Fen

The modern English word fen is derived from Old English fenn ("fen; marsh; mud; dirt"), itself derived from Proto-Germanic *fanja- ("swamp; marsh"), from the Proto-Indo-European root *pen- ("swamp"). == Distribution and extent ==

Fens are distributed around the world, but are most frequently found at the mid-high latitudes of the Northern Hemisphere. 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. Because wetland definitions vary regionally, not all countries define fens the same way. In addition, wetland data is not always available or of high quality. Fens are also difficult to rigidly delineate and measure, as they are located between terrestrial and aquatic ecosystems. == Definition ==

Rigidly defining types of wetlands, including fens, is difficult for a number of reasons. First, wetlands are diverse and varied ecosystems that are not easily categorized according to inflexible definitions. They are often described as a transition between terrestrial and aquatic ecosystems with characteristics of both. 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. • Peat is present. • The surface of the wetland is level with the water table. Water flows on the surface and through the subsurface of the wetland. • The water table fluctuates. It may be at the surface of the wetland or a few centimeters above or below it. • The wetland receives a significant amount of its water from mineral-rich groundwater or surface water. • Decomposed sedges or brown moss peat are present. • The vegetation is predominantly graminoids and shrubs. Wetland Ecology: Principles and Conservation (Keddy) definition The textbook Wetland Ecology: Principles and Conservation offers a somewhat simpler definition of a fen as "a wetland that is usually dominated by sedges and grasses rooted in shallow peat, often with considerable groundwater movement, and with pH greater than 6." This definition differentiates fens from swamps and marshes by the presence of peat. The Biology of Peatlands (Rydin) definition In The Biology of Peatlands fens are defined by the following criteria: • The wetland is not flooded by lake or stream water. • Woody vegetation 2 meters or taller is absent or canopy cover is less than 25%. • The wetland is minerotrophic (it receives its nutrients from mineral-rich groundwater). A further distinction is made between open and wooded fens, where open fens have canopy cover less than 10% and wooded fens have 10–25% canopy cover. If tall shrubs or trees dominate, the wetland is instead classified as a wooded bog or swamp forest, depending on other criteria. == Hydrology and biogeochemistry ==

Hydrology Hydrological conditions are a major determinant of fen biota and biogeochemistry. 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. Poor fens Poor fens are, in many ways, an intermediate between rich fens and bogs. Hydrologically, they are more similar to rich fens than to bogs, but regarding vegetation composition and chemistry, they are more similar to bogs than rich fens. They are much more acidic than their rich counterparts, with a pH of approximately 5.5 to 4. Peat in poor fens tends to be thicker than that of rich fens, which cuts off vegetation access to the mineral-rich soil underneath. In addition, the thicker peat reduces the influence of mineral-rich groundwater that buffers the pH. This makes the fen more ombrotrophic, or dependent on nutrient-poor precipitation for its water and nutrients. Poor fens may also form in areas where the groundwater supplying the fen flows through sediments that don't dissolve well or have low buffering capacity when dissolved. Species richness tends to be lower than that of rich fens but higher than that of bogs. Poor fens, like bogs, are dominated by Sphagnum mosses, which acidify the fen and decrease nutrient availability. == Threats and conservation ==

Threats to fens Fens face many threats to their existence. Historically, a major cause of fen loss has been conversion to agricultural lands. 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. Peat cutting, while much more common in bogs, does happen in fens. Peat cut from fens has many uses, including burning as a fuel. Pollutants can alter the chemistry of fens and facilitate invasion by invasive species. Common pollutants of fens include road salts, nutrients from septic tanks, and runoff of agricultural fertilizers and pesticides. Conservation of fens Significant losses of fens, particularly via conversion to agricultural lands, has reduced their prevalence, especially across Europe and North America. As such, remaining fens are often highly valued by conservationists and are the subject of many conservation efforts, such as the Habitats Directive. There is also interest in restoring fens that have been damaged or degraded. Often, this is attempted by altering or restoring the hydrological conditions at a site (e.g., blocking of drains or ditches). Restoration is not always possible or successful. The particular disturbances that have degraded a particular fen may cause irreversible changes that make restoration unfeasible. Initially, efforts to conserve fens were driven primarily by a desire to protect these unique ecosystems. However, it is increasingly recognized that fens, like other wetlands, deliver a number of services valuable to society (e.g., carbon sequestration, influencing water quality), fostering further support for fen conservation and restoration. == Ecosystem services ==

Once commonly drained for conversion to agricultural purposes or for urban development, wetlands, including fens, are now recognized to offer society a range of services that may include: 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. One major application of this is treatment of agricultural runoff rich in nitrogen. Fens accepting this agricultural runoff can discharge water containing lower levels of nitrogen and other pollutants, improving water quality in nearby aquatic environments. However, fens may be sensitive to pollutants and nutrient loading. Some fens in which plant growth was previously limited by nitrogen availability may have significantly increased primary production with a subsequent decrease in species richness. ==Images ==

File:Kakerdaja fen in spring.JPG|Kakerdaja Fen, Estonia File:Dernford Fen 4.jpg|Dernford Fen, Cambridgeshire File:Sugar Fen 7.jpg|Sugar Fen, Norfolk File:Illustrated diagram of a fen.jpg ==See also==