Why is overturn and stagnation of lakes important

We are now near to the summer season on our lakes again. This marks the fourth installment of my lake stratification layers and mixing articles. Once the ice melts off the lake, the lake goes through another period of mixing like it did back in the fall. First, I'll recap the underlying cause of the lake mixing phenomenon. The layering of lakes has to do with the relationship between water density and temperature. Water is most dense at 39 Fahrenheit 4 degrees Celsius , and as water warms or cools from that mark it gets less dense.

This has implications for a lake's structure because the denser water is heavier and will be at the bottom of a lake while the less dense water is lighter and will generally be at the top of the lake. In the winter, most of the water under the ice is 39 Fahrenheit; however, there is a thin layer of water under the ice that is colder than 39 and therefore less dense.

Decomposing organic materials are churned up from the bottom of the lake, bringing a signature sign of lake turnover. The distinct layers established over the summer disappear as the top layer of the lake cools, and temperatures equalize. The lake is once again able to refresh nutrients and oxygen.

Later, when temperatures stabilize, it becomes stratified through the winter, until spring. Oxygen is most commonly depleted at the bottom of the lake by oxygen-hungry bacteria.

These bacteria consume dead algae that sinks to lake floor. If oxygen is not replenished, the amount of viable habitat for fish would drastically decline. Hypoxic zones or dead zones are areas of low oxygen. These zones are only suitable for certain bacteria. The biannual cycle of lake turnover is essential in mitigating the negative impacts of low oxygen dead zones and providing suitable habitat for fish and aquatic organisms to thrive. Read more about our five Yahara lakes.

Did I say simple? But I'll try to describe the process in as simple and non-technical a way as possible. Water is fascinating, for a number of reasons. One of the more interesting reasons is due to a water molecule's amazing structure - its chemistry if you will. In nature, heat and cold change the physical properties of all substances in predictable ways. This principal holds for solids, liquids and gases.

Able to exist in all three forms, water, as a liquid, contracts and becomes more dense as it gets colder And that point is where the oddity owing to water's special chemistry comes into play, the one that keeps your favorite lake -- and your favorite gamefish -- healthy and happy. Something strange and wonderful happens when water reaches a temperature of approximately 39 degrees Fahrenheit 40 Celsius.

Well, it is wonderful for lakes and the life in them, not so for the roads on which we drive -- more on that later. In any case, as water cools to that temperature, it does, as predicted, contract and become more dense, ultimately sinking to the bottom of the lake and pushing the water it has displaced to the surface, where it too can cool.

This is a consequence of heat exchange with the atmosphere and the seasonal variation of meteorological parameters, such as incoming solar radiation. The temperatures in the deep water follow the surface temperatures only for the time when the lake is homo-thermal, as in our example Lake Goitsche, Germany Figure 1 , during winter, from November until April. Throughout summer, temperatures vary from the surface to the lake bed, and the lake remains stratified. Warmer and less dense water floats on top of colder, denser water Figure 1.

Thus, Lake Goitsche is called stably stratified, as overturning water parcels would require energy. On the contrary, during winter, no density differences obstruct the vertical transport. These seasons are commonly referred to as the stagnation period and the circulation period.

Lakes that experience a complete overturn during the year are called holomictic. During the circulation period, dissolved substances, such as oxygen or nutrients, get distributed over the entire water body Figure 2. Hence, the circulation pattern is a decisive factor for the evolution of water quality and the biocenosis of the lake.

In conclusion, the commonly used classification of lakes is according to their circulation patterns. As a consequence, a chemically different layer of bottom water is formed, the monimolimnion see below , and remains there for at least 1 year.

Usually permanently ice-covered lakes are included in this class. Lakes, however, can circulate underneath an ice sheet by external forcing, such as solar radiation that penetrates to the lake bed and geothermal heat flux, or salinity gradients created when ice is forming on a salt lake. The deep water however is partially replaced in episodic events. The entire lake is mixed by sporadic strong wind events over the year or even on a daily basis in response to strong diurnal temperature variation.

A closer look at the lakes, however, reveals that in most cases an ice cover or a great maximum depth is required to guarantee a stratification period during the cold season see Figure 2. Many lakes in the temperate climate zone belong into this class, if they do not develop an ice cover during winter. Sometimes such lakes are also referred to as warm monomictic to distinguish them from cold monomictic lakes, which show an ice cover for most of the year and circulate during the short period without ice.

As a consequence of the natural variability of the weather conditions between years, the circulation patterns of the lakes also vary.

A usually monomic-tic lake, for example, can show a dimictic circulation pattern when it freezes in an unusually cold winter. As another example, late during the twentieth century, Mono Lake turned meromictic for intermittent periods of 5 or 7 years, respectively, because of inflowing freshwater, but in other years showed a holomictic circulation.

Although the surface water is exposed to solar radiation and thermal contact with the atmosphere, the. Figure 1 Temperatures 24 h mean on several depths in Lake Goitsche near Bitterfeld, Germany during the year Reviews in Geophysics, 46, RG, doi Diffusive heat transport on a molecular level is very slow and requires a month for the transport of heat over a vertical distance of 1 m.

A much more efficient heat transport can be accomplished by turbulent transport. The energy for the turbulence is mainly supplied by wind stress at the lake surface and transferred via instabilities through friction at the side walls and internal current shear.

As a consequence, transport of heat to greater depths requires energy. The limited budget of kinetic energy available for mixing limits the depth to which a certain amount of heat can be forwarded over the stratification period. In sufficiently deep lakes , the thermal stratification holds until cooler autumn and winter temperatures permit a deeper circulation.

The warm surface water layer is called epilimnion, while the colder water layer beneath, which has not been mixed into the epilim-nion is called hypolimnion. A sharp temperature gradient thermocline separates both layers Figure 3.

Epilimnion and atmosphere are in thermal contact and exchange volatile substances with each other. In addition, the epilimnion is recirculated by wind events or periods of lower temperatures during the stratification period. During those periods, dissolved substances are distributed within the epilimnion. On the contrary, the hypolimnion is insulated from exchange with the atmosphere during the stratification period.