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Types of Wetlands and Their Roles in the Watershed

Introduction

This section focuses on wetlands as water resources within a watershed and, specifically, how they are both a product of and an influence on watershed hydrology and water quality. One useful way to categorize wetlands, for those interested in water quality management and watershed management, is by dominant water source. Wetlands may be precipitation dominated, surface flow dominated, or ground water dominated (Brinson 1993).

Wetlands may have different functions as a result of their position in the landscape and their dominant water source. Although all wetlands receive precipitation, precipitation events serve as the sole source of water for some wetlands. Precipitation-dominated wetlands may supply water to headwater streams and ground water by infiltration.

Riparian wetlands, marshes, and tidal wetlands, which are dominated by surface flow, may remove, store, or release water, nutrients, and sediments.

Mangrove wetlands are not included in this discussion because they are marine or estuarine systems that have little significant function in watershed freshwater dynamics. They do, however, provide aquatic life support similar to that of a tidal salt marsh (described below).

The terms used below for types of wetlands are commonly recognized terms in the United states and are typically used in the wetlands literature.

Precipitation Dominated Wetlands

Bogs

Bogs are waterlogged peatlands in old lake basins or depressions in the landscape, forming where peat accumulation exceeds decomposition as a result of climatic conditions. Bogs in the lower 48 states are found largely in the glaciated northeast, Wisconsin, Minnesota, and Michigan; in the southeast ( pocosins and Carolina bays); and on mountains (Mitsch and Gosselink 1993). Bogs are precipitation dominated because the accumulated peat formations elevate the system surfaces sufficiently relative to the surrounding landscape that there are few or no surface inflows. Since bogs do not receive nutrients or organic matter transported by surface water, they have low rates of primary productivity and decomposition.

Bogs are typically acidic because the dominant living plant matter, Sphagnum moss, releases H+ ions (acidity), and the peat releases organic acids (Mitsch and Gosselink 1993). The pH in bogs can be as low as 3.0 - 4.0 (Camp, Dresser, and McKee 1981). Bogs have specialized and unique flora that has evolved in their nutrient-poor and acidic conditions. An example of this unique flora is the carnivorous pitcher plant, which obtains nutrients from the flies it traps.

Values related to watershed management

Bogs generally have no significant inflows or outflows (Mitsch and Gosselink 1993). Some bogs, however, may act as headwaters, supplying water to downstream reaches; recharge ground water; and maintain the hydraulic pressure of the water table (Brinson 1993).

The role wetlands play in ground water recharge depends in large part on substrate permeability. A characteristic of bogs is a fairly impermeable layer of peat; thus, most bogs do not recharge ground water. If the edges of a bog do consist of permeable soil, recharge into the ground water can occur. One Minnesota bog converts 55% of water input to water yield (stream and ground water), while the adjacent upland hardwood forest converts only 34% (Verry and Timmons 1982).

Life support

Bogs do not support large populations of animals because productivity is low and the water can be quite acidic. However, bogs provide important habitat for such species as moose, deer, black bear, beaver, lynx, fishers, snowshoe hare, otter, and mink, either because bogs occur in remote areas due to the climate or altitude, or because they are not suitable for agriculture, forestry, or development (Mitsch and Gosselink 1993). As the land use in surrounding areas changes, wildlife species are driven to these relatively undisturbed habitats.

Migratory birds use bogs on their flight paths. From warblers to wood ducks, many bird species breed, nest, and feed in bog habitats. The greater sandhill crane, great gray owl, short eared owl, sora rail, and sharp-tailed sparrow depend completely on bogs and fens for survival (Mitsch and Gosselink 1993). Bogs with a pH greater than 4.5 may provide habitat for game fish species such as pike, walleye, bluegill, and smallmouth bass (Camp, Dresser, and McKee 1981; Novotony and Olem 1995).

Pocosins

Pocosins are evergreen shrub bogs found on the Coastal Plain of the southeastern United States, with an estimated 70% occurring in North Carolina. Typically, pocosins are found on high areas of a flat, "water-logged, acidic, nutrient-poor landscape" (the name means swamp on a hill) (Richardson 1991). Pocosins retain rainfall for long periods of time, releasing water slowly through sheet flow. Thus, they may remove nutrients and other compounds from atmospheric deposition. Because of their slightly raised position on the landscape, their contribution to surface water quality improvement is generally limited to situations where the landscape has been modified to permit surface water inflow (for example, pine plantation ditches) (Richardson 1991). Pocosins producing sustained discharges that provide an important contribution to surface water supplies moderate water flow from storm events in the southeastern Coastal Plain. They also provide an important contribution to the global water cycle through evapotranspiration (Richardson and Gibbons 1993; Richardson and McCarthy 1994).

