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 Holomictic lakes are lakes that have a uniform temperature and density from surface to bottom at a specific time during the year, which allows the lake waters to mix in the absence of stratification. Details Holomictic lakes mix at least occasi ...

, 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 The epilimnion or surface layer is the top-most layer in a thermally stratified lake. It sits above the deeper metalimnion and hypolimnion. It is typically warmer and has a higher pH and higher dissolved oxygen concentration than the hypolimnion ...

), from the colder bottom waters (the

hypolimnion The hypolimnion or under lake is the dense, bottom layer of water in a thermally- stratified lake. The word hypolimnion is derived from the Greek "limnos" meaning "lake". It is the layer that lies below the thermocline. Typically the hypolimni ...

). 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.

Examples of dimictic lakes

Lake Mendota Lake Mendota is a freshwater eutrophic lake that is the northernmost and largest of the four lakes in Madison, Wisconsin. The lake borders Madison on the north, east, and south, Middleton on the west, Shorewood Hills on the southwest, Maple Bl ...

Lake Superior Lake Superior in central North America is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. and the third-largest by volume, holding 10% of the world's surface fresh wa ...

Lake Simcoe Lake Simcoe is a lake in southern Ontario, Canada, the fourth-largest lake wholly in the province, after Lake Nipigon, Lac Seul, and Lake Nipissing. At the time of the first European contact in the 17th century the lake was called ''Ouentironk' ...

* Lake Opeongo *

Loch Lomond Loch Lomond (; gd, Loch Laomainn - 'Lake of the Elms'Richens, R. J. (1984) ''Elm'', Cambridge University Press.) is a freshwater Scottish loch which crosses the Highland Boundary Fault, often considered the boundary between the lowlands of C ...

Seasonal cycles of mixing and stratification

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.

Spring overturn

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 Lake stratification is the tendency of lakes to form separate and distinct thermal layers during warm weather. Typically stratified lakes show three distinct layers, the Epilimnion comprising the top warm layer, the thermocline (or Metalimnion): ...

starts to develop.

Summer stratification

During summer, the heat fluxes from the atmosphere to a lake warms the surface layers. This results in dimictic lakes have a strong thermal stratification, with a warm

separated from the cold

by the metalimnion. Within the

metalimnion A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...

there is a

thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...

, usually defined as the region where temperature gradients exceed 1 °C/m. Due to the stable density gradient, mixing is inhibited within the thermocline, which reduces the vertical transport of

dissolved oxygen Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It ca ...

. If a lake is

eutrophic Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytoplan ...

and has a high sediment oxygen demand, the hypolimnion in dimictic lakes can become hypoxic during summer stratification, as often seen in

Lake Erie Lake Erie ( "eerie") is the fourth largest lake by surface area of the five Great Lakes in North America and the eleventh-largest globally. It is the southernmost, shallowest, and smallest by volume of the Great Lakes and therefore also has t ...

. During summer stratification, most lakes are observed to experience

internal wave Internal waves are gravity waves that oscillate within a fluid medium, rather than on its surface. To exist, the fluid must be stratified: the density must change (continuously or discontinuously) with depth/height due to changes, for example, in ...

s due to energy input from winds. If the lake is small (less than 5 km in length), then the period of the internal seiche is well predicted by the Merian formulae. Long period internal waves in larger lakes can be influenced by Coriolis forces (due to the Earth's rotation). This is expected to occur when the period of internal seiche becomes comparable to the local inertial period, which is 16.971 hours at a latitude of 45 °
(link to Coriolis utility).
In large lakes (such a

Lake Geneva , image = Lake Geneva by Sentinel-2.jpg , caption = Satellite image , image_bathymetry = , caption_bathymetry = , location = Switzerland, France , coords = , lake_type = Glacial lak ...

Lake Michigan Lake Michigan is one of the five Great Lakes of North America. It is the second-largest of the Great Lakes by volume () and the third-largest by surface area (), after Lake Superior and Lake Huron. To the east, its basin is conjoined with that o ...

Lake Ontario Lake Ontario is one of the five Great Lakes of North America. It is bounded on the north, west, and southwest by the Canadian province of Ontario, and on the south and east by the U.S. state of New York. The Canada–United States border sp ...

) the observed frequencies of internal seiches are dominated by Poincaré waves and

Kelvin wave A Kelvin wave is a wave in the ocean or atmosphere that balances the Earth's Coriolis force against a topographic boundary such as a coastline, or a waveguide such as the equator. A feature of a Kelvin wave is that it is non-dispersive, i.e., the ...

Fall overturn

In late summer, air temperatures drop and the surface of lakes cool, resulting in a deeper mixed layer, until at some point the water column becomes isothermal, and generally high in dissolved oxygen. During fall a combination of wind and cooling air temperatures continue to keep the water column mixed. The water continues to cool until the temperature reaches 4 °C. Often fall overturn can last for 3–4 months.

Winter inverse stratification

After the water column reaches the temperature of maximum density at 4°C, any subsequent cooling produces less dense water due to non-linearity of equation of state of water. Early winter is thus a period of restratification. If there is relatively little wind, or the lake is deep, only a thin layer of buoyant cold water forms above denser 4°C waters and the lake will be "cryostratified" once ice forms. If the lake experiences strong winds or is shallow, then the whole water column can cool to near 0°C before ice forms, these colder lakes are termed "cryomictic". Once ice forms on a lake, the heat fluxes from the atmosphere are largely shut down and the initial cyrostratified or cryomictic conditions are largely locked in. The development of thermal stratification during winter is then defined by two periods: Winter I and Winter II. During the early winter period of Winter I the major heat flux is due to heat stored in sediment; during this period the lake heats up from beneath forming a deep layer of 4 °C water. During late winter, the surface ice starts to melt and with the increased length of the day, there is increased sunlight that penetrates through the ice into the upper water column. Thus during Winter II, the major heat flux is now from above, and the warming causes an unstable layer to form, resulting in solar driven convection. This mixing of the upper water column is important for keeping plankton in suspension, which in turn influences the timing of under-ice algal blooms and levels of dissolved oxygen. Coriolis forces can also become important in driving circulation patterns due to differential heating by solar radiation. The winter period of lakes is probably the least studied, but the chemistry and biology are still very active under the ice.

References

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External links

"Circulation: annual patterns of dimictic lakes" at Encyclopædia Britannica Online
Lakes by type