NATURAL WETLANDS

6.2 NATURAL WETLANDS

Natural wetlands can be broadly classi fied as either coastal wetlands or inland wetlands, where coastal wetlands are in fluenced by alternate floods and ebbs of tides, and inland wet- lands are nontidal. Coastal wetlands in the United States are found along the Atlantic, Paci fic, Alaskan, and Gulf coasts, and there are a wide variety of estuarine and marine fish, shell fish, birds, and mammals that must have coastal wetlands to survive. Coastal wetlands may be further categorized as either marine or estuarine wetlands (Chang, 2002), depend- ing on whether they are adjacent to an open ocean or adjacent to an estuary. In contrast to coastal wetlands, inland wetlands are most common on floodplains along rivers and streams (riverine wetlands), in isolated depressions surrounded by dry land (palustrine wetlands), along littoral zones of lakes and ponds (lacustrine wetlands), and in other low-lying areas where the ground water intercepts the soil surface or where precipitation su fficiently satu- rates the soil. Riverine and lacustrine wetlands are sometimes collectively referred to as riparian wetlands . Wetlands are typically classi fied as marshes, swamps, bogs, or fens.

6.2.1 Marshes

Marshes are wetlands that are frequently or continually inundated with water and charac- terized by emergent soft-stemmed vegetation that is adapted to saturated-soil conditions.

A typical marsh is illustrated in Figure 6.1. There are many di fferent kinds of marshes,

FIGURE 6.1 Typical marsh. (Courtesy of David A. Chin.)

WETLANDS

ranging from prairie potholes in the central and northern United States to the vast Everglades in Florida. Marshes are found in coastal and inland areas, and can contain fresh or salt water. Marshes receive most of their water from surface runo ff, and many marshes are directly connected to the ground water. Marshes typically recharge the ground water and moderate stream flow by providing water to streams, which is an especially important function during periods of drought. The presence of marshes in a watershed reduces dam- age caused by floods because they slow and store floodwater. Marshes are very important in preserving the quality of surface waters. As water moves slowly through a marsh, sedi- ment and other pollutants settle to the bottom of the marsh. Vegetation and microorgan- isms found in marshes utilize excess nutrients that can otherwise pollute surface water.

Freshwater marshes have larger and more diverse plant populations than saltwater or tidal marshes, and irregularly flooded salt marshes exhibit the fewest species of plants (DeBarry, 2004). Salt marshes are typically composed mainly of rushes, sedges, and grasses. Animals seek refuge from predators in the thick marsh vegetation. After marsh plants die, microorganisms break the plants down into detritus, which serves as a food source for many small animals.

6.2.2 Swamps

A swamp is any wetland dominated by wood plants, and are characterized by saturated soils during the growing season and standing water during portions of the year. Swamps serve vital roles in flood protection, nutrient removal, and sediment removal. Swamps are divided into two major categories, depending on the type of vegetation present: forested swamps and shrub swamps.

Forested Swamps Forested swamps are found throughout the United States and are often inundated with floodwater from nearby rivers and streams. A typical forested swamp is illustrated in Figure 6.2. In very dry years they may represent the only shallow water for

FIGURE 6.2 Typical forested swamp.

NATURAL WETLANDS

kilometers, and their presence is critical to the survival of wetland-dependent species such as wood ducks (Aix sponsa), river otters (Lutra canadensis), and cottonmouth snakes (Agkistrodon piscivorus). Common species of trees found in forested wetlands are red maple and pin oak (Quercus palustris) in the northern United States, overcup oak (Quercus lyrata ) and cypress (Taxodium spp.) in the south, and willows (Salix spp.) and western hemlock (Tsuga spp.) in the northwest. Bottomland hardwood swamp is a type of forested swamp found in the south-central United States.

Bottomland hardwood forests are found along rivers and streams in the southeast and south-central United States, generally in broad floodplains. These forested wetlands are typically made up of di fferent species of gum (Nyssa spp.), oak (Quercus spp.), and bald cypress (Taxodium distichum); these trees have the ability to survive in areas that are either seasonally flooded or covered with water much of the year. Bottomland hardwoods serve

a critical role in a watershed by reducing the risk and severity of flooding to downstream communities by providing areas to store floodwater. In addition, hardwood forests improve water quality by filtering and flushing nutrients, processing organic wastes, and attenuat- ing sediment before it reaches open water.

