User:Ruite006/Fen

To do
We need more on nutrient cycling, especially in the phosphorus section. Beyond that, general improvements/revisions, nearing completion.

> I inserted info and 3 more sources for phosphorus and nutrient cycling. I also pulled some more info from a source I'd already uploaded.

Biogeochemistry notes
(Some of this stuff is about wetlands in general, but it can still be applied to fens.)

Fens are often found on top of calcareous rocks. Wetland soils are unique because of their anoxic conditions. Because there's oxygen in the atmosphere, pretty much everything tends to be oxidized. Not so in wetlands, whose constant inundation creates anoxic conditions. Anoxic conditions result in reduced chemistry. The major driver of fen chemistry is hydrology.

Reduced soils in anoxic conditions change the availability of certain nutrients (for example, NO3 may become NH4). Therefore, plants need special adaptations to live in reduced soils.

Carbon in a wetland has three main fates: normal respiration, burial as peat, or decomposition to methane. Wetlands tend to be sinks of N, P, and S, whereas most terrestrial ecosystems are sources of these. If peat accumulates to a depth of greater than 10 cm, roots can be cut off from the mineral-rich soil underneath the peat. The result is that the peatland becomes more dependent on rain for nutrients. Peatlands require a stable water level. If water levels get too low, then peat can burn or decompose. In order for peat to accumulate, primary production by plants has to be greater than decomposition. When peat is thin, groundwater is the main water source. Calcium in the groundwater prevents big drops in pH. Nitrogen and phosphorous control how fertile the wetland is. As peat accumulates, groundwater can get cut off, turning fens into bogs. Peat accumulation can raise the bog above the surrounding landscape, resulting in a dome-shaped bog. Melting snow can maintain fens by supplying oxygen rich water?

Root decomposition plays a crucial role in chemical cycling within fens, and decomposition usually is performed by resident mosses. Within temperate fens, root decomposition by mosses plays a role in carbon cycling alongside other nutrients.

Groundwater characteristics play a huge role in determining fen composition. These guys looked at isolated vs. connected fens (isolated meaning isolated from surface waters such as lakes/rivers) and found that isolated fens are richer than connected fens. Rich and extremely rich fens tend to be limited by phosphorous. Medium fens tend to be limited by NO3-.

Why do we care about calcium concentrations so much? https://mysite.science.uottawa.ca/idclark/GEO4342/2009/Weathering.pdf. Calcium-rich minerals such as limestone (CaCO3) can dissolve in small concentrations in groundwater. When limestone dissolves, it dissociates into Ca2+ and CO3(2-). CO3(2-) is in equilibrium with bicarbonate and carbonic acid by picking up protons out of solution. Carbonic acid is in equilibrium with water and and carbon dioxide, and carbon dioxide leaves the system by returning to the atmosphere. In this way, the dissolution of calcium carbonate results in the loss of two protons from the system, increasing pH.

Accumulation of peat can isolate vegetation from mineral-rich groundwater, making the peatland more dependent on precipitation for water. There are a few important terms associated with peatlands:


 * 1) Ombrogenous: the wetland water supply is dominated by precipitation
 * 2) Minerogenous/geogenous: the wetland water supply is dominated by mineral-rich groundwater
 * 3) Ombrotrophic: the wetland receives nutrients through precipitations and airborne dust
 * 4) Minerotrophic: the wetland receives nutrients through mineral-rich groundwater

A common convention among peatland ecologists is to call the ombrotrophic peatlands bogs and the minerotrophic peatlands fens.

"Rich" and "poor" are referencing biodiversity, not nutrient status. A rich fen can be nutrient-poor, and a poor fen can be nutrient rich. However, the two tend to be correlated: a rich fen tends to be nutrient-rich as well. These descriptions have pH values associated with them (note: these pH values are not the exact same as what other sources say!):


 * 1) bog: pH 3.5-4.2
 * 2) poor fen: pH 4-5.5
 * 3) intermediate/moderately rich fen: 5-7
 * 4) Extremely rich fen: 6.8-8

Rich fens tend to occur in places where the soil is calcium-rich, such as limestone.

