Lake turnover is a natural phenomenon where a lake’s entire volume of water mixes completely from its surface down to the bottom sediment. This process involves the upper and lower layers of water exchanging places, resetting the lake’s internal environment. Seasonal changes in air temperature drive this mixing by altering the density of the surface water. This annual or biannual event significantly affects the lake’s physical and chemical characteristics.
Why Lake Water Separates into Layers
The separation of lake water into distinct layers is called thermal stratification, occurring because water density changes with temperature. Water is densest at approximately 4° Celsius, meaning it takes up the least space at this temperature. Above or below 4°C, water becomes less dense, a property that allows lakes to stratify and freeze at the surface. During the warm summer months, the lake develops three primary zones based on temperature and density differences.
The top layer, the epilimnion, is warm, well-lit, and easily mixed by wind action. Beneath this lies the metalimnion, which contains the thermocline, marking a rapid temperature drop over a small change in depth. The bottom layer, the hypolimnion, remains uniformly cold, often near 4°C, and is isolated from the surface. The density difference between the warm epilimnion and the cold hypolimnion creates a strong barrier that resists mixing throughout the summer, preventing the layers from exchanging heat, oxygen, or nutrients.
The Physical Process of Seasonal Mixing
Turnover is triggered when the density barrier breaks down, which often happens twice a year in temperate lakes. In the spring, after the ice melts, the cold surface water warms. As the surface water approaches 4°C, its density increases until it matches the density of the deeper water. Wind then easily pushes the surface water, causing a circulation pattern that completely mixes the water column.
Fall turnover begins when the air temperature cools in late autumn, chilling the surface layer. This cooling increases the surface water’s density, making it heavier than the water below. As this dense surface water sinks, it displaces the warmer, less dense water in the hypolimnion, forcing it upward in a complete exchange of layers. This continuous sinking and rising action, assisted by wind, ensures the lake achieves a uniform temperature of approximately 4°C throughout its depth.
The wind plays a significant role in driving the circulation once the water column reaches a uniform density. Without the resistance of stratified layers, even moderate winds can generate currents strong enough to ensure that the deep, isolated hypolimnion is completely exchanged with the surface waters. This full circulation continues until the surface layer either warms significantly in the spring or freezes in the fall, initiating the next period of temperature-driven stability.
What Happens to the Lake Ecosystem After Turnover
The complete mixing of the water column affects the lake’s ecosystem. During the summer, the isolated hypolimnion often becomes depleted of dissolved oxygen as bacteria consume it while decomposing organic matter that sinks from the surface. Turnover replenishes this oxygen-starved bottom water by bringing oxygenated surface water deep into the lake, restoring conditions for aquatic life.
The mixing event also redistributes nutrients accumulated at the lake bottom throughout the stratification period. Sediments often store high concentrations of phosphorus and nitrogen, which are brought up to the surface layer during turnover. This influx of nutrients into the sunlit epilimnion can sometimes trigger an increase in phytoplankton growth, leading to temporary surface algae blooms. The intensity of this nutrient release influences the lake’s productivity.
Fish and other mobile organisms respond quickly to the newly uniform conditions of temperature and oxygen. Before turnover, fish may be restricted to a narrow band of water with suitable temperatures and oxygen levels, avoiding the cold, anoxic bottom layer. Afterward, they can move freely across the entire depth of the lake, expanding their habitat and feeding opportunities.
The disturbance of the bottom sediments during turnover can lead to temporary aesthetic changes. Fine particulate matter stirred up from the lake floor can temporarily decrease the water’s clarity. Additionally, if the bottom sediments were highly anoxic, the mixing may release dissolved gases like hydrogen sulfide, which can produce a noticeable rotten-egg odor near the shore until the gases dissipate.