Lake stratification

Short description: Separation of water in a lake into distinct layers

Lakes are stratified into three separate sections:
I. The Epilimnion
II. The Metalimnion
III. The Hypolimnion
The scales are used to associate each section of the stratification to their corresponding depths and temperatures. The arrow is used to show the movement of wind over the surface of the water which initiates the turnover in the epilimnion and the hypolimnion.

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), the middle layer, whose depth may change throughout the day; and the colder hypolimnion, extending to the floor of the lake.

Every lake has a set mixing regime that is influenced by lake morphometry and environmental conditions. However, changes to human influences in the form of land use change, increases in temperature, and changes to weather patterns have been shown to alter the timing and intensity of stratification in lakes around the globe.[1][2] Rising air temperatures have the same effect on lake bodies as a physical shift in geographic location, with tropical zones being particularly sensitive.[2][1] These changes can further alter the fish, zooplankton, and phytoplankton community composition, in addition to creating gradients that alter the availability of dissolved oxygen and nutrients.[3][4]

Typical mixing pattern for many lakes, caused by the fact that water is less dense at temperatures other 4°C (the temperature where water is most dense). Lake stratification is stable in summer and winter, becoming unstable in spring and fall when the surface waters cross the 4°C mark.

## Contents

* 1 Definition
* 2 De-stratification
* 3 Anthropogenic influences
* 4 See also
* 5 References

## Definition

The thermal stratification of lakes refers to a change in the temperature at different depths in the lake, and is due to the density of water varying with temperature.[5] Cold water is denser than warm water and the epilimnion generally consists of water that is not as dense as the water in the hypolimnion.[6] However, the temperature of maximum density for freshwater is 4 °C. In temperate regions where lake water warms up and cools through the seasons, a cyclical pattern of overturn occurs that is repeated from year to year as the cold dense water at the top of the lake sinks (see stable and unstable stratification). For example, in dimictic lakes the lake water turns over during the spring and the fall. This process occurs more slowly in deeper water and as a result, a thermal bar may form.[5] If the stratification of water lasts for extended periods, the lake is meromictic.

In shallow lakes, stratification into epilimnion, metalimnion, and hypolimnion often does not occur, as wind or cooling causes regular mixing throughout the year. These lakes are called polymictic. There is not a fixed depth that separates polymictic and stratifying lakes, as apart from depth, this is also influenced by turbidity, lake surface area, and climate.[7]

The lake mixing regime (e.g. polymictic, dimictic, meromictic)[8] describes the yearly patterns of lake stratification that occur in most years. However, short-term events can influence lake stratification as well. Heat waves can cause periods of stratification in otherwise mixed, shallow lakes,[9] while mixing events, such as storms or large river discharge, can break down stratification.[10] Recent research suggests that seasonally ice-covered dimictic lakes may be described as "cryostratified" or "cryomictic" according to their wintertime stratification regimes.[11] Cryostratified lakes exhibit inverse stratification near the ice surface and have depth-averaged temperatures near 4°C, while cryomictic lakes have no under-ice thermocline and have depth-averaged winter temperatures closer to 0°C.[12]

The accumulation of dissolved carbon dioxide in three meromictic lakes in Africa (Lake Nyos and Lake Monoun in Cameroon and Lake Kivu in Rwanda) is potentially dangerous because if one of these lakes is triggered into limnic eruption, a very large quantity of carbon dioxide can quickly leave the lake and displace the oxygen needed for life by people and animals in the surrounding area.

## De-stratification

In temperate latitudes, many lakes that become stratified during the summer months de-stratify during cooler windier weather with surface mixing by wind being a significant driver in this process. This is often referred to as "autumn turn-over". The mixing of the hypolimnium into the mixed water body of the lake recirculates nutrients, particularly phosphorus compounds, trapped in the hypolimnion during the warm weather. It also poses a risk of oxygen sag as a long established hypolimnion can be anoxic or very low in oxygen.

Lake mixing regimes can shift in response to increasing air temperatures. Some dimictic lakes can turn into monomictic lakes, while some monomictic lakes might become meromictic, as a consequence of rising temperatures.[13]

Many types of aeration equipment have been used to thermally de-stratify lakes, particularly lakes subject to low oxygen or undesirable algal blooms.[14] In fact, natural resource and environmental managers are often challenged by problems caused by lake and pond thermal stratification.[6][15][16] Fish die-offs have been directly associated with thermal gradients, stagnation, and ice cover.[17] Excessive growth of plankton may limit the recreational use of lakes and the commercial use of lake water. With severe thermal stratification in a lake, the quality of drinking water also can be adversely affected.[6] For fisheries managers, the spatial distribution of fish within a lake is often adversely affected by thermal stratification and in some cases may indirectly cause large die-offs of recreationally important fish.[17] One commonly used tool to reduce the severity of these lake management problems is to eliminate or lessen thermal stratification through water aeration.[15] Aeration has met with some success, although it has rarely proved to be a panacea.[16]