Which plants are present is most often used to classify how rich a fen is. Plants that are sensitive to minerotrophic conditions allow for identification of richness. For example, a plant that requires a basic pH to grow will not be found in a bog or poor fen.

Nitrogen: The main form of nitrogen available in peat is NH4+ because peat is acidic and reducing. Because nitrogen is an integral part of organic matter, there is a lot of nitrogen in peat. There are three forms of nitrogen found in peatlands (should include fens): organic, oxidized (NO3 and NO2), and reduced (NH4). Nitrogen fixation occurs in peatlands, taking atmospheric N2 and transforming it into NH3 and incorporating that into organic molecules. Ammonification also happens.

The type of peat present can be a good indicator of where along the gradient a certain mire lies. If the peat is dominated by dead sedges (species from the genus Carex), the mire is likely a fen. If the peat is instead dominated by dead Sphagnum mosses, the mire is likely a very poor fen or bog. By sampling the peat at different locations in a mire and determining which species make up the peat, a boundary can be determined between a fen and bog.

Rich fens have been shown to have limited plant growth due to saturation of Phosphorus within the fen. This is counteracted by accumulation of mosses and mycorrhiza. The two drive phosphorus cycling within fens through redox reactions and stimulate growth of forbs and bacteria, supporting the idea that the definition of a "rich" fen doesn't always mean biodiversity is low.

Fen pH is buffered by the presence of carbonate and bicarbonate. This tends to keep the pH stable. Calcium content in the water is often used to determine extent of contact with groundwater. The pH values listed here for bogs/poor/rich fens are different from that laid out by The Biology of Peatlands. The concentration of the four important base ions (Ca, Mg, K, Na) in rich fens was determined to be five times that of poor fens. Rich fens also have high concentrations of sulfate (SO4) and bicarbonate (HCO3-).

Extreme rich fens and marl fens often have marl deposits. Because extremely rich fens will often accumulate marl, the two terms are sometimes used interchangeably. Poor fens may be nutrient-poor because they have little groundwater input, the groundwater they do receive flows through materials that aren't very soluble (like granite), or the minerals the water does flow through don't act as a buffer (like sand).

Marl is a soft, unconsolidated deposit of mostly calcium carbonate mixed with clay and organic matter. In fens, it occurs when calcium carbonate precipitates out of solution when the partial pressure of carbon dioxide in the solution falls. This drop in carbon dioxide levels is due to direct loss to the atmosphere or removal by plants for photosynthesis. Marl is often found in extremely rich fens.

Fens, as a type of wetland, share many biogeochemical characteristics and processes with other wetlands. The most important determinant of wetland characteristics is hydrology because it creates conditions unique to wetlands. They have high primary productivity due to water flow through the wetland. Bogs, on the other hand, are stagnant (don't have water flow), so they tend to have much lower primary productivity. In most wetlands, there is a very thin oxidized layer on top of the anaerobic layers. This is important in nutrient cycling, as oxidized species such as Fe3+, Mn4+, NO3-, and SO42- are often found here. Under reduced conditions in the anaerobic layer, a number of reactions take place, including denitrification, manganese reduction, iron reduction, sulfate reduction, and methanogenesis.

This textbook divides wetland biogeochemistry into two major areas: intersystem cycling and exchange with other ecosystems. We probably won't have enough to make these two separate sections. It's difficult to make broad statements about wetland biogeochemistry or even about individual types of wetlands because they're so varied.

Fens emit substantially more methane than bogs do. The high water table typical of fens prevents oxidation of methane, so it escapes into the atmosphere.

Fens where the dominant cation is calcium and the dominant anion is bicarbonate are rich fens.They are characterized by brown mosses of the family Amblystegiaceae and lots of sedges. Extreme rich fens have pH > 7 and often have marl. Moderate-rich (moderately rich) fens have pH 5.5-7. Poor fens are acidic and dominated by Sphagnum mosses, which decrease nutrient availability and acidify. Poor fens are more similar to rich fens than bogs hydrologically, but more similar to bogs in terms of vegetation and chemistry.

