Wetlands play a critical role in regulating the movement of water within watersheds as well as in the global water cycle (Richardson 1994; Mitsch and Gosselink 1993). Wetlands, by definition, are characterized by water saturation in the root zone, at, or above the soil surface, for a certain amount of time during the year. This fluctuation of the water table (hydroperiod) above the soil surface is unique to each wetland type.
Wetlands store precipitation and surface water and then slowly release the water into associated surface water resources, ground water, and the atmosphere. Wetland types differ in this capacity based on a number of physical and biological characteristics, including: landscape position, soil saturation, the fiber content/degree of decomposition of the organic soils, vegetation density and type of vegetation (Taylor et al. 1990):
Diagram of a waterbalance, where P=precipitation, Pn= net precipitation, ET=evapotranspiration, I=Interception, Si=surface water inflow, So=surface water outflow, Gi= groundwater inflow, Go=groundwater outflow, T= Tide, V=change in storage, and t=time (Mitsch and Gosselink, 2000).
Landscape position affects the amount and source of water in a wetland. For example, wetlands that are near a topographical height, such as a mountain bog, will not receive as much runoff as a marsh in a low area amidst fields. Wetlands can be precipitation dominated, ground water dominated, or surface flow dominated. Wetlands on local topographic heights are often precipitation dominated. Precipitation dominated wetlands may also be in flat or slightly elevated areas in the landscape, where they receive little or no surface runoff. Generally such wetlands have a clay and peat layer that retains the precipitation and also prevents discharge from ground water. Wetlands also form in landscape positions at which the water table actively discharges, particularly at the base of hills and in valleys. Such groundwater dominated wetlands may also receive overland flow but they have a steady supply of water from and to groundwater. Most wetlands in low points on the landscape or within other water resources are dominated by overland flow. Such riverine, fringe (marsh), and tidal wetlands actively play a role in the landscape since they come in contact with, store, or release large quantities of water and act upon sediments and nutrients. These wetlands may be recharged by ground water as well, but surface water provides the major source of water.
Discharge-recharge interchanges between wetlands and groundwater systems including: a. marsh as a depression receiving groundwater flow, a ‘discharge’ wetland. b. groundwater spring or seep wetland or groundwater slope wetland at the base of a steep slope. c. floodplain wetland fed by groundwater. d. marsh as a ‘recharge wetland’ adding water to groundwater. e. perched wetland or surface water depressin wetland. f. groundwater flow through a tidal wetland (Mitsch and Gosselink, 2000).
Soil saturation and fiber content
Soil saturation and fiber content are important factors in determining the capacity of a wetland in retaining water. Like a sponge, as the pore spaces in wetland soil and peat become saturated by water, they are able to hold less additional water and are also able to release the water more easily. Clay soils retain more water than loam or sand, and hold the water particles more tightly through capillary action since pore spaces are small and the water particles are attracted to the negatively charged clay. Pore spaces between sand particles are large and water drains more freely since less of the water in the pore is close enough to be attracted to the soil particle.
Water drains more freely from the least decomposed (fibric) peat because pore spaces are large and the surface area for capillary action is small. Sapric peat (most decomposed, fibers unrecognizable) and hemic peat (intermediate) have very small pores. Water moves very slowly in such peats. Water in wetlands, as a result, flows over the surface or close to the surface in the fibric layer and root zone (acrotelm) (Boelter and Verry 1977). Thus wetlands with sapric peat and clay substrate will store water but will have no ground water discharge (inflow) or outflow (recharge).
Vegetation density and type
Stems cause friction for the flow of the water, thus reducing water velocity. As density of vegetation increases, velocity decreases. Plants that are sturdy, such as shrubs and trees are more important in this function than grasses.
During the growing season, plants actively take up water and release it to the atmosphere through evapotranspiration. This process reduces the amount of water in wetland soil and increases the capacity for absorption of additional precipitation or surface water flow. As a result, water levels and outflow from the wetland are less than when plants are dormant. Larger plants and plants with more surface area will transpire more.