## Anthropogenic influences

Every lake has a set mixing regime that is influenced by lake morphometry and environmental conditions. However, changes to human influences in the form of land use change, increases in temperatures, and changes to weather patterns have been shown to alter the timing and intensity of stratification in lakes around the globe.[1][2] These changes can further alter the fish, zooplankton, and phytoplankton community composition, in addition to creating gradients that alter the availability of dissolved oxygen and nutrients.[3][4]

There are a number of ways in which changes in human land use influence lake stratification and consequently water conditions. Urban expansion has led to the construction of roads and houses close to previously isolated lakes, sometimes causing increased runoff and pollution. The addition of particulate matter to lake bodies can lower water clarity, resulting in stronger thermal stratification and overall lower average water column temperatures, which can eventually affect the onset of ice cover.[18] Water quality can also be influenced by the runoff of salt from roads and sidewalks, which often creates a benthic saline layer that interferes with vertical mixing of surface waters.[4] Further, the saline layer can prevent dissolved oxygen from reaching the bottom sediments, decreasing phosphorus recycling and affecting microbial communities.[4]

On a global scale, rising temperatures and changing weather patterns can also affect stratification in lakes. Rising air temperatures have the same effect on lake bodies as a physical shift in geographic location, with tropical zones being particularly sensitive.[2][1] The intensity and scope of impact depends on location and lake morphometry, but in some cases can be so extreme as to require a reclassification from monomictic to dimictic (e.g. Great Bear Lake).[2] Globally, lake stratification appears to be more stable with deeper and steeper thermoclines, and average lake temperature as a main determinant in the stratification response to changing temperatures.[1] Further, surface warming rates are much greater than bottom warming rates, again indicating stronger thermal stratification across lakes.[1]

Changes to stratification patterns can also alter the community composition of lake ecosystems. In shallow lakes, temperature increases can alter the diatom community; while in deep lakes, the change is reflected in the deep chlorophyll layer taxa.[3] Changes in mixing patterns and increased nutrient availability can also affect zooplankton species composition and abundance, while decreased nutrient availability can be detrimental for benthic communities and fish habitat.[3][4]

## See also

* Aquatic Science
* Stratification (water)
* Hypoxia
* Freshwater ecosystems
* Water column
* Lake aeration

