Fen


A fen is a type of peat-accumulating wetland fed by mineral-rich ground or surface water. It is one of the main types of wetland along with marshes, swamps, and bogs. Bogs and fens, both peat-forming ecosystems, are also known as mires. The unique water chemistry of fens is a result of the ground or surface water input. Typically, this input results in higher mineral concentrations and a more basic pH than found in bogs. As peat accumulates in a fen, groundwater input can be reduced or cut off, making the fen ombrotrophic rather than minerotrophic. In this way, fens can become more acidic and transition to bogs over time.
Fens can be found around the world, but the vast majority are located at the mid to high latitudes of the Northern Hemisphere. They are dominated by sedges and mosses, particularly graminoids that may be rarely found elsewhere. Fens are highly biodiverse ecosystems and often serve as habitats for endangered or rare species, with species composition changing with water chemistry. They also play important roles in the cycling of nutrients such as carbon, nitrogen, and phosphorus due to the lack of oxygen in waterlogged organic fen soils.
Fens have historically been converted to agricultural land. Aside from such conversion, fens face a number of other threats, including peat cutting, pollution, invasive species, and nearby disturbances that lower the water table in the fen, such as quarrying. Interrupting the flow of mineral-rich water into a fen changes the water chemistry, which can alter species richness and dry out the peat. Drier peat is more easily decomposed and can even burn. International conservation efforts were originally targeted at preserving fens as unique ecosystems that harbor significant biodiversity. These efforts have been augmented more recently by the realization that fens may provide other services to society, such as carbon sequestration and water quality enhancement.

Etymology

The modern English word fen is derived from Old English fenn, itself derived from Proto-Germanic *fanja-, from the Proto-Indo-European root *pen-.

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. Fens are also spread across the northern latitudes of Eurasia, including Britain and Ireland, as well as Japan, but east-central Europe is especially rich in fens. Further south, fens are much rarer, but do exist under specific conditions. In Africa, fens have been found in the Okavango Delta in Botswana and the highland slopes in Lesotho. Fens can also be found at the colder latitudes of the Southern Hemisphere. They are found in New Zealand and southwest Argentina, but the extent is much less than that of the northern latitudes. Locally, fens are most often found at the intersection of terrestrial and aquatic ecosystems, such as the headwaters of streams and rivers.
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. This makes it difficult to delineate the exact extent of a wetland. Second, terms used to describe wetland types vary greatly by region. The term bayou, for example, describes a type of wetland, but its use is generally limited to the southern United States. 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. The result is a large number of wetland classification systems that each define wetlands and wetland types in their own way. However, many classification systems include four broad categories that most wetlands fall into: marsh, swamp, bog, and fen. While classification systems differ on the exact criteria that define a fen, there are common characteristics that describe fens generally and imprecisely. A general definition provided by the textbook Wetlands describes a fen as "a peat-accumulating wetland that receives some drainage from surrounding mineral soil and usually supports marsh like vegetation."
Three examples are presented below to illustrate more specific definitions for the term fen.

Canadian Wetland Classification System definition

In the Canadian Wetland Classification System, fens are defined by six characteristics:
  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.
  5. Decomposed sedges or brown moss peat are present.
  6. 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:
  1. The wetland is not flooded by lake or stream water.
  2. Woody vegetation 2 meters or taller is absent or canopy cover is less than 25%.
  3. The wetland is minerotrophic.
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

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 soils due to the slow rate at which oxygen diffuses into waterlogged soil. 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 reduced, rather than oxidized, soil chemistry.
A hallmark of fens is that a significant portion of their water supply is derived from groundwater. 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. 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." 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.
Fens tend to be found above rocks that are rich in calcium, such as limestone. When groundwater flows past calcareous rocks like limestone, a small amount dissolves and is carried to the fen supplied by the groundwater. When calcium carbonate dissolves, it produces bicarbonate and a calcium cation according to the following equilibrium:
CaCO3 + H2CO3 <=> Ca^2+ + 2HCO3^-
where carbonic acid is produced by the dissolution of carbon dioxide in water. In fens, the bicarbonate anion produced in this equilibrium acts as a pH buffer, which keeps the pH of the fen relatively stable. 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.
In extreme rich fens, calcium carbonate can precipitate out of solution to form marl deposits. Calcium carbonate precipitates out of solution when the partial pressure of carbon dioxide in the solution falls. The decrease in carbon dioxide partial pressure is caused by uptake by plants for photosynthesis or direct loss to the atmosphere. This reduces the availability of carbonic acid in solution, shifting the above equilibrium back towards the formation of calcium carbonate. The result is the precipitation of calcium carbonate and the formation of marl.

Nutrient cycling

Fens share many biogeochemical characteristics with other wetlands. Like all wetlands, they play an important role in nutrient cycling because they are located at the interface of aerobic and anaerobic environments. Most wetlands have a thin top layer of oxygenated soil in contact with the atmosphere or oxygenated surface waters. Nutrients and minerals may cycle between this oxidized top layer and the reduced layer below, undergoing oxidation and reduction reactions by the microbial communities adapted to each layer. Many important reactions take place in the reduced layer, including denitrification, manganese reduction, iron reduction, sulfate reduction, and methanogenesis. Because wetlands are hotspots for nutrient transformations and often serve as nutrient sinks, they may be constructed to treat nutrient-rich waters created by human activities.
Fens are also hotspots for primary production, as the continuous input of groundwater stimulates production. Bogs, which lack this input of groundwater, have much lower primary production.