Editing Fen (section)

=== Hydrological conditions ===
[[Hydrology|Hydrological]] conditions, as seen in other wetlands, are a major determinant of fen biota and [[biogeochemistry]].<ref name=":23">{{Cite book|last=Keddy|first=Paul A. |title=Wetland ecology: principles and conservation |date=2010 |publisher=Cambridge University Press |isbn=978-1-139-22365-2 |edition=2nd |location=Cambridge |oclc=801405617}}</ref> Fen soils are constantly inundated because the water table is at or near the surface.<ref name=":6">{{Cite book|last=Rydin|first=Håkan |title=The biology of peatlands |date=2013 |others=J. K. Jeglum |isbn=978-0-19-150828-8 |edition=Second |location=Oxford, UK |oclc=861559248}}</ref> The result is anaerobic (oxygen-free) soils due to the slow rate at which oxygen diffuses into waterlogged soil.<ref name=":23"/> Anaerobic soils are ecologically unique because Earth's atmosphere is oxygenated, while most terrestrial ecosystems and surface waters are aerobic. The anaerobic conditions found in wetland soils result in [[Redox|reduced]], rather than [[oxidized]], soil chemistry.<ref name=":23"/>

A hallmark of fens is that a significant portion of their water supply is derived from [[groundwater]] (minerotrophy).<ref name=":6" /> Because hydrology is the dominant factor in wetlands, the chemistry of the groundwater has an enormous effect on the characteristics of the fen it supplies.<ref name=":3">{{Cite journal |last1=Godwin |first1=Kevin S. |last2=Shallenberger |first2=James P. |last3=Leopold |first3=Donald J. |last4=Bedford |first4=Barbara L. |date=December 2002 |title=Linking landscape properties to local hydrogeologic gradients and plant species occurrence in minerotrophic fens of New York State, USA: A Hydrogeologic Setting (HGS) framework |journal=Wetlands |volume=22 |issue=4 |pages=722–737 |doi=10.1672/0277-5212(2002)022[0722:llptlh]2.0.co;2 |s2cid=20623975 |issn=0277-5212 |url=https://link.springer.com/article/10.1672/0277-5212%282002%29022%5B0722%3ALLPTLH%5D2.0.CO%3B2 |url-access=subscription |access-date=2021-04-05 |archive-date=2018-06-04 |archive-url=https://web.archive.org/web/20180604201340/https://link.springer.com/article/10.1672%2F0277-5212%282002%29022%5B0722%3ALLPTLH%5D2.0.CO%3B2 |url-status=live }}</ref> Groundwater chemistry, in turn, is largely determined by the geology of the rocks that the groundwater flows through.<ref>{{cite book |last=Fitts |first=Charles R. |chapter=10 – Groundwater Chemistry |date=2013 |doi=10.1016/B978-0-12-384705-8.00010-8 |title=Groundwater Science |edition=Second |pages=421–497 |editor-last=Fitts|editor-first=Charles R. |location=Boston |publisher=Academic Press |language=en |isbn=978-0-12-384705-8}}</ref> Thus, the characteristics of a fen, especially its pH, are directly influenced by the type of rocks its groundwater supply contacts. pH is a major factor in determining fen species composition and richness, with more basic fens called "rich" and more acidic fens called "poor."<ref name=":6" /> Rich fens tend to be highly biodiverse and harbor a number of rare or endangered species, and biodiversity tends to decrease as the richness of fen decreases.<ref name=":3" /><ref name=":6" />

Fens tend to be found above rocks that are rich in calcium, such as [[limestone]].<ref name=":23"/> When groundwater flows past calcareous (calcium-rich) rocks like limestone ([[calcium carbonate]]), a small amount dissolves and is carried to the fen supplied by the groundwater.<ref name=":12">{{Cite book|last=Clark|first=Ian|title=Environmental Geochemistry of Isotopes|publisher=Unpublished|year=2006|location=University of Ottawa|pages=1–7|chapter=Chapter 6: Weathering}}</ref> When calcium carbonate dissolves, it produces [[bicarbonate]] and a [[calcium]] [[cation]] according to the following equilibrium:<ref name=":12" />

<chem>CaCO3 + H2CO3 <=> Ca^2+ + 2HCO3^-</chem>

where [[carbonic acid]] (H<sub>2</sub>CO<sub>3</sub>) is produced by the dissolution of [[carbon dioxide]] in water.<ref name=":12" /> In fens, the bicarbonate anion produced in this equilibrium acts as a pH buffer, which keeps the pH of the fen relatively stable.<ref name=":11">{{Cite journal|last=Bourbonniere|first=Richard A.|date=January 2009 |title=Review of Water Chemistry Research in Natural and Disturbed Peatlands |journal=Canadian Water Resources Journal |language=en |volume=34 |issue=4 |pages=393–414 |doi=10.4296/cwrj3404393|bibcode=2009CaWRJ..34..393B |issn=0701-1784 |doi-access=free}}</ref> Fens supplied by groundwater that doesn't flow through minerals and act as a [[Buffer solution|buffer]] when dissolved tend to be more acidic.<ref name=":10">{{Cite journal|last1=Bedford|first1=Barbara L.|last2=Godwin|first2=Kevin S.|date=September 2003 |title=Fens of the United States: Distribution, characteristics, and scientific connection versus legal isolation |journal=Wetlands|volume=23|issue=3|pages=608–629|doi=10.1672/0277-5212(2003)023[0608:fotusd]2.0.co;2 |s2cid=24228048 |issn=0277-5212}}</ref> The same effect is observed when groundwater flows through minerals with low solubility, such as sand.<ref name=":10" />

In extreme rich fens, calcium carbonate can [[Precipitation (chemistry)|precipitate]] out of solution to form [[marl]] deposits.<ref name=":10" /> Calcium carbonate precipitates out of solution when the [[partial pressure]] of carbon dioxide in the solution falls.<ref name=":13">{{Cite journal|last=Bartigs|first=Rodney|date=March 1984|title=Marl Wetlands in Eastern West Virginia: Distribution, Rare Plant Species, and Recent History|journal=Castanea|volume=49|pages=17–25}}</ref> The decrease in carbon dioxide partial pressure is caused by uptake by plants for photosynthesis or direct loss to the atmosphere.<ref name=":13" /> This reduces the availability of carbonic acid in solution, [[Le Chatelier's principle|shifting the above equilibrium]] back towards the formation of calcium carbonate. The result is the precipitation of calcium carbonate and the formation of marl.<ref name=":13" />