the year.
Why Lakes Form Layers
The stratification, or layering, of a lake is a direct consequence of a unique physical property of water: its anomalous density behavior. Unlike most liquids, which become progressively denser as they cool, fresh water reaches its maximum density at approximately 4 degrees Celsius (39.2 degrees Fahrenheit). Water that is warmer or colder than this temperature is less dense, a characteristic that dictates how lakes structure themselves seasonally.
During the summer, solar radiation heats the surface water, causing it to become significantly less dense than the cooler water below. This warm surface layer is known as the epilimnion, and it floats atop the colder, denser water mass beneath it. The epilimnion is constantly exposed to wind and is well-mixed, but its density difference creates a strong barrier against the deeper water.
The unique temperature of maximum density means that water near the lake bottom, the hypolimnion, will remain near 4°C throughout the summer. This water is the densest in the lake, which is why it settles at the bottom, shielded from the sun’s heat and surface mixing. The stability of this layering is entirely dependent on the temperature gradient.
The transition zone between the warm epilimnion and the cold hypolimnion is called the metalimnion, or thermocline. This middle layer is characterized by a rapid decrease in temperature with increasing depth, often defined as a change of one degree Celsius or more per meter. The density differential across the thermocline is substantial, requiring significant energy to overcome and preventing the wind from circulating water deeper than the epilimnion.
This thermal barrier isolates the hypolimnion from the surface, effectively separating the lake into two distinct chemical and biological environments for months. This stability persists until external forces, primarily seasonal temperature changes, weaken the density gradient enough to allow for complete mixing.
The Seasonal Cycle of Mixing
The most common mixing pattern in temperate regions is found in dimictic lakes, meaning they mix completely twice per year: once in the spring and again in the fall. These lakes experience periods of stratification during both summer and winter, making the turnover events predictable seasonal transitions. The mechanism that initiates turnover is the uniform cooling or warming of the surface water to the temperature of maximum density.
As autumn air temperatures decrease, the surface water of the lake begins to cool, causing its density to increase. This denser, cooled water sinks, displacing the slightly warmer, less dense water below it in a process called convective cooling. This continuous sinking and displacement gradually deepens the epilimnion, pushing the thermocline deeper and weakening the stratification barrier.
The fall turnover is ultimately triggered when the surface temperature cools to the point where it matches the temperature of the hypolimnion, around 4°C. At this point, the density difference is eliminated, and the lake becomes isothermal. With the density constraint removed, even moderate wind energy can circulate the water freely from top to bottom.
A similar process unfolds in the spring, following the inverse stratification that occurs under winter ice cover. During winter, the densest water (4°C) settles at the bottom, while colder, less dense water (0°C to 4°C) floats beneath the ice. The spring turnover begins once the ice melts and the surface water starts to warm from 0°C toward 4°C.
As the surface water warms, it increases in density and begins to sink, displacing the colder water below. This convective sinking continues until the entire water column reaches a uniform temperature of approximately 4°C, eliminating the density gradient. At this isothermal point, wind and convection currents again work together to mix the lake completely, marking the end of the winter stratification period.
The full circulation achieved during both the spring and fall events transforms the lake into a temporarily uniform environment. This brief period of instability is the main physical driver that allows for the necessary redistribution of materials that have accumulated in the separated layers over the previous season.
What Happens to the Ecosystem During Turnover
The physical act of turnover has profound and immediate consequences for the lake’s ecology by redistributing accumulated chemical resources. During stratification, the deep hypolimnion becomes isolated from the atmosphere and sunlight, leading to the consumption of dissolved oxygen by decomposing organic matter. This results in the formation of an oxygen-depleted, or anoxic, zone near the lakebed.
When the lake mixes completely, the oxygen-rich water from the surface is rapidly moved down to the deep hypolimnion, replenishing the oxygen supply for deep-water organisms. Simultaneously, the nutrient-rich water that accumulated at the bottom is brought up to the surface epilimnion. This bottom water typically contains high concentrations of nutrients like phosphorus and nitrogen, which are released from the sediments in low-oxygen conditions.
This sudden upwelling of nutrients to the sunlit surface layer can fuel rapid growth of phytoplankton and algae, often resulting in a noticeable surge in biological activity. This pulse of resources is particularly pronounced during the fall turnover and can lead to late-season algal blooms. The mixing also temporarily affects water clarity and odor.
The disturbance stirs up fine bottom sediments, causing the water to become turbid for a short period. The release of dissolved gases like hydrogen sulfide and methane from the anoxic deep water can also cause a temporary odor near the surface. While these changes are short-lived, the redistribution of oxygen and nutrients is an important mechanism that supports the overall health and diversity of the lake ecosystem.