Fen

thumb|upright=1.3|[[Avaste Nature Reserve|Avaste Fen, Estonia. Sedges dominate the landscape. Woody shrubs and trees are sparse.]]

thumb|upright=1.3|[[Wicken Fen, England. Grasses in the foreground are typical of a fen.]]

A fen is a type of peat-accumulating wetland fed by mineral-rich ground or surface water. Fens are highly biodiverse ecosystems and often serve as habitats for endangered or rare species, with species composition changing with water chemistry.

Distribution and extent

Fens are distributed around the world, but are most frequently found at the mid-high latitudes of the Northern Hemisphere.<!-- This sentence is repeated. --> They are found throughout the temperate zone and boreal regions, but are also present in tundra and in specific environmental conditions in other regions around the world. In the United States, fens are most common in the Midwest and Northeast, but can be found across the country. In Canada, fens are most frequent in the lowlands near Hudson Bay and James Bay, but can also be found across the country.

It is estimated that there are approximately 1.1 million square kilometers of fens worldwide, but quantifying the extent of fens is difficult. Third, different languages use different terms to describe types of wetlands. For instance, in Russian, there is no equivalent word for the term swamp as it is typically used in North America.

1. Peat is present.
2. The surface of the wetland is level with the water table. Water flows on the surface and through the subsurface of the wetland.
3. The water table fluctuates. It may be at the surface of the wetland or a few centimeters above or below it.
4. The wetland receives a significant amount of its water from mineral-rich groundwater or surface water. Fen soils are constantly inundated because the water table is at or near the surface. The result is anaerobic (oxygen-free) soils due to the slow rate at which oxygen diffuses into waterlogged soil. Groundwater chemistry, in turn, is largely determined by the geology of the rocks that the groundwater flows through. 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." When calcium carbonate dissolves, it produces bicarbonate and a calcium cation according to the following equilibrium: Fens supplied by groundwater that doesn't flow through minerals and act as a buffer when dissolved tend to be more acidic. The same effect is observed when groundwater flows through minerals with low solubility, such as sand. The decrease in carbon dioxide partial pressure is caused by uptake by plants for photosynthesis or direct loss to the atmosphere. 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. These peat stores sequester an enormous amount of carbon. This is because fens emit methane, which is a more potent greenhouse gas than carbon dioxide. Peatlands dominated by brown mosses and sedges such as fens have been found to emit a greater amount of methane than Sphagnum-dominated peatlands such as bogs.

Nitrogen, along with phosphorus, controls how fertile a wetland is. Helophytes have been shown to bolster phosphorus cycling within fens, especially in fen reestablishment, due to their ability to act as a phosphorus sink, which prevents residual phosphorus in the fen from being transferred away from the it. Under normal conditions, phosphorus is held within soil as dissolved inorganic phosphorus, or phosphate, which leaves trace amounts of phosphorus in the rest of the ecosystem.

Iron is important in phosphorus cycling within fens. Iron can bind to high levels of inorganic phosphate within the fen, leading to a toxic environment and inhibition of plant growth. Peat soils play a role in preventing the bonding of irons to phosphate by providing high levels of organic anions for iron to bind to instead of inorganic anions such as phosphate. In this context, "rich" and "poor" refer to the species richness, or how biodiverse a fen or bog is. In general, rich fens are minerotrophic, or dependent on mineral-rich groundwater, while bogs are ombrotrophic, or dependent on precipitation for water and nutrients. These conditions promote high biodiversity. Within rich fens, there is a large amount of variability. The richest fens are the extreme rich (marl) fens, where marl deposits are often build up. Compared to poor fens, rich fens have higher concentrations of bicarbonate, base cations (Na⁺, Ca²⁺, K⁺, Mg²⁺), and sulfate. Where climates are suitable, fens have been drained for agricultural use alongside crop production, grazing, and hay making. Draining a fen directly is particularly damaging because it lowers the water table. Habitat fragmentation threatens fen species, especially rare or endangered species that are unable to move to nearby fens due to fragmentation. Rich and calcareous fens in particular tend to have high biodiversity and contain rare plants.

Water quality

Wetlands, including fens, are known to modify the chemistry of water flowing through them, removing pollutants and nutrient excess.