Wetlands help maintain the level of the water table and exert control on the hydraulic head (O'Brien 1988; Winter 1988). This provides force for ground water recharge and discharge to other waters as well. The extent of ground water recharge by a wetland is dependent upon soil, vegetation, site, perimeter to volume ratio, and water table gradient (Carter and Novitzki 1988; Weller 1981). Ground water recharge occurs through mineral soils found primarily around the edges of wetlands (Verry and Timmons 1982) The soil under most wetlands is relatively impermeable. A high perimeter to volume ratio, such as in small wetlands, means that the surface area through which water can infiltrate into the ground water is high (Weller 1981). Ground water recharge is typical in small wetlands such as prairie potholes, which can contribute significantly to recharge of regional ground water resources (Weller 1981). Researchers have discovered ground water recharge of up to 20% of wetland volume per season (Weller 1981).
Climate control is another hydrologic function of wetlands. Many wetlands return over two-thirds of their annual water inputs to the atmosphere through evapotranspiration (Richardson and McCarthy 1994). Wetlands may also act to moderate temperature extremes in adjacent uplands (Brinson 1993).
The fluctuating water levels (also known as hydrologic flux) that are characteristic of wetlands control the oxidation-reduction (redox) conditions that occur. These redox conditions governed by hydroperiod play a key role in: nutrient cycling, availability, and export; pH; vegetation composition; sediment and organic matter accumulation; decomposition and export; and metal availability and export.
When wetland soil is dry, microbial and chemical processes occur using oxygen as the electron acceptor. When wetland soil is saturated with water, microbial respiration and biological and chemical reactions consume available oxygen. This shifts the soil from an aerobic to an anaerobic, or reduced, condition. As conditions become increasingly reduced, other electron acceptors than oxygen must be used for reactions. These acceptors are, in order of microbial preference, nitrate, ferric iron, manganese, sulfate, and organic compounds.
Wetland plants are adapted to changing redox conditions. Wetland plants often contain arenchymous tissue (spongy tissue with large pores) in their stems and roots that allows air to move quickly between the leaf surface and the roots. Oxygen released from wetland plant roots oxidizes the rhizosphere (root zone) and allows processes requiring oxygen, such as organic compound breakdown, decomposition, and denitrification, to occur (Steinberg and Coonrod 1994).
Changes in frequency, duration, and timing of hydroperiod may impact spawning, migration, species composition, and food chain support of the wetland and associated downstream systems (Crance 1988). Normal hydrologic flux allows exchange of nutrients, detritus, and passage of aquatic life between systems.
Values of wetlands as a result of the functions of hydrologic flux and storage include: water quality, water supply, flood control, erosion control, wildlife support , recreation, culture, and commercial benefits.
Wetlands may be a sink for, or transform, nutrients, organic compounds, metals, and components of organic matter. Wetlands may also act as filters of sediments and organic matter. A wetland may be a permanent sink for these substances if the compounds become buried in the substrate or are released into the atmosphere; or a wetland may retain them only during the growing season or under flooded conditions. Wetland processes play a role in the global cycles of carbon, nitrogen, and sulfur by transforming them and releasing them into the atmosphere.
The values of wetland functions related to biogeochemical cycling and storage include: water quality and erosion control.
The biological and chemical process of nitrification/denitrification in the nitrogen cycle transforms the majority of nitrogen entering wetlands, causing between 70% and 90% to be removed (Reilly 1991; Gilliam 1994).
In aerobic substrates, organic nitrogen may mineralize to ammonium, which plants and microbes can utilize, adsorb to negatively charged particles (e.g., clay), or diffuse to the surface. As ammonia diffuses to the surface, the bacteria Nitrosomonas can oxidize it to nitrite. The bacteria Nitrobacter oxidizes nitrite to nitrate. This process is called nitrification. Plants or microorganisms can assimilate nitrate, or anaerobic bacteria may reduce nitrate (denitrification) to gaseous nitrogen (N2) when nitrate diffuses into anoxic (oxygen depleted) water. The gaseous nitrogen volatilizes and the nitrogen is eliminated as a water pollutant. Thus, the alternating reduced and oxidized conditions of wetlands complete the needs of the nitrogen cycle and maximize denitrification rates (Johnston 1991).
