Lake turnover is a natural, seasonal phenomenon where the entire body of water in a lake mixes from top to bottom. This process ensures the redistribution of essential elements throughout the aquatic environment. It helps maintain the health and balance of a lake’s ecosystem, sustaining diverse aquatic life.
Understanding Lake Layers
Lakes often develop distinct layers based on temperature, a phenomenon known as thermal stratification. Water density changes with temperature, becoming denser as it cools until it reaches its maximum density at approximately 4 degrees Celsius (39.2 degrees Fahrenheit). Beyond this point, water becomes less dense as it approaches freezing. This density variation leads to the formation of three primary layers during stratification.
The uppermost layer, warmed by the sun and rich in oxygen, is called the epilimnion. Below this is the metalimnion, also known as the thermocline, characterized by a rapid decrease in temperature with increasing depth. This sharp temperature gradient acts as a barrier, limiting the exchange of water and dissolved substances. The deepest and coldest layer, the hypolimnion, remains near 4 degrees Celsius and can become depleted of oxygen over time due to decomposition. These distinct layers prevent the full mixing of the lake’s waters for extended periods.
The Annual Mixing Cycle
Lake turnover occurs when temperature differences between layers diminish, allowing the entire water column to mix. This process typically happens twice a year in many temperate lakes, in spring and fall. These lakes are referred to as dimictic.
Spring turnover begins as ice melts and surface water warms, gradually reaching 4 degrees Celsius. As the entire water column approaches this uniform temperature, its density becomes nearly equal. Wind action then easily circulates the water, allowing for thorough mixing. This spring event reoxygenates deeper waters and redistributes nutrients settled during winter.
Fall turnover is initiated as surface water cools in autumn due to decreasing air temperatures. This cooler, denser surface water sinks, displacing warmer, less dense water below. This continuous sinking, combined with strong autumn winds, drives the mixing process. The lake continues to mix until its temperature is nearly uniform throughout, before winter stratification or freezing. This autumnal mixing replenishes oxygen in the hypolimnion and brings accumulated nutrients from the bottom to the surface.
Impacts on Lake Life
Lake turnover significantly impacts aquatic life by redistributing oxygen and nutrients throughout the water column. During stratified periods, the hypolimnion can become oxygen-depleted, creating challenging conditions for bottom-dwelling organisms and fish. Turnover events alleviate this by circulating oxygen-rich surface waters to the depths, making the entire lake habitable. This oxygenation supports the respiration of diverse aquatic species.
The mixing also brings accumulated nutrients, such as phosphorus and nitrogen, from the hypolimnion to the surface epilimnion. These nutrients, released from decaying organic matter on the lakebed, become accessible to primary producers like phytoplankton and algae. While this nutrient influx can sometimes lead to temporary increases in algal growth, it generally supports the base of the lake’s food web. These periodic mixing events are essential for maintaining a healthy and balanced lake ecosystem.