Hypolimnion

The deepest portions of the hypolimnion often have lower oxygen concentrations than the surface waters (i.e., epilimnion).{{Cite journal|last1=Sadchikov|first1=A. P.|last2=Ostroumov|first2=S. A.|date=October 2019|title=Epilimnion, Metalimnion, and Hypolimnion of a Mesotrophic Aquatic Ecosystem: Functional Role of the Vertical Structure of the Reservoir Ecosystem in Terms of Hydrochemical and Biological Parameters|journal=Russian Journal of General Chemistry|language=en|volume=89|issue=13|pages=2860–2864|doi=10.1134/S107036321913019X|s2cid=211138964 |issn=1070-3632}} While oxygen can typically exchange between surface waters and the atmosphere (i.e., in the absence of ice cover), bottom waters are comparatively isolated from atmospheric replenishment of oxygen. In particular, during periods of thermal stratification, gas exchange between the epilimnion and hypolimnion is limited by the density difference between these two layers. Consequently, decomposition of organic matter in the water column and sediments can cause oxygen concentrations to decline to the point of hypoxia (low oxygen) or anoxia (no oxygen).{{Cite journal|last1=Weinke|first1=Anthony D.|last2=Biddanda|first2=Bopaiah A.|date=2019-12-01|title=Influence of episodic wind events on thermal stratification and bottom water hypoxia in a Great Lakes estuary|journal=Journal of Great Lakes Research|language=en|volume=45|issue=6|pages=1103–1112|doi=10.1016/j.jglr.2019.09.025|s2cid=209571196 |issn=0380-1330|doi-access=free|bibcode=2019JGLR...45.1103W }} In dimictic, eutrophic lakes, the hypolimnion is often anoxic throughout a majority of the stratified period.{{Cite journal|last1=Su|first1=Xiaoxuan|last2=He|first2=Qiang|last3=Mao|first3=Yufeng|last4=Chen|first4=Yi|last5=Hu|first5=Zhi|date=2019-01-01|title=Dissolved oxygen stratification changes nitrogen speciation and transformation in a stratified lake|journal=Environmental Science and Pollution Research|language=en|volume=26|issue=3|pages=2898–2907|doi=10.1007/s11356-018-3716-1|pmid=30499088|s2cid=54168543 |issn=1614-7499|doi-access=free|bibcode=2019ESPR...26.2898S }} However, hypolimnetic oxygen concentrations are replenished in the fall and early winter in many temperate lakes, as lake turnover allows mixing of oxic surface waters and anoxic bottom waters.{{Cite journal|last1=Sánchez-España|first1=Javier|last2=Mata|first2=M. Pilar|last3=Vegas|first3=Juana|last4=Morellón|first4=Mario|last5=Rodríguez|first5=Juan Antonio|last6=Salazar|first6=Ángel|last7=Yusta|first7=Iñaki|last8=Chaos|first8=Aida|last9=Pérez-Martínez|first9=Carmen|last10=Navas|first10=Ana|date=2017-12-01|title=Anthropogenic and climatic factors enhancing hypolimnetic anoxia in a temperate mountain lake|url=http://www.sciencedirect.com/science/article/pii/S0022169417307254|journal=Journal of Hydrology|language=en|volume=555|pages=832–850|doi=10.1016/j.jhydrol.2017.10.049|bibcode=2017JHyd..555..832S |issn=0022-1694}}

Notably, anoxic conditions in temperate lakes have the potential to create a positive feedback, whereby anoxia during a given year begets increasingly severe and frequent occurrences of anoxia in future years.{{cite journal |last1=Lewis |first1=Abigail S. L. |last2=Lau |first2=Maximilian P. |last3=Jane |first3=Stephen F. |last4=Rose |first4=Kevin C. |last5=Be'eri-Shlevin |first5=Yaron |last6=Burnet |first6=Sarah H. |last7=Clayer |first7=François |last8=Feuchtmayr |first8=Heidrun |last9=Grossart |first9=Hans-Peter |last10=Howard |first10=Dexter W. |last11=Mariash |first11=Heather |last12=DelgadoMartin |first12=Jordi |last13=North |first13=Rebecca L. |last14=Oleksy |first14=Isabella |last15=Pilla |first15=Rachel M. |last16=Smagula |first16=Amy P. |last17=Sommaruga |first17=Ruben |last18=Steiner |first18=Sara E. |last19=Verburg |first19=Piet |last20=Wain |first20=Danielle |last21=Weyhenmeyer |first21=Gesa A. |last22=Carey |first22=Cayelan C. |title=Anoxia begets anoxia: A positive feedback to the deoxygenation of temperate lakes |journal=Global Change Biology |date=January 2024 |volume=30 |issue=1 |pages=e17046 |doi=10.1111/gcb.17046|pmid=38273535 |hdl=10919/118062 |hdl-access=free }} Anoxia can lead to release of nutrients from sediment, which contribute to increased phytoplankton growth. Increased phytoplankton growth subsequently increases decomposition, perpetuating hypolimnetic oxygen declines. This positive feedback effect has been termed the Anoxia Begets Anoxia feedback.

In eutrophic lakes where the hypolimnion is anoxic, hypolimnetic aeration may be used to add oxygen to the hypolimnion. Adding oxygen to the system through aeration can be costly because it requires significant amounts of energy.

• Epilimnion
• Metalimnion

{{Reflist}}

• [http://wow.nrri.umn.edu/wow/teacher/thermal/teaching.html Water on the Web] {{Webarchive|url=https://web.archive.org/web/20040510151706/http://wow.nrri.umn.edu/wow/teacher/thermal/teaching.html |date=2004-05-10 }}

Category:Limnology

Category:Aquatic ecology