See the section on Nitrogen in the Water Quality and Land Treatment Information section for an in-depth discussion of this compound and the nitrogen cycle.
Phosphorus can enter wetlands with suspended solids or as dissolved phosphorus. Significant quantities of phosphorus associated with sediments are deposited in wetlands (Walbridge and Struthers 1993). Phosphorus removal from water in wetlands occurs through use of phosphorus by plants and soil microbes; adsorption by aluminum and iron oxides and hydroxides; precipitation of aluminum, iron, and calcium phosphates; and burial of phosphorus adsorbed to sediments or organic matter (Richardson 1985; Johnston 1991; Walbridge and Struthers 1993). Wetland soils can, however, reach a state of phosphorus saturation, after which phosphorus may be released from the system (Richardson 1985). Phosphorus export from wetlands is seasonal, occurring in late summer, early fall, and winter as organic matter decomposes and phosphorus is released into surface water.
Dissolved phosphorus is processed by wetland soil microorganisms, plants, and geochemical mechanisms. (Walbridge and Struthers 1993) Microbial removal of phosphorus from wetland soil or water is rapid and highly efficient, however, following cell death, the phosphorus is released again. Similarly, for plants, litter decomposition causes a release of phosphorus. Burial of litter in peat can, however, provide long term removal of phosphorus. Harvesting of plant biomass is needed to maximize biotic phosphorus removal from the wetland system.
The potential for long-term storage of phosphorus through adsorption to wetland soil is greater than the maximum rates of phosphorus accumulation possible in plant biomass (Walbridge and Struthers 1988; Johnston 1991). In alkaline wetlands, such as found in the West, phosphorus precipitates with calcium as calcium phosphate (Novotony and Olem 1994; Walbridge and Struthers 1988). However, the presence of aluminum is the significant predictor of dissolved phosphorus sorption and removal from water in most wetland systems (Richardson 1985; Gale et al. 1994; Walbridge and Struthers 1993). The capacity for phosphorus adsorption by a wetland, however, can be saturated in a few years if it has low amounts of aluminum and iron or calcium (Richardson 1985).
Wetlands along rivers have a high capacity for phosphorus adsorption because as clay is deposited in the floodplain, aluminum (Al) and iron (Fe) in the clay accumulate as well (Gambrell 1994). Thus floodplains tend to be important sites for phosphorus removal from the water column, beyond that removed as sediments are deposited (Walbridge and Struthers 1993).
See the section on Phosphorus in the Water Quality and Land Treatment Information section for an in-depth discussion of this compound.
Wetlands store carbon within peat and soil. Storing carbon is an important function within the carbon cycle, particularly given observations of increasing levels of carbon dioxide in the atmosphere and concerns about global warming. When wetlands are drained, the oxidizing conditions increase organic matter decomposition, thus increasing the release of carbon dioxide. When wetlands are preserved or restored, the wetlands act as a sink for carbon since organic matter decomposition is stable or slowed.
Wetlands are capable of reducing sulfate to sulfide. Sulfide is released to the atmosphere as hydrogen, methyl, and dimethyl sulfides or is bound in insoluble complexes with phosphate and metal ions in wetland sediments (Mitsch and Gosselink 1993). Dim ethyl sulfide released from wetlands may act as a seed for cloud formation (Hader et al. 1991). Sulfate may exist in soils or may enter wetlands through tidal flow or atmospheric deposition.
Wetlands filter suspended solids from water that comes into contact with wetland vegetation. Stems and leaves provide friction for the flow of the water, thus allowing settling of suspended solids and removal of related pollutants from the water column (Johnston 1991). Wetlands may retain sediment in the peat or as substrate permanently (Johnston 1991). Sediment deposition is variable across individual wetlands and wetland types, as deposition depends upon the rate and type of water flow (channelized or sheet flow), particulate size, and vegetated area of the wetland (Aust et al. 1991;Johnston 1991; Crance 1988; USEPA 1993c; Hemond and Benoit 1988).
All soils contain at least a low concentration of metals but in some locations human activities have resulted in metal levels high enough to cause health or ecological risks in water resources. Metals may exist in wetland soils or enter wetlands through surface or ground water flow.
