When a lake “turns over,” it undergoes a natural, seasonal mixing process where surface water exchanges places with bottom water. This phenomenon is fundamental for maintaining the health and balance of a lake’s ecosystem, redistributing resources to support diverse aquatic life.
Water’s Behavior and Lake Layers
Water exhibits unique properties that influence how lakes layer and mix. Unlike most substances, water reaches its maximum density at approximately 4 degrees Celsius (39.2 degrees Fahrenheit). Water becomes less dense as it warms above 4°C or cools below it, which is why ice floats. This density characteristic leads to thermal stratification, where lakes form distinct temperature-based layers seasonally.
During warmer months, lakes stratify into three main layers. The uppermost layer, the epilimnion, consists of warmer, less dense water heated by the sun and well-mixed by wind. Below this is the metalimnion, also known as the thermocline, a transitional middle layer where temperature rapidly changes with increasing depth. The deepest and coldest layer is the hypolimnion, which contains denser water and receives little to no sunlight or wind mixing.
The Seasonal Turnover Process
Lake turnover is driven by seasonal changes in air temperature and wind, diminishing density differences between water layers. In autumn, as surface temperatures cool, the epilimnion becomes denser and sinks. This cooling and sinking, combined with wind, causes the entire water column to reach a uniform temperature and density, allowing for complete mixing known as fall turnover. This process can last for several months as water cools towards 4°C.
In spring, as ice melts and surface water warms from near 0°C, it approaches 4°C, becoming denser. This warming, along with wind, triggers spring turnover, mixing the lake from top to bottom. Lakes that undergo two such mixing periods annually, in spring and fall, are referred to as dimictic lakes, common in temperate regions with four distinct seasons.
Ecological Effects of Lake Turnover
Lake turnover impacts the lake ecosystem by redistributing vital components. During stratification, the deeper hypolimnion becomes depleted of oxygen due to decomposition of organic matter, while the surface epilimnion remains oxygen-rich. Turnover replenishes oxygen in these deeper waters, creating suitable habitats for aquatic organisms like fish that seek colder temperatures. Without this replenishment, oxygen-depleted zones can form, limiting viable habitat for fish.
The mixing brings nutrient-rich water from the lake bottom, accumulated from decaying organic material, to the surface. This influx of nutrients, such as phosphorus and nitrogen, supports the growth of algae and phytoplankton, forming the base of the aquatic food web. This can lead to temporary changes in water clarity, as stirred-up sediments and organic matter can make the water appear murky. A telltale sign of turnover can be a temporary, unpleasant odor, resulting from gases like hydrogen sulfide released from bottom sediments. While turnover is beneficial for lake health, rapid or extreme events can cause temporary stress or even fish kills due to sudden changes in oxygen levels.