Lake Turnover
Lake Turnover is the process of the dense lower layer of a lake rising to become the upper, less-dense layer1
A lake’s turnover can be affected by many factors
Wind
solar radiation
presence of ice
air temperature
lake depth, size and topography
air temperature
lake tributaries
During the Summer, thermal stratification occurs within the lake due to layers separating by temperature and density
The upper layer, the epilimnion, is exposed to solar radiation and thermal contact with the atmosphere, keeping it warmer
During the Winter, inverse stratification occurs within the lake due to a layer of ice forming on top which prevents mixing
The lower layer, the hypolimnion, is usually too deep to be affected by wind, solar radiation and atmospheric heat exchanges, which causes this layer to have a stable temperature and keeps it roughly around water’s maximum density
During the transition from Summer to Fall and Winter to Spring, a turnover occurs within the lake
Thermal stratification is broken down by seasonal temperature changes, and the lake begins to take a roughly uniform temperature, and thus, uniform density profile
At this point, wind energy will thoroughly mix the water layers.3
Due to regional climatic differences affecting separate lake systems, their turnovers can have contrasting features
Lakes can be categorized based on their stratification and circulation patterns
In terms of dissolved oxygen, lake turnover is extremely important for aquatic organisms. Since thermal stratification causes density differences that do not allow water layers to mix, the hypolimnion becomes completely separated from any gas exchange with the atmosphere and the overlying layers. This separation causes the hypolimnion to contain a “finite” level of dissolved oxygen that may be quickly used up by benthic organisms
When lake turnover occurs, however, the layers begin to mix and exchange dissolved oxygen throughout the water column. The oxygen requirements for some freshwater organisms becomes restored until thermal stratification separates the layers in the next season
Minimum dissolved oxygen requirements of freshwater fish
Dissolved oxygen levels often stratify in the winter and summer, turning over in the spring and fall as lake temperatures align.
A technological innovation that has been used to avoid the oxygen depletion of the hypolimnion is called Hypolimnetic Aeration. A deep water aeration system that stretches from above the water column to the hypolimnion creates a medium that allows atmospheric oxygen and hypolimnetic water to mix. The oxygen and water mixture is injected into a degassing chamber that removes residual gases to the atmosphere and then releases the freshly oxygenated water back into the hypolimnion3
An advantage to this system is that it does not affect the natural stratification of the lake. Thus, organisms that prefer a certain temperature range will not have their habitat disturbed3
Scientists can use thermistor temperature strings to create a temperature profile of a lake. This profile will allow scientists to monitor turnover as it occurs
A PTC thermistor has a non-linear, analogous relationship between temperature and resistance5
An NTC thermistor has a non-linear, inverse relationship between temperature and resistance. Most thermistors are of this type5
Thermistors are metallic oxides encased in a water-tight material. Their electrical resistance is dependent upon the surrounding temperature. Depending on the thermistor material, the thermistor can be considered one that is a Negative Temperature Coefficient (NTC) or a Positive Temperature Coefficient (PTC)5
Additional References
- National Geographic Society. (2012, November 09). Lake Turnover. Retrieved February 18, 2019, from https://www.nationalgeographic.org/media/lake-turnover/
- Agnes. (2014, September 10). When Water Rolls Over. Retrieved February 18, 2019, from https://www.slideserve.com/agnes/when-water-rolls-overhttps://www.iisd.org/ela/blog/commentary/lakes-stratify-turn-explain-science-behind-phenomena/
- Approaches To Aeration. (n.d.). Retrieved February 18, 2019, from https://www.aquaticbiologists.com/new-page/https://canadianpond.ca/product/tibean-hypolimnetic-deep-water-aerators/
- Research Gate. (n.d.). (Retrieved February 18,2019, from https://www.researchgate.net/figure/The-process-of-thermal-stratification-and-turnover-in-a-typical-Adirondack-Lake_fig3_324895847
- THERMISTOR BASICS. (2018, January 05). Retrieved February 22, 2019, from https://www.teamwavelength.com/thermistor-basics/All references can be viewed in the reference section of the Fondriest Environmental Website “Learning Center” under “References”https://www.fondriest.com/environmental-measurements/references/
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