It is difficult to determine if peatlands as a whole are sinks or sources of carbon. Primary production in peatlands is greater than decomposition, so carbon builds up in peat, sequestering it. However, peatlands do emit methane, which is a potent greenhouse gas. Peatlands are a major stores of carbon. Peatlands dominated by brown mosses and sedges produce much more CH4 than Sphagnum-dominated peatlands.

Beavers can have a huge impact on fens. Beavers can flood or drain fens by dam building or channel excavation, respectively. This can significantly alter the vegetation or turn it into a completely different ecosystem. It has also been observed that beaver flooding of a bog has "reverted" it back into a minerotrophic fen with characteristic fen vegetation.

Vegetation and biodiversity notes
Most have brown mosses. They are often home to rare species. One such species is the bog turtle, which lives in wet meadows and fens.

,, , Mosses, particularly Sphagnum mosses, have been shown to play a crucial role in the development of vegetation and increase of biodiversity of fens. They play a crucial role in nutrient cycling, particularly in phosphorus and nitrogen cycling. Furthermore, Sphagnum mosses can invade and proliferate within rich fens, gradually acidifying the waters and converting rich fens to poor fens.

Poor fen vegetation is an intermediate between

Fen vegetation generally includes graminoids, herbs, low shrubs, few to no trees, and bryophytes, especially brown mosses and in some cases Sphagnum mosses. Aerobic methanotrophs live near the water surface where they have access to methane from the reducing peat below and oxygen from the atmosphere above. The methane is made by methanogenic Archaea. (Note: We should find a lot more on this, I bet we could find a ton about microbial metabolism vs. location in peat). Fungi are abundant in the upper layers of peat where the peat is aerated. Microalgae are also abundant in peatlands.

Sphagnum mosses dominate in bogs and poor fens, while brown mosses dominate in rich fens. Graminoids are characteristic of fens, making fens attractive places to graze and make hay. Fens can be rich in herbs (leafy plants that aren't woody), especially when peat is thin and nutrients are available. Grazing and mowing of fens can actually increase diversity of herbs by reducing the dominant graminoids. Trampling also helps by promoting seed germination. Fens can also be home to some aquatic plants in open water patches within fens. One example would be a fen that borders a lake: aquatic species can grow along the edges of the fen. There are many short to intermediate-height shrubs that can grow in fens, but remember that fens are characterized by few large shrubs and trees.

Generally, species richness increases as you go from bog to poor fen to rich fen. Herbs and graminoids contribute the most to biodiversity in fens.

Generally, as the distance between the surface and the water table (the height of the water table) increases, the height of the plants will increase as well. For example, in highly saturated conditions, short mosses and liverworts may dominate, whereas trees may grow in soils high above the water table.

Fens are among the most diverse wetland types.

Threats to fens notes
Fens are particularly vulnerable ecosystems because they depend on calcium-rich groundwater that can be cut off through drainage. They are also generally small and isolated.

Fens are threatened by conversion to fields, use for hay, and grazing. They are also threatened by drainage. Because fens rely on calcium-rich groundwater, draining groundwater near the fen can lower the water table and sap the fen of its water source. This is typically done for agriculture. The result is that the fen becomes increasingly dependent on rainwater for its water and nutrients. This acidifies the fen and changes its water chemistry. The end result is a change in species composition. Many of the signature fen species end up disappearing.

Drainage is particularly dangerous for fens because it lowers the water table. Lowering the water table reduces the saturation of peat, which increases aeration. This allows for decomposition of the peat.

Global warming has been demonstrated to adversely affect peatlands. A changing climate may mean warmer and drier summers, which can lower the water table. The result is an increase in carbon dioxide emitted from peatlands.

Other notes
The position of a wetland in the landscape can have a huge effect on what it looks like. For example, fens are sometimes found on slopes because they have access to nutrient-rich water running down the slope and groundwater discharge to the surface. Peatlands, including fens, play a huge role in carbon sequestration. Fens, however, do emit a significant amount of methane, which is a potent greenhouse gas.

Fens are super valuable because they are highly biodiverse. Fens often contain endangered or rare species.