Wetlands can remove metals from surface and ground water as a result of the presence of clays, humic materials (peats), aluminum, iron, and/or calcium (Gambrell 1994). Metals entering wetlands bind to the negatively ionized surface of clay particles, precipitate as inorganic compounds (includes metal oxides, hydroxides, and carbonates controlled by system pH), complex with humic materials, and adsorb or occlude to precipitated hydrous oxides. Iron hydroxides are particularly important in retaining metals in salt marshes. Wetlands remove more metals from slow flowing water since there is more time for chemical processes to occur before the water moves out of the wetland. Burial in the wetland substrate will keep bound metals immobilized. Neutral pH favors metal immobilization in wetlands (Gambrell 1994). With the exception of very low pH peat bogs, as oxidized wetland soils are flooded and reduced, pH converges toward neutrality (6.5 to 7.5) whether the wetland soils were originally acidic or alkaline (Ponnamperuna 1972).
See the Heavy Metal section for more general information on metals.
Wetlands are among the most productive ecosystems in the world (Mitsch and Gosselink 1993). Immense varieties of species of microbes, plants, insects, amphibians, reptiles, birds, fish, and other wildlife depend in some way on wetlands. Wetlands with seasonal hydrologic pulsing are the most productive.
Wetland plants play an integral role in the ecology of the watershed. Wetland plants provide breeding and nursery sites, resting areas for migratory species, and refuge from predators (Crance 1988). Decomposed plant matter (detritus) released into the water is important food for many invertebrates and fish both in the wetland and in associated aquatic systems (Crance 1988). Physical and chemical characteristics such as climate, topography, geology, hydrology, and inputs of nutrients and sediments determine the rate of plant growth and reproduction (primary productivity) of wetlands (Brinson 1993; Mitsch and Gosselink 1993; Weller 1981; Crance 1988).
A wetland with more vegetation will intercept more runoff and be more capable of reducing runoff velocity and removing pollutants from the water than a wetland with less vegetation (Demissie and Khan 1993; Richardson and McCarthy 1994; NC DEM 1993). Wetland plants also reduce erosion as their roots hold the streambank, shoreline, or coastline.
Values associated with biological productivity of wetlands include: water quality, flood control, erosion control, community structure and wildlife support, recreation, aesthetics, and commercial benefits.
Decomposition rates vary across wetland types, particularly as a function of climate, vegetation types, available carbon and nitrogen, and pH (Johnston 1991).
A pH above 5.0 is necessary for bacterial growth and survival (Richardson 1995). Liming, to increase pH, accelerates decomposition, causing the release of carbon dioxide from wetlands and land subsidence (Richardson 1995).
The nutrients and compounds released from decomposing organic matter may be exported from the wetland in soluble or particulate form, incorporated into the soil, or eventually transformed and released to the atmosphere. Decomposed matter (detritus) forms the base of the aquatic and terrestrial food web.
Decomposition requires oxygen and thus reduces the dissolved oxygen content of the water. High rates of decomposition -- such as occur after algae has bloomed -- can reduce water quality and impair aquatic life support. For more information on low dissolved oxygen see DO.
The inundated or saturated conditions occurring in wetlands limit plant species composition to those that can tolerate such conditions. Beaver, muskrat and alligators create or manipulate their own wetland habitat that other organisms, such as fish, amphibians, waterfowl, insects, and mammals can then use or inhabit (Weller 1981; Mitsch and Gosselink 1993).
Wetland shape and size affect the wildlife community and the wetland's function as suitable habitat (Kent 1994b; Brinson 1993; Harris 1988). The shape of the wetland varies the perimeter to area ratio. The amount of perimeter versus area has importance for the success of interior and edge species (Kent 1994b). Shape is also important for the possibility of movement of animals within the habitat and between habitats. Wetland size is particularly important for larger and wide ranging animals that utilize wetlands for food and refuge, such as black bear or moose, since in many locations wetlands may be the only undeveloped and undisturbed areas remaining.
Values associated with community structure and wildlife support in wetlands include: fish and wildlife support, recreation, aesthetics, and commercial benefits.
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