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Concept
Dimictic lake
Summary
A dimictic lake is a body of freshwater whose difference in temperature between surface and bottom layers becomes negligible twice per year, allowing all strata of the lake's water to circulate vertically. All dimictic lakes are also considered holomictic, a category which includes all lakes which mix one or more times per year. During winter, dimictic lakes are covered by a layer of ice, creating a cold layer at the surface, a slightly warmer layer beneath the ice, and a still-warmer unfrozen bottom layer, while during summer, the same temperature-derived density differences separate the warm surface waters (the epilimnion), from the colder bottom waters (the hypolimnion). In the spring and fall, these temperature differences briefly disappear, and the body of water overturns and circulates from top to bottom. Such lakes are common in mid-latitude regions with temperate climates.
Lake Mendota
Lake Superior
Lake Simcoe
Lake Opeongo
Loch Lomond
Lake Altaussee
Mixing (overturning) typically occurs during the spring and autumn, when the lake is "isothermal" (i.e. at the same temperature from the top to the bottom). At this time, the water throughout the lake is near 4 °C (the temperature of maximum density), and, in the absence of any temperature or density differences, the lake readily mixes from top to bottom. During winter any additional cooling below 4 °C results in stratification of water column, so dimictic lakes usually have an inverse thermal stratification, with water at 0 °C below ice and then with temperatures increasing to near 4 °C at the lake's base.
Once the ice melts, the water column can be mixed by the wind. In large lakes the upper water column is often below 4 °C when the ice melts, so that spring is characterized by continued mixing by solar driven convection, until the water column reaches 4 °C. In small lakes, the period of spring overturn can be very brief, so that spring overturn is often much shorter than the fall overturn. As the upper water column warms past 4 °C a thermal stratification starts to develop.
Official source
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