Calcareous fens are one of the most species-rich ecosystems in the temperate zone. They often contain endangered species.

Peat in poor fens is generally one to three meters deep. The surface waters are generally acidic, cool in temperature, and low in nutrients. This is because there is low input from groundwater, so the fen gets its water and nutrients from both mineral-rich groundwater and rainwater.

In fens, the water table is either just below, at, or just above the surface. Peat depth in fens is usually greater than 30 cm, but can be less. Succession in fens often results in fens becoming more ombrotrophic; that is, fens tend to become bogs over time as peat accumulates and cuts off mineral-rich groundwater. However, this is not always the case. Outside influences such as fire, flooding, climate change, or grazing, can turn bogs back into fens. One common example is flooding by beavers, which can revert a bog to a fen.

Paludification and infilling/terrestrialization are major processes that form peatlands.


 * 1) Infilling/terrestrialization: when peat develops along the shores of open water. The formation of peat in open waters allows plants to colonize, and a floating mat of peat can gradually spread over the open water. As peat accumulates, it will fill in the water, and the ecosystem will transition from open water to marsh to fen. As peat continues to develop, the accumulation and raising of the fen surface can separate the fen from the surface waters, increasing acidity. The fen may eventually transition to a bog.
 * 2) Paludification: when peat develops over previously dry soils. There are a number of ways in which this can happen, but one of the most common is through encroachment by existing peatlands. A rise in the water table can saturate soils higher up than the existing peatland, and as a result the peatland may expand upward.

Ombrotrophication: the transition from minerotrophic to ombrotrophic.

Fens do not always transition to bogs over time. While many do, some fens in Western Canada have been shown to be over 10,000 years old. While not as common as fens transitioning to bogs, bogs can transition back into fens due to changes in the hydrology of the area.

Most peatlands that form at high latitudes start off as fens, then transition to bogs as peat accumulates and cuts the ecosystem off from groundwater.

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 the Hudson Bay and James Bay, but can also be found across the country. Fens are also spread across the northern latitudes of Eurasia, including the British Isles and 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 southern Chile and 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 isn't 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 defined by 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
In the textbook Wetland Ecology: Principles and Conservation, Paul A. Keddy offers a somewhat simpler definition of fens 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." It should be noted that this definition differentiates fens from swamps and marshes by the presence of peat.

The Biology of Peatlands (Rydin) definition
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 (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
The major determinant of fen biota and biogeochemistry in fens, like other wetlands, is hydrology. Fen soils are constantly inundated because the water table is just below, just above, or at 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, 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 (minerotrophy). 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 fen richness decreases.

Fens tend to be found above rocks that are rich in calcium, such as limestone. When groundwater flows past calcareous (calcium-rich) rocks like limestone (CaCO3), 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:

where carbonic acid (H2CO3) 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 that 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
As a type of wetland, 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.

Carbon
Most carbon arrives in wetlands, including fens, as organic carbon, either from adjacent upland ecosystems or by photosynthesis in the wetland itself. Once In the wetland, organic carbon generally has three main fates: oxidation to CO2 by aerobic respiration, burial as organic matter in peat, or decomposition to methane. In peatlands, including fens, primary production by plants is greater than decomposition, which results in accumulation of organic matter as peat. These peat stores sequester an enormous amount of carbon. Nevertheless, it is difficult to determine whether fens net take up or emit greenhouse gases. This is because fens emit methane, which is a more potent greenhouse gas than carbon dioxide. Methanogenic Archaea that reside in the anaerobic layers of peat combine carbon dioxide and hydrogen gas to form methane and water. This methane can then escape to the atmosphere and exert its warming effects. 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
Fens play an important role in the global nitrogen cycle due to the anaerobic conditions found in their soils, which facilitate the oxidation or reduction of one form of nitrogen to another. Most nitrogen arrives in wetlands as nitrate from runoff, in organic matter from other areas, or by nitrogen fixation in the wetland. There are three main forms of nitrogen found in wetlands: nitrogen in organic matter, oxidized nitrogen (nitrate or nitrite), and ammonium.