## References

1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Kraemer, Benjamin M.; Anneville, Orlane; Chandra, Sudeep; Dix, Margaret; Kuusisto, Esko; Livingstone, David M.; Rimmer, Alon; Schladow, S. Geoffrey et al. (2015-06-28). "Morphometry and average temperature affect lake stratification responses to climate change: LAKE STRATIFICATION RESPONSES TO CLIMATE" (in en). Geophysical Research Letters 42 (12): 4981–4988. doi:10.1002/2015GL064097. https://corescholar.libraries.wright.edu/biology/571.
2. ↑ 2.0 2.1 2.2 2.3 2.4 Meyer, Gabriela K.; Masliev, Ilya; Somlyódy, László (1996), "Impact of Climate Change on Sensitivity of Lake Stratification: A Global Perspective", Water Resources Management in the Face of Climatic/Hydrologic Uncertainties (Springer Netherlands): pp. 225–270, doi:10.1007/978-94-009-0207-7_9, ISBN 978-94-010-6577-1
3. ↑ 3.0 3.1 3.2 3.3 Edlund, Mark; Almendinger, James; Fang, Xing; Hobbs, Joy; VanderMeulen, David; Key, Rebecca; Engstrom, Daniel (2017-09-07). "Effects of Climate Change on Lake Thermal Structure and Biotic Response in Northern Wilderness Lakes" (in en). Water 9 (9): 678. doi:10.3390/w9090678. ISSN 2073-4441.
4. ↑ 4.0 4.1 4.2 4.3 4.4 Novotny Eric V.; Stefan Heinz G. (2012-12-01). "Road Salt Impact on Lake Stratification and Water Quality". Journal of Hydraulic Engineering 138 (12): 1069–1080. doi:10.1061/(ASCE)HY.1943-7900.0000590.
5. ↑ 5.0 5.1 "Density Stratification". Water on the Web. October 7, 2015. http://www.waterontheweb.org/under/lakeecology/05_stratification.html.
6. ↑ 6.0 6.1 6.2 "Lake Lanier Turnover Facts". Georgia Department of Natural Resources. https://georgiawildlife.com/lake-lanier-turnover-facts.
7. ↑ Kirillin, G.; Shatwell, T. (October 2016). "Generalized scaling of seasonal thermal stratification in lakes". Earth-Science Reviews 161: 179–190. doi:10.1016/j.earscirev.2016.08.008. Bibcode: 2016ESRv..161..179K.
8. ↑ Lewis Jr., William M. (October 1983). "A Revised Classification of Lakes Based on Mixing". Canadian Journal of Fisheries and Aquatic Sciences 40 (10): 1779–1787. doi:10.1139/f83-207.
9. ↑ Wilhelm, Susann; Adrian, RITA (4 October 2007). "Impact of summer warming on the thermal characteristics of a polymictic lake and consequences for oxygen, nutrients and phytoplankton". Freshwater Biology 53 (2): 226–37. doi:10.1111/j.1365-2427.2007.01887.x.
10. ↑ Yang, Bernard; Wells, Mathew G.; McMeans, Bailey C.; Dugan, Hilary A.; Rusak, James A.; Weyhenmeyer, Gesa A.; Brentrup, Jennifer A.; Hrycik, Allison R. et al. (2021-02-16). "A New Thermal Categorization of Ice‐Covered Lakes" (in en). Geophysical Research Letters 48 (3): e91374. doi:10.1029/2020GL091374. ISSN 0094-8276. Bibcode: 2021GeoRL..4891374Y. https://onlinelibrary.wiley.com/doi/10.1029/2020GL091374.
11. ↑ Yang, Bernard; Wells, Mathew G.; McMeans, Bailey C.; Dugan, Hilary A.; Rusak, James A.; Weyhenmeyer, Gesa A.; Brentrup, Jennifer A.; Hrycik, Allison R. et al. (2021). "A New Thermal Categorization of Ice-Covered Lakes" (in en). Geophysical Research Letters 48 (3): e2020GL091374. doi:10.1029/2020GL091374. ISSN 1944-8007. Bibcode: 2021GeoRL..4891374Y. https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020GL091374.
12. ↑ Yang, Bernard; Wells, Mathew G.; McMeans, Bailey C.; Dugan, Hilary A.; Rusak, James A.; Weyhenmeyer, Gesa A.; Brentrup, Jennifer A.; Hrycik, Allison R. et al. (2021). "A New Thermal Categorization of Ice-Covered Lakes" (in en). Geophysical Research Letters 48 (3): e2020GL091374. doi:10.1029/2020GL091374. ISSN 1944-8007. Bibcode: 2021GeoRL..4891374Y. https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020GL091374.
13. ↑ Woolway, R. Iestyn; Merchant, Christopher J. (18 March 2019). "Worldwide alteration of lake mixing regimes in response to climate change". Nature Geoscience 12 (4): 271–276. doi:10.1038/s41561-019-0322-x. Bibcode: 2019NatGe..12..271W. http://centaur.reading.ac.uk/82814/1/Woolway_Merchant_accepted.pdf.
14. ↑ Cooke, G. Dennis; Welch, Eugene B.; Peterson, Spencer et al., eds (2005). Restoration and Management of Lakes and Reservoirs (Third ed.). Boca Raton: CRC Press. p. 616. ISBN 9781566706254.
15. ↑ 15.0 15.1 Lackey, Robert T. (February 1972). "A technique for eliminating thermal stratification in lakes". Journal of the American Water Resources Association 8 (1): 46–49. doi:10.1111/j.1752-1688.1972.tb05092.x. Bibcode: 1972JAWRA...8...46L.
16. ↑ 16.0 16.1 Lackey, Robert T. (June 1972). "Response of physical and chemical parameters to eliminating thermal stratification in a reservoir". Journal of the American Water Resources Association 8 (3): 589–599. doi:10.1111/j.1752-1688.1972.tb05181.x. Bibcode: 1972JAWRA...8..589L.
17. ↑ 17.0 17.1 Lackey, Robert T.; Holmes, Donald W. (July 1972). "Evaluation of Two Methods of Aeration to Prevent Winterkill". The Progressive Fish-Culturist 34 (3): 175–178. doi:10.1577/1548-8640(1972)34[175:EOTMOA2.0.CO;2].
18. ↑ Heiskanen, Jouni J.; Mammarella, Ivan; Ojala, Anne; Stepanenko, Victor; Erkkilä, Kukka-Maaria; Miettinen, Heli; Sandström, Heidi; Eugster, Werner et al. (2015). "Effects of water clarity on lake stratification and lake-atmosphere heat exchange" (in en). Journal of Geophysical Research: Atmospheres 120 (15): 7412–7428. doi:10.1002/2014JD022938. ISSN 2169-8996. Bibcode: 2015JGRD..120.7412H.

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