Fen
A fen is a type of peat-accumulating wetland fed by mineral-rich ground or surface water.
Typically, this input results in higher mineral concentrations and a more basic pH than found in bogs.
[2] Fens can be found around the world, but the vast majority are located at the mid to high latitudes of the Northern Hemisphere.
[2] They also play important roles in the cycling of nutrients such as carbon, nitrogen, and phosphorus due to the lack of oxygen (anaerobic conditions) in waterlogged organic fen soils.
[1][2] Fens are distributed around the world, but are most frequently found at the mid-high latitudes of the Northern Hemisphere.
In Africa, fens have been found in the Okavango Delta in Botswana and the highland slopes in Lesotho.
[2][6] Locally, fens are most often found at the intersection of terrestrial and aquatic ecosystems, such as the headwaters of streams and rivers.
[2] Fens are also difficult to rigidly delineate and measure, as they are located between terrestrial and aquatic ecosystems.
[2] Rigidly defining types of wetlands, including fens, is difficult for a number of reasons.
[1] The term bayou, for example, describes a type of wetland, but its use is generally limited to the southern United States.
[1] However, many classification systems include four broad categories that most wetlands fall into: marsh, swamp, bog, and fen.
If tall shrubs or trees dominate, the wetland is instead classified as a wooded bog or swamp forest, depending on other criteria.
[11] Anaerobic soils are ecologically unique because Earth's atmosphere is oxygenated, while most terrestrial ecosystems and surface waters are aerobic.
[11] A hallmark of fens is that a significant portion of their water supply is derived from groundwater (minerotrophy).
[12] 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.
[14] Thus, the characteristics of a fen, especially its pH, are directly influenced by the type of rocks its groundwater supply contacts.
[16] Fens supplied by groundwater that doesn't flow through minerals and act as a buffer when dissolved tend to be more acidic.
[17] In extreme rich fens, calcium carbonate can precipitate out of solution to form marl deposits.
[18] The decrease in carbon dioxide partial pressure is caused by uptake by plants for photosynthesis or direct loss to the atmosphere.
[19] Like all wetlands, they play an important role in nutrient cycling because they are located at the interface of aerobic (oxic) and anaerobic (anoxic) environments.
[11] In peatlands, including fens, primary production by plants is greater than decomposition, which results in the accumulation of organic matter as peat.
[19] Methanogenic archaea that reside in the anaerobic layers of peat combine carbon dioxide and hydrogen gas to form methane and water.
[11] Denitrification in the reduced layer produces nitrogen gas and some nitrous oxide, which then exit the wetland to the atmosphere.
[11] Nitrous oxide is a potent greenhouse gas whose production is limited by nitrate and nitrite concentrations in fens.
Iron can bind to high levels of inorganic phosphate within the fen, leading to a toxic environment and inhibition of plant growth.
[24] In iron-rich fens, the area can become vulnerable to acidification, excess nitrogen and potassium, and low water levels.
[11][12] Rich bogs with limited primary production can stabilize with the accumulation of mosses and mycorrhiza, which promote phosphorus cycling and can support the growth of new vegetation and bacteria.
[4] Where climates are suitable, fens have been drained for agricultural use alongside crop production, grazing, and hay making.
[5] This can make the fen more ombrotrophic (dependent on precipitation), which results in acidification and a change in water chemistry.
[5] Common pollutants of fens include road salts, nutrients from septic tanks, and runoff of agricultural fertilizers and pesticides.