Nitrogen is abundant in peat. When the organic matter in peat is decomposed in the absence of oxygen, ammonium is produced via ammonification. In the oxidized surface layer of the wetland, this ammonium is oxidized to nitrite and nitrate by nitrification. The production of ammonium in the reduced layer and its consumption in the top oxidized layer drives upward diffusion of ammonium. Likewise, nitrate production in the oxidized layer and nitrate consumption in the reduced layer by denitrification drives downward diffusion of nitrate. Denitrification in the reduced layer produces molecular nitrogen and some nitrous oxide, which which then exit the wetland to the atmosphere. Nitrous oxide is a potent greenhouse gas whose production is limited by nitrate and nitrite concentrations in fens.

Nitrogen, along with phosphorus, controls how fertile a wetland is.

Phosphorus
Almost all of the phosphorus that arrives in a wetland does so through sediments or plant litter arriving from other ecosystems. Along with nitrogen, phosphorus limits wetland fertility. Under basic conditions like those found in extremely rich fens, calcium will bind to phosphate anions to make calcium phosphates, which are unavailable for uptake by plants. Mosses also play a considerable role in aiding plants in phosphorus uptake by decreasing soil phosphorus stress and stimulating phosphatase activity in organisms found below the moss cover. Helophytes have been shown to also bolster phosphorus cycling within fens, especially in fen reestablishment, due to their ability to act as a phosphorus sink—this prevents the any residual phosphorus in the fen from being transferred away from the fen. 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 plays a large role in phosphorus cycling within fens. The ability for iron to bind to the high levels of inorganic phosphate within the fen can lead to a toxic environment and can inhibit plant growth. In iron-rich fens, the area can become vulnerable to acidification, excess nitrogen and potassium, and low water levels. 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.

Bog-rich fen gradient
Bogs and fens can be thought of as two ecosystems on a gradient from poor to rich, with bogs at the poor end, extremely rich fens at the rich end, and poor fens in between. In this context, "rich" and "poor" refer to the species richness, or how biodiverse a fen or bog is. Species richness is strongly influenced by pH and concentrations of calcium and bicarbonate, so these factors together may be used to classify where along the gradient a particular fen falls. In general, rich fens are minerotrophic, or dependent on mineral-rich groundwater, while bogs are ombrotrophic, or dependent on precipitation for water and nutrients. Poor fens fall between these two.

Rich fens
Rich fens are strongly minerotrophic; that is, a large proportion of their water comes from mineral-rich ground or surface water. This water is dominated by calcium and bicarbonate, resulting in a slightly acidic to slightly basic pH, which is characteristic of rich fens. 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. These are often pH 7 or greater. Rich and intermediate rich fens are generally neutral to slightly acidic, with a pH of approximately 7 to 5. Rich fens are not always very productive; at high calcium concentrations, calcium ions bind to phosphate anions, reducing the availability of phosphorous and decreasing primary production. Brown mosses (family Amblystegiaceae) and sedges (genus Carex) are the dominant vegetation. 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 alike to rich fens than to bogs, but in terms of vegetation composition and chemistry, they are more similar to bogs than rich fens. They 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
Fens face many threats, but they are most commonly lost to conversion to agricultural lands. In climates where agriculture is possible, fens have been drained for agricultural use, including crop production, grazing, and hay making. Directly draining a fen is particularly damaging because it lowers the water table. A lower water table can improve aeration and dry out peat, allowing for aerobic decomposition or burning of the organic matter in peat. Indirectly draining a fen or decreasing its water supply can be just as damaging. Disrupting groundwater flow into the fen with nearby human activities such as quarrying or residential development changes how much water and nutrients enter the fen. This can make the fen more ombrotrophic (dependent on precipitation), which results in acidification and a change in water chemistry. Species composition changes often follow, and many signature fen species disappear.

Fens are also threatened by invasive species, fragmentation, peat cutting, and pollution. Non-native invasive species, such as the common buckthorn in North America, can invade fens and outcompete rare fen species, reducing biodiversity. 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 of invasive species. Common pollutants of fens include road salts, nutrients from septic tanks, and runoff of agricultural fertilizers and pesticides.