Vernal Pools, Playas, Prairie Potholes, Wet Meadows, and Wet Prairies

These five types of wetlands are often categorized as marshes, but their dependence on precipitation rather than surface water inputs is important enough to distinguish them from typical surface water dominated marshes.

Vernal (spring) pools are small, shallow, intermittently flooded depressions in grasslands or forests, and are usually wet only in the winter and early spring. Vernal pools are found wherever precipitation in the winter and early spring exceeds evapotranspiration and soil infiltration rates.

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Photo courtesy of USDA NRCS

Playas are marshlike ponds found in the arid Southern Great Plains of Texas, New Mexico, Kansas, Oklahoma, and Colorado. Prairie potholes are marshlike ponds that have formed in shallow basins caused by glaciation in the Dakotas, Iowa, and the Canadian prairies. Both playas and prairie potholes receive runoff from surrounding land uses because of their depressional nature. However, water levels fluctuate seasonally as a result of dependence on precipitation, and these two wetland types may be periodically dry for up to several years.

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Photo courtesy of USDA NRCS

Two other wetland types, wet meadows and wet prairies, derive a significant amount of their water supply from precipitation. Wet meadows are grasslands with soil that is waterlogged after precipitation events. Wet prairies are hydrologically intermediate between marshes and wet meadows: standing water occurs for a shorter duration and frequency than in marshes. Wet prairies may receive water from intermittent streams as well as from ground water and precipitation.

Wet meadows and prairies are often categorized as marshes because their vegetative communities are similar to those of marshes. However, they are drier than most marshes and are typically found in lower areas on flat landscapes, surrounded by upland meadows or prairie grasses. These systems may also exist as broad transition zones surrounding or leading downgradient to deeper marshes. Because wet meadows and wet prairies depend largely on precipitation for water inputs, they are generally dry during the summer.

Values related to watershed management

Because these four wetland types are largely isolated from other surface water resources, they typically contribute little to watershed surface water quality. When they do receive surface water inflow, they function like marshes, removing nutrients and other pollutants (Mitsch and Gosselink 1993; Rickerl et al. 1993).

Prairie potholes, wet meadows, and wet prairies generally contribute to ground water recharge (Weller 1981; Mitsch and Gosselink 1993). Potholes also appear to provide water quality improvement values for agricultural runoff (Jacobsen 1994).

Life support

Vernal pools are critical habitats for the life cycles of some animals, including certain amphibians, which rely on them exclusively. The playas of the Southern Great Plains provide refuge to several million migrating ducks, geese, shorebirds, and wading birds each year, as well as habitat for mammals, amphibians, and reptiles. Playas are particularly critical wetland habitat in the intensively farmed, dry plains of New Mexico and Texas (Ducks Unlimited 1991). The Southern Great Plains area is within the migratory corridor known as the Central Flyway; the birds rely on playas for staging, resting, and breeding areas. The prairie potholes form the northern staging, resting, and breeding part of the Central Flyway. An estimated 50 to 75 percent of all the waterfowl in North America are hatched in the prairie potholes (Mitsch and Gosselink 1993).

Ground Water Dominated Wetlands

Fens

Fens are peat-accumulating wetlands that form at low points in the landscape or near slopes where ground water intercepts the soil surface (Mitsch and Gosselink 1993). Water levels are fairly constant all year because the water supply is provided by ground water inputs. Fens, like bogs, tend to be glacial in origin and are found in the northern United States or on mountains and mountainsides. Fens are dominated by herbaceous plants, such as grasses and sedges, typically lack the Sphagnum moss that predominates in bogs, and look like meadows.