Shrub Swamps Shrub swamps are similar to forested swamps except that shrubby vegetation such as buttonbush, willow, dogwood (Cornus spp.), and swamp rose (Rosa palustris ) predominates. Mangrove swamps are a common type of shrub swamp domi- nated by mangroves, as illustrated in Figure 6.3. Most mangrove swamps are denser than shown in Figure 6.3(a), and Figure 6.3(b) shows a close-up view of a typical dense man- grove swamp. Mangrove swamps are coastal wetlands found in tropical and subtropical regions. The word mangrove refers to both the wetland itself and to the salt-tolerant trees that dominate these wetlands. Florida’s southwest coast supports one of the largest man- grove swamps in the world. In the continental United States, only three species of mangrove grow: red, black, and white mangroves. Red mangrove (Rhizophora mangle) is easily rec- ognized by its distinctive arching roots. Black mangrove (Avicennia spp.), which often grows more inland, has root projections (pneumatophores), which help to supply the plant with air in submerged soils. White mangroves (Laguncularia racemosa) grow farther inland with no outstanding root structures. In Florida, mangrove swamps are dominated by

FIGURE 6.3 Mangrove swamp.

WETLANDS

red and black mangroves. Mangrove swamps are constantly replenished with nutrients transported by freshwater runo ff from the land and flushed by the ebb and flow of tides. Mangroves perform a variety of important functions, such as bu ffering the shoreline against heavy wave action, filtering sediment- and pollutant-laden water, preventing shoreline ero- sion, and serving as rookeries for a variety of wading birds. Mangrove trees support a signi ficant fish habitat for many juvenile fish species, including barracuda and snapper.

6.2.3 Bogs

Bogs are one of North America’s most distinctive types of wetland. They are characterized by spongy peat deposits, acidic waters, and a floor covered by a thick carpet of sphagnum moss. A typical bog (the Männikjärve bog in Estonia) is illustrated in Figure 6.4. Bogs receive all or most of their water from precipitation rather than from surface runo ff, ground water, or streams. Consequently, bogs are low in the nutrients needed for plant growth, a condition that is enhanced by acid-forming peat mossses. The unique and demanding physical and chemical characteristics of bogs result in the presence of plant and animal communities that demonstrate many adaptations to low nutrient levels, waterlogged con- ditions, and acidic waters, such as carnivorous plants. In the United States, bogs are found in the glaciated northeast and Great Lakes regions, where they are referred to as northern bogs , and bogs are also found in the southeast, where they are referred to as pocosins. Bogs serve an important ecological function in preserving downstream flooding by absorbing precipitation, and bogs have been recognized for their role in regulating the global climate by storing large amounts of carbon in peat deposits.

6.2.4 Fens

Fens are peat-forming wetlands that receive nutrients from sources other than precipita- tion, usually from upslope sources through surface runo ff over surrounding mineral soils

FIGURE 6.4 Aerial view of a bog wetland. (From Aber, 2001. Copyright: J. S. Aber.)

DELINEATION OF WETLANDS

FIGURE 6.5 Typical fen. (From McGlynn, 2005. Photo by Steve Cook.)

and from ground-water recharge. A typical fen is illustrated in Figure 6.5, where the slop- ing land above the fen is shown clearly in the background. Fens di ffer from bogs in that they are less acidic and have higher nutrient levels, which make them able to support a much more diverse plant and animal community. Fens are often covered by grasses, sedges, rushes, and wild flowers. Fens are mostly a northern hemisphere phenomenon, occurring in the northeastern United States, Great Lakes region, the Rocky Mountains, and much of Canada. Fens are generally associated with low temperatures and short growing seasons, where ample precipitation and high humidity cause excessive amounts of mois- ture to accumulate. Fens provide important bene fits in a watershed, including preventing or reducing flood risks, improving water quality, and providing habitat for unique plant and animal communities.