Fens may represent an earlier successional stage of peat accumulation than bogs, and over geologic time, fens may become bogs. Unlike bogs, fens receive minerals and nutrients from ground water, because they have built up less peat and ground water is still sufficiently close to the surface. Fens are less acidic than bogs because they have little or no Sphagnum, and because ground water inputs tend to be neutral or alkaline. The pH of fens ranges from 4.0 - 8.0, depending on vegetation and peat type (Camp, Dresser, and McKee 1981). Fens provide less stressful growing conditions for plants and microbes and thus have higher primary productivity and a greater variety of flora and fauna than bogs.

Fens may depend on aquifers that are recharged in uplands. These upland recharge areas may be distant from the wetlands (Brinson 1993). Thus, excessive withdrawal or interception of ground water for municipal and agricultural uses, and reduced urban ground water recharge as a result of increased impervious surfaces can decrease water supply to fens, potentially leading to degradation of these wetland communities (USEPA 1993b).

Values related to watershed management

In addition to their ground water inputs and precipitation, fens may receive runoff and other surface water. They tend to contribute more to downgradient surface water supplies than do bogs because of additional ground and surface water inputs to fens. Fens may help maintain surrounding water tables, exerting influence on the recharge and discharge of local aquifers and thus on the hydrology of other water resources (O'Brien 1988).

Life support

Fens, like bogs, support a great diversity of wildlife species in relatively limited quantities. Like bogs, they are increasingly important habitat for moose, deer, black bear, beaver, lynx, fishers, snowshoe hare, otter, and mink because of development elsewhere (Mitsch and Gosselink 1993). Because fens are more productive than bogs, they support a greater variety of small mammals and are host to greater numbers of their predators. Many bird species (including migratory birds) breed, nest, feed, and find refuge in fens. The greater sandhill crane, great gray owl, short eared owl, sora rail, and sharp-tailed sparrow depend on fens and bogs for survival. Fens provide habitat for more fish species than bogs because fens may have inflow and outflow streams. Species such as pike, walleye, bluegill, smallmouth bass, brook trout, brown trout, and killifish may inhabit fens or streams fed by fens (Camp, Dresser, and McKee 1981). One third of fish taxa in the deserts of the southwestern United States are completely dependent on ground water-fed wetlands and downstream riparian marshes (cienegas) (Meffe, 1989).

Surface Water Dominated Wetlands

Marshes (freshwater)

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Photo courtesy of USDA NRCS

Marshes are one of the broadest categories of wetlands and in general harbor the greatest biological diversity. They are characterized by shallow water, little or no peat deposition, and mineral soils. Marshes are dominated by floating-leafed plants (such as water lilies and duckweed) or emergent soft-stemmed aquatic plants (such as cattails, arrowheads, reeds, and sedges). Marshes form in depressions in the landscape, as fringes around lakes, and along slow-flowing streams and rivers (such riparian marshes are also referred to as sloughs). Marshes are frequently or continually inundated with water (Mitsch and Gosselink 1993). Marshes derive most of their water from surface water, including streams, runoff, and overbank flooding; however, they receive inputs from ground water as well (Mitsch and Gosselink 1993; Brinson 1993). Environmental conditions in marshes lead to high productivity since the pH of most marshes is generally circumneutral and nutrients derived from runoff are plentiful (Mitsch and Gosselink 1993).

Values related to watershed management

Water supply: Marshes may recharge ground water, depending on soil permeability and wetland size. As noted in the Functions section, ground water recharge is related to the perimeter: volume ratio and soil permeability. The frequent lack of a perched water table resulting from peat accumulation and compression allows marshes to recharge more ground water than a peat-accumulating wetland such as a fen or a pocosin. Recharge is relatively plentiful in marshes, and may contribute significantly (up to 20% of volume) to regional ground water supplies (Weller 1981; O'Brien 1988).

Marshes may help reduce local peak and flood flows and moderate stream flow (Demissie and Khan 1993; Mitsch and Gosselink 1993; Gosselink et al. 1990).

Water Quality: As a gross estimation, removal of nitrogen from surface water by marshes is approximately 50% and phosphorus removal is approximately 10 - 15% of inputs (Mitsch and Gosselink 1993). Marshes slow the flow of water moving through the system and facilitate the settling of suspended solids and pollutants adhering to sediment. As noted in the Functions section, vegetation quantity and type are important factors in determining the ability of a wetland to reduce water velocity. Marsh vegetation utilizes nutrients, and, more importantly, provides attachment surfaces and a carbon source for organisms that assimilate and transform nutrients. Plant roots oxygenate the soil and provide additional microbial habitat, facilitating such processes. The greater the amount of open water present, such as when marshes border lakes, the less water quality improvement functions will dominate, and the more sediment-attached pollutants will remain suspended in the water column (Whigham et al., 1988).

Life support

Mammals, reptiles, amphibians, and birds depend on marshes for food, water, and habitat. Although waterfowl such as ducks and herons are commonly associated with marshes, other birds, such as songbirds and hawks, also feed on the life generated within wetlands.

Marshes with deeper water or riparian marshes that are open to rivers (riverine marshes) and lakes (lacustrine marshes) support more diverse and abundant fish life (Mitsch and Gosselink 1993). The cienegas, or riparian marshes of the Southwest, provide essential habitat for one-third of all fish species in this arid region, and many more depend on these small marshes for habitat (Meffe 1989). Species such as pike, muskellunge, largemouth bass and other sunfish species, gar, and bullhead are a few of the game fish that can be found in marshes (Levine and Willard 1990; Camp, Dresser, and McKee 1981). Shallower marshes without predatory fish such as bass may support diverse fish species and amphibians which would not survive if carnivorous fish were stocked.

Riparian Forested Wetlands

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Photo courtesy of
US Army Corps of Engineers

Riparian forested wetlands are dominated by surface water; they are linear systems found along lakes, streams, and rivers from headwaters down to the sea. Riparian forested wetlands are saturated or inundated with water during the winter, when evapotranspiration is low, because plants are dormant and precipitation is high; and during the early part of the growing season, when precipitation and runoff are still abundant. These wetlands are generally not wet in the summer or fall except during flood conditions.

Riparian wetlands do not have particular characteristics of pH or nutrient load, but differ based on inputs, substrate, and vegetation type (Mitsch and Gosselink 1993). However, riparian wetlands are particularly productive ecosystems, receiving large inputs of water and nutrients from upstream sources during flooding. This feature has led to their conversion for agricultural use, a practice that has contributed to water quality degradation.

Southern deepwater swamps are riparian systems notable for the standing water present during much of the year. While they may be traversed by rivers or streams, which provide seasonal water inputs, these systems may also be headwaters. A cypress dome is an anomalous southern deepwater swamp type that is typically precipitation versus surface water, dominated. Cypress domes typically exist as isolated depressions in very gently sloping landscapes.

Other examples of riparian forested wetlands include maple swamps, bottomland hardwood forests, and cottonwood riparian areas.

Values related to watershed management

Riparian systems provide a continuum of water quality benefits. Headwater wetlands are the source of water; the forested wetlands and marshes along low order streams protect water quality and aquatic life; and wetlands along higher order streams provide flood control, water quality maintenance, and life support.

Water supply: Riparian forested wetlands and swamps have a significant water storage and ground water recharge role, and thus are valuable in water supply and flood control (Reilly et al. 1991; Hook et al. 1988; Ewel 1990; Brinson 1993; Demissie and Khan 1993; Brown and Sullivan 1988; Gosselink et al. 1990). The wider the floodplain, the greater the storage action and reduction of flood peaks that can occur. Large floodplains with long retention times can be important ground water recharge areas, depending on substrate permeability (Taylor et al. 1990; O'Brien 1988). A forested wetland overlaying permeable soil may produce 100,000 gallons of water per acre per day (Anderson and Rockel 1991).

Water quality: Riparian wetlands are important sinks for pollutants carried in upland runoff and from upstream areas (Brinson 1993). Riparian wetlands that are adjacent to small streams are particularly valuable in trapping pollutants and preventing nonpoint source pollution from ever reaching larger water resources (Gilliam 1994; Walbridge 1993). Riparian wetlands also serve as valuable transformers of pollutants. They are noted for processing large fluxes of energy and materials from upstream sources, and they typically show high primary productivity, functions that make them important ecological links and valuable habitat.

Examples of the importance of forested wetlands in nutrient removal from water resources:

Life support

Forested riparian wetlands provide cover, spawning, and nursery habitat for numerous fish and shellfish species (Crance 1988). For instance, 90 fish species use the wooded floodplains of the Atchafalaya River in Louisiana. Deepwater swamps can be important refuges for fish during dry periods. In the western United States, healthy fisheries are related to perennial streams with undisturbed riparian wetland zones (Mitsch and Gosselink 1993). Transpiration by coniferous trees maintains a low soil and water temperature that is critical to the survival of cold water fish in streams fed by or within such forested wetlands (Sharitz and Gibbons 1989).

Many animal and bird species depend on riparian forested wetlands for habitat (Mitsch and Gosselink 1993). For example, at least 88 species of birds are completely dependent on western riparian systems. Other bird species use forested wetlands throughout the United States for food and rest during migration, or for breeding and nesting habitat (Mitsch and Gosselink 1993).

Tidal Freshwater Marshes

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Photo courtesy of
US Army Corps of Engineers

Tidal freshwater marshes are found upstream of estuaries where the tides still influence water levels, but where the water is predominantly fresh. Tidal freshwater marshes receive substantial water and nutrients from upstream water resources, as well as inputs from runoff and precipitation. High nutrient inputs contribute to the extremely high primary productivity and biodiversity of these systems.

Values related to watershed management

The physical, biological, and chemical processes occurring in tidal freshwater marshes provide valuable water quality improvement for flows entering estuaries (Mitsch and Gosselink 1993). Tidal freshwater marshes remove approximately 50% of the nitrogen entering the estuarine systems (Novotny and Olem 1995). Nitrogen removal by tidal freshwater marshes is extremely valuable, since most estuaries are nitrogen limited systems. When high levels of nitrogen enter estuaries, algal blooms and fish kills may result. Since the nutrients are used to fuel high productivity in the marshes, they tend to act as sinks for nitrate and phosphorus during the growing season. Nutrients may be exported as detritus in the fall and winter. Like inland marshes, tidal marshes slow the flow of water moving through the system and facilitate the settling of sediment and pollutants adhering to sediment.

Tidal freshwater marshes may help maintain the water table level and protect ground water against saltwater intrusion (OTA 1993).

Life support

Tidal freshwater marshes provide habitat, food, shelter, and nurseries for many fish and shellfish. In addition, approximately half of the organic matter produced in these marshes is transported downstream to the estuary or the sea as detritus, forming the base of the food web.

Some fish species, such as minnows, carp, sunfish, bass, and catfish, spend their entire life cycle in tidal freshwater marshes. (Mitsch and Gosselink 1993). Other fish and shellfish rely on the freshwater marshes for parts of their life cycle and spend the remainder of their lives in the marine environment. Killifish, anchovy, herring, salmon, shad, striped bass, menhaden, spot, tarpon, juvenile brown shrimp, and juvenile white shrimp are examples (Mitsch and Gosselink 1993). Coastal freshwater marshes may support the largest and most diverse bird populations of all wetland habitats.

Tidal Salt Marshes

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Photo courtesy of
US Army Corps of Engineergs

Salt marshes are tidally influenced systems that may receive inflow of fresh water from rivers, runoff, or ground water. Freshwater inflow is important in diluting the salinity of the system. Salinity is the major stressor in this wetland system and limits species to those that have evolved adaptive mechanisms. Readily available nutrients and organic matter from upstream sources and runoff, and the alternating aerobic and anaerobic conditions caused by the tides result in the very significant productivity of salt marsh ecosystems. Salt marshes have the highest primary productivity of all wetland systems and have higher primary productivity than most upland systems.

Values related to watershed management

Tidal salt marshes are efficient nitrogen transformers because of the daily hydrologic cycle, which removes a significant portion of total inputs from the aquatic system in the form of gaseous nitrogen (Mitsch and Gosselink 1993). Marshes act as sinks for total phosphorus, although there may be discharge of inorganic phosphorus into the marine system (Mitsch and Gosselink 1993).

The high rates of sulfur retention in salt marshes (sulfur enters from sea water) may play an important role in immobilization and detoxification of toxic metals (Novotny and Olem 1995).

Life support

The unparalleled primary productivity of salt marshes yields abundant habitat and food for both resident species and marine species that utilize the marshes for only portions of their life cycles. Mussels, oysters, and the majority of the other commercially and recreationally important fish and shellfish of the southeast Atlantic and Gulf coasts utilize salt marshes and meadows of submerged aquatic plants for habitat, refuge, and food (NOAA 1990b, NOAA 1995b). Salt marshes provide critical food and refuge for migrating waterfowl and shore birds as well (Mitsch and Gosselink